Hand-Eye Coordination in Tennis. Exploration of Manually Influenced Eye Dominance and its Association with the Shoulder Position in the Tennis Serve

Contents

List of abbreviatons

List of figure

List of tables

Acknowledgements

Preface

Abstract (English language)

Abstrakt (German language)

List of scientific papers

Background
The tennis serve
Energy transfer and shoulder loading position
Tennis serve and injury
The biomechanics and the energy flow of the kinetic chain in the tennis serve
The visual control mechanisms
The role of visuality and the interactive hand-eye
coordination
Hardware or software – „the abilities and skills“
Vision and sport
Eye dominance testing
Assessment of the shoulder loading position in the tennis serve
The manual influence of the eye dominance and the ability to switch eye dominance in a frontal gaze direction
The role of the shoulder position in the serve
The association between the SLOP and the MIED in the
tennis serve
Rationale of the thesis

Aims

Methods and materials
Overview study I and study II
Study population
Data collection
Data analysis
Study I – T1 (SI/E1a and SI/E1b).
Study I – T2 (SI/E2a and SI/E2b).
Statistical methods
Ethical considerations
Study I - Specific procedure
T1 – one-handed validity test (SI/E1a and SI/E1b)
T2 – two-handed validity test (SI/E2a and SI/E2b)
Description of the PT-Test performance
Description of the CT-Test performance
Validity and reliability T1 and T2
Interlude
Study II - Specific procedure of the SLOP in the tennis serve
System/MatLab
Study population
Data collection
Data analysis
In situ capture
Protocol
Kinetic values
Statistical methods – experiment SII/E1

Results
Study I (SI)
T1 - one-handed validity test; PT (SI/E1a)
T1 - one-handed validity test; CT (SI/E1b)
T1 - SI/E1a and SI/E1b; comparative analysis
T2 - two-handed validity test; both eyes CT and PT (SI/E2a and SI/E2b)
Study II (SII)
Experiment SII/E1 – SLOP Test
Analysis 1 - SII/A1; comparison of closed SLOP
with one-handed crossed and consistent P-MIED
Analysis 2 - SII/A2; comparison of the CSCr1PM with
CSCr1CM and CSCn1PM with CSCn1CM
Tables 1a-g

Discussion
Strengths and limitations
Future perspective
Additional perspectives and considerations
Practical applications
C-MIED Procedure

Conclusions
Epilogue
Summary
Table 2
Table 3
Table 4

References

Declaration

Student agreement:

This is to certify that this dissertation is my original work. All external sources have become the proper attribution and all needed copyright permissions have been obtained. I hereby grant to the University of Central Nicragua the non-exclusive license to archive and access my work in whole or in part in media. This thesis may be available for worldwide access. I retain all other ownership rights to the copyright of my work. I also retain the right to use the thesis, fully or partly in future works like articles or books. I understand that I am free to register the copyright to my work.

Håkan Dahlbo

List of abbreviatons

A1 Anaylsis 1 (2nd study)

A2 Analysis 2 (2nd study)

C-MIED Circular manually influenced eye dominance

Cn1PM One-handed consistent P-MIED

Cn1CM One-handed consistent C-MIED

Cn2PM Two-handed consistent P-MIED

Cn2CM Two-handed consistent C-MIED

Cr1PM One-handed crossed P-MIED

Cr1CM One-handed crossed C-MIED

CS Closed SLOP

CSCn1PM Closed SLOP/Consistent P-MIED

CSCn1CM Closed SLOP/Consistent C-MIED

CSCr1PM Closed SLOP/Crossed P-MIED

CSCr1CM Closed SLOP/Crossed C-MIED

CT Circular Test

E1 Experiment 1 (2nd study)

E1a Experiment 1a (1st study)

E1b Experiment 1b (1st study)

E2a Experiment 2a (1st study)

E2b Experiment 2b (1st study)

EV Expected value

FB Foot-back

FU Foot-Up

H1 1st hyopthesis

H2 2nd hypothesis

HEC Hand-eye coordination

HED Hand-eye dominance

LL Right hand/Left eye, left hand/left eye

LR Right hand/Left eye, left hand/right eye

MIED Manually influenced eye dominance

OS Open SLOP

OV Observed value

P-MIED Porta manually influenced eye dominance

PT Porta Test

RL Right hand/right eye, left hand/left eye

RR Right hand/right eye, left hand/right eye

S1 Study 1

S2 Study 2

SLOP Shoulder loading position

Sw2PM Two-handed switched P-MIED

X2 Chi square

Sw2CM Two-handed switched C-MIED

T1 Test 1

T2 Test 2

p p-value

List of figures

Figure 1a Distance to the target

Figure 1b Circular grip of the racket Figure 1c Circular grip of the ball

Figure 2a One-handed PT

Figure 2b One-handed CT

Figure 3 Comparison; One-handed PT vs. CT

Figure 4a Two-handed PT

Figure 4b Two-handed CT

Figure 5a Closed (5a,c und d) open (5b,d) and shoulder loading positions (SLOPs)

Figure 5a i. Closed SLOP

Figure 5a ii. Open SLOP

Figure 5a iii. Axis of SLOP / Acromion (marker)

Figure 5a iv. Closed SLOP

Figure 5b Shoulder orientation lines description Loading position Baseline: line indicating the boundary of the area of play. (See: In Situ Capture)

Figure 6 Lab documentation-Body marker positions

Figure 7 Lab documentation-Aligned position - no disc pressure

Figure 8 Lab documentation -Not aligned position - disc pressure

List of tables

Table 1a Study I / Experiment 1a One-handed Porta Test (PT)

Table 1b Study I / Experiment 1b One-handed Circular Test (CT)

Table 1c Study I/Experiment 2a Two-handed Porta Test (PT)

Table 1d Study I / Experiment 2b Two-handed Circular Test (CT)

Table 1e Study II / Experiment 1

Table 1f Study II /Analysis 1

Table 1g Study II / Analysis 2

Table 2 Eye dominance Porta´s Test vs. Circular Test One-handed and Two-handed (Right-handers) evaluation sheet – study 1

Table 3 Shoulder position in relation to the baseline – CT evaluation sheet – study 2

Table 4 Shoulder position in relation to the baseline - PT evaluation sheet – study 2

Acknowledgements

No matter how independent we think we are everybody needs assistance of some kind. Writing a thesis is exciting, but also a great challenge with many ups and downs. Despite the fact that doubts and joy are constant companions it is a great privilege to be able to pull your thoughts together and try your wings in an unexplored field. Obviously, there are many people behind such an achievement and I would like to express my gratitude to some of them. First of all, I want to thank my first coordinator Ass. Univ.-Prof. Dr. Christian Haid of the Department of Orthopaedics, Medical University of Innsbruck for giving me the opportunity to write this thesis in the first place. Dr. Haid also played a key role in finding my first thesis supervisor Univ.-Prof. Dr.med. Martin Krismer of the Department of Orthopaedics, Medical University of Innsbruck. Professor Krismer provided excellent support and together with Prof. Dr. Peter Federolf of the Department for Sport Science, University of Innsbruck, Austria, brought to light many of my weak spots, always making me a little better each time. However, without the final support of Prof. Dr. Dr. Dr. Gerhard Berchtold of the Universidad Azteca and University of Central Nicaragua–Innsbruck, Austria in Innsbruck this thesis would not have seen the light of the day. Without all of you I would not have been where I am today. Thank you all! Next, I want to express my gratitude to the Department of Sport Science of the University of Innsbruck, Austria, for permitting us to use the necessary facilities. I am also very grateful to the Medical University of Innsbruck where I did all my coursework in research between 2008 and 2020. I also feel it is important to express my gratitude to the players, parents and coaches of the Alexander Raschke Tennis Academy in Munich, Germany, and the Estess Academy in Tyrol, Austria, who intensely supported this project.

Next, I want to thank my lifetime companion Lena and our beautiful children Sarah and David as well as my deceased parents Sven and Marianne for standing by me with such patience and trust through all these years in which I indulged my passion for tennis, medicine and research. In one way or another you were all a source of inspiration for me to continue the striving for achievement and exploration. I love you all! It deserves to be mentioned that without the achievement of my long-time friend Fredrik Johansson who completed his PhD in 2017, I am not sure I would have finished my PhD. His drive and dedication for his research motivated me to continue my work. Furthermore, I want to express my gratitude to Prof. Dr. Eva Skillgate, an amazing person and a good friend who has been extremely helpful. Independently of our phone calls and our chats in the office, she has always been there with good advice. Eva made this process even more fun and opened many doors in my head. It is beyond imagining how much Eva and Fredrik have meant to me in this process. To the complete Team ESTESS: our many discussions and adventures have kept my mind on track for so many years. That deserves a huge thank you not only for being my colleagues but also my good friends for so many years. Thank you for being patient and interested in a topic that is not widely known and too often ignored. The personal achievement of all of you throughout our common journey has motivated me to be a better person. Thank you for your unconditional friendship and support, not only during my academic and coaching voyage but on my own personal journey as well. Michael Flatz, Thomas Haid, Johannes Schullern, Daniel Lochbihler and Sarah Johansson, each of you have offered a unique perspective and provided invaluable feedback during the testing and writing process. The input each of you has provided to me has been critical to my success as a student, researcher, and teacher. The time, energy, and effort that each of you expended is greatly appreciated, and never went unrecognized. I would also like to take this opportunity to thank all the unmentioned colleagues, friends and partners who helped me through this project. You know who you are. I hope our paths will cross again soon, in the pursuit of new interesting goals regarding visual processing and musculoskeletal behaviour.

Preface

This thesis is a summary of findings observed during the last three decades. However, since September 2016 scientific experiments have followed a group of players with regard to manually influenced eye dominance (MIED) and shoulder loading position (SLOP) in their tennis serve. The thesis describes, explains and discusses the possible influence of manual motor tasks on eye dominance as the basis for a new perspective on hand-eye dominance and its possible influence on the serve position .

The outcome is the result of collaboration with Professor Dr.med. Martin Krismer of the Medical University of Innsbruck and Professor Dr. Peter Federolf of the Department of Sport Science of the University of Innsbruck. The research has been conducted in players from renowned tennis academies with tremendous support received from the academy coaches. The author’s own extensive experience as a professional coach and player plays an essential role in realisation of the project. However, the role of the author over more than three decades has not been limited to professional tennis coaching and playing. The experience of tutoring, developing and constructing professional structures for international coaches has been an additional important ingredient for the validity of the project.

The topic visuality and hand-eye coordination is extremely exciting and this thesis may be considered a door-opener for more fundamental functional research projects concerning the manually influenced eye dominance and its influence on the human locomotive system. Hopefully, the results of the study will be of value for tennis students and educators around the world, who are looking for an additional way to discover fundamental innate abilities that support the skill performance of the tennis serve. This thesis presents a new complex theory around the serve and its association with eye dominance and recommends certain applications worth trying out. The aim of this thesis is to offer a functional solution to a difficult and dynamic topic and to integrate the greatest possible degree of practical thoughts and background knowledge. This thesis offers new knowledge on the hand-eye association and a scientifically based functional solution in an effort to support observational processes and emphasizes hand-eye coordination in the assessment of the tennis serve. Thus, the overall aim is to share knowledge about manually influenced eye dominance and its association with SLOP with the players, coaches, physiotherapists and fitness coaches in the tennis industry in order to optimize players’ serving technique.

Abstract

Shoulder motion is considered one of the most stressful movements in the tennis serve, but no research has been done with regard to the shoulder loading position (SLOP) in the tennis serve and its association with eye dominance (ED). Understanding the hand-eye coordination (HEC) system and how the hand and the eye mutually control and depend on each other remains challenging for visuomotor performance research. The visual influence of the hand and its importance in sports has commonly been documented, but scientific evidence is lacking with regard to the manual influence of the eye. A circular manual motor task with the thumb and the index finger, similar to the grip used on a tennis racket, seems to influence ED. The aim of this thesis is to contribute to a better understanding of the circular manually influenced eye dominance (C-MIED) and its association with the personal SLOP in the tennis serve. This thesis is based on two studies (SI and SII) with data from a primary exploratory research study conducted at the Department of Sport Science, University of Innsbruck, with a total of thirty-one right-handed healthy tennis players. Study I (SI) compared two different hand-eye dominance (HED) screening tests: a. the conventional Porta Test (PT) and b. the new Circular Test (CT). Both tests were performed separately one-handed, namely with the right hand (T1), as well as two-handed, meaning additionally with the left hand (T2). T1 and T2 investigated the data validity and reliability of the CT in comparison to the PT as to whether crossed ED is more common than consistent ED. The T1 classifications were defined as: a. crossed (right-hand/left-eye) and b. consistent (right-hand/right-eye). As an additional validity test T2 studied whether the PT and the CT can influence an eye dominance switch in frontal gaze direction. The T2 classifications were defined as: a. crossed-switch (eye dominance switches with the test hand); b. consistent (same dominance in eye and hand). Study II (SII) explored the SLOPs that were recorded with an eight-camera VICON system. The SLOPs were defined as closed (≥90°) or open (<90°) in relation to the baseline. Firstly, the most common SLOP, closed or open, was evaluated. Secondly, SII studied whether the most common SLOP is encountered more frequently with a crossed or consistent PT in comparison with CT classification. The results of the SI and the one-handed T1-PT (SI/E1a) confirmed the findings of previous research and detected 14 (45%) players out of 31 with crossed EDs (right hand/left eye) and 17 (55%) players with consistent EDs (right hand/right eye) (Table 1a). Evaluation of the T1-CT (SI/E1b) contradicts previous research findings and revealed 22 players with crossed EDs, while nine players showed consistent EDs. The T1 comparative analysis of the PT (n=14/n=17) and the CT (n=22/n=9) revealed a significant difference (p =0.004) in crossed and consistent EDs. The double-sided experiment (T2; SI/E2a and SI/E2b) showed that no players switched eyes when they performed the PT, whereas 19 of the 31 players switched eyes when the CT was performed. The PT results confirmed previous research, whereas the results of the CT contradict previous research. SII revealed that the closed SLOP was the most common (n=22) position. The comparative analyses of this stratified sample of SLOP (n=22) with the PT or with the CT revealed that of the 22 closed SLOP players ten showed crossed P-MIED (CSCr1PM), whereas in the same sample 16 of the 22 players showed a crossed C-MIED (CSCr1CM). The analysis of the SLOP/PT is aligned with the results of previous ED studies and the result of SI/E1a (p =0.375; Table 1f), whereas the SLOP/CT analysis shows a significantly different result (p =0.010; Table 1g) as compared to the SLOP/PT. Thus, six (50%) of the 12 closed players with a consistent PT result changed their ED from consistent to crossed when they performed the one-handed CT. The chi-squared test was used to compare the groups. The experiments demonstrate that the CT can influence ED in frontal gaze direction and that C-MIED may be associated with a player’s choice of SLOP in the tennis serve. The results may have an impact on serving technique.

Abstract (German)

Die Schulterbewegung gilt als eine der stressigsten Bewegungen beim Tennisaufschlag. Es wurde jedoch nie Studien im Bezug auf Schulterposition (SLOP) und die Relation mit Augendominanz (ED). durchgeführt. Die Bedeutung der visuelle Einfluss der Hand im Sport ist allgemein dokumentiert, jedoch fehlen wissenschaftliche Beweise für den manuellen Einfluss des Auges. Eine kreisförmige manuelle motorische Aufgabe mit Daumen und Zeigefinger, ähnlich dem Griff eines Tennisschlägers, scheint die ED zu beeinflussen. Das Ziel dieser Arbeit ist es, zu einem besseren Verständnis der manuell beeinflussten Augendominanz (MIED) und ihrer Assoziation mit dem persönlichen SLOP beim Tennisaufschlag beizutragen. Die Dissertation basiert auf 2 Studien (SI und SII) mit Daten einer primären explorativen Forschungsstudie am Institut für Sportwissenschaft der Universität Innsbruck mit insgesamt einunddreißig gesunden Rechtshändern. Studie I (SI) verglich zwei verschiedene Hand-Augen-Dominanz (HED) Screening-Tests; a. der konventionelle Porta Test (PT) und b. der neue Zirkulartest (CT). Beide Tests wurden separat einhändig durchgeführt, mit der rechten Hand (T1) sowie zusätzlich mit der linken Hand (T2) kombiniert. Die T1 und T2 untersuchten die Datenvalidität und Reliabilität der CT im Vergleich zu PT, ob gekreuzte ED häufiger vorkommt als konstante ED. Die T1-Klassifikationen wurden definiert als: a. gekreuzt (rechte Hand/linkes Auge) und b. konstant (rechte Hand/rechtes Auge). Als zusätzlicher Validitätstest wurde im T2 untersucht, ob PT und CT einen Augendominanzwechsel in frontaler Blickrichtung beeinflussen können. Die T2-Klassifikationen wurden definiert als: a. gekreuzt (Augendominanz wechselt mit der Testhand); b. konstant (gleiche Dominanz in Augen und Händen). Studie II (SII) untersuchte die SLOPs, die mit einem VICON-System mit acht Kameras aufgezeichnet wurden. Die SLOPs wurden als geschlossen (≥90°) oder offen (<90°) bezogen auf die Basislinie definiert. Zunächst wurde der gängigste SLOP, geschlossen oder offen, ausgewertet. Zweitens untersuchte SII, ob der häufigste SLOP bei gekreuzten bzw. konstante PT im Vergleich zur CT-Klassifikation häufiger vorkommt. Die Ergebnisse des SI und des einhändigen T1-PT (SI/E1a) bestätigten frühere Forschungen und entdeckten 14 Spieler (45%) von 31 mit gekreuzten EDs (rechte Hand/linkes Auge) und 17 Spieler (55%) mit konstante EDs (rechte Hand/rechtes Auge) (Tabelle 1a). Die Auswertung des T1-CT (SI/E1b) widerspricht früheren Forschungen und ergab 22 Spieler mit gekreuzten EDs, während 9 Spieler konstante EDs zeigten. Die T1-Vergleichsanalyse des PT (n=14/n=17) und des CT (n=22/n=9) zeigte einen signifikanten Unterschied (p =0,004) in gekreuzten und konstanten EDs. Das doppelseitige Experiment (T2; SI/E2a und SI/E2b) zeigte, dass kein Spieler die Augen wechselte, wenn sie die PT durchführten, während 19 Spieler von 31 Spielern die Augen wechselten, wenn die CT durchgeführt wurde. Das PT-Ergebnis bestätigte frühere Forschungen, während das Ergebnis der CT der vorherigen Forschung widersprach. SII ergab, dass der geschlossene SLOP die häufigste (n=22) Position war. Die vergleichenden Analysen dieser stratifizierten Stichprobe von SLOP (n=22) mit dem PT bzw. mit dem CT ergaben, dass 10 von 22 geschlossenen SLOP-Spielern gekreuzte P-MIED (CSCr1PM) aufwiesen, während in derselben Stichprobe 16 von 22 Spielern eine gekreuzte C-MIED (CSCr1CM) zeigte. Die Analyse des SLOP/PT orientiert sich an den Ergebnissen früherer ED-Studien und dem Ergebnis von SI/E1a (p =0,375; Tabelle 1f), während die SLOP/CT-Analyse ein signifikant anderes Ergebnis zeigt (p =0,010; Tabelle 1g) im Vergleich zu SLOP/PT. Somit änderten 6 Spieler (50 %) von 12 geschlossenen Spielern mit einem konstanten PT-Ergebnis ihre ED von konstant zu gekreuzt, wenn sie die einhändige CT durchführten. Der Chi-Quadrat-Test wurde verwendet, um die Gruppen zu vergleichen. Die Experimente beschreiben, dass CT die ED in frontaler Blickrichtung beeinflussen kann und dass C-MIED und die Wahl des SLOP eines Spielers beim Tennisaufschlag zusammenhängen können. Die Ergebnisse können einen Einfluss auf die Aufschlagtechnik haben.

List of scientific papers

I. Dahlbo, H., Flatz, M., Federolf, P.

Eye dominance testing: An exploration of the conventional standard eye dominance Porta Test in comparison with a circular manually influenced eye dominance Test

Department of Orthopedics - Medical University Innsbruck / Department of Sports Science - University of Innsbruck 2020a

II. Dahlbo, H., Flatz, M., Haid T., Federolf, P., Krismer, M.

Eye dominance in tennis serve: An exploration of shoulder loading position in the tennis serve and its association with manually influenced eye dominance

Department of Orthopedics - Medical University of Innsbruck / Department of Sports Science - University of Innsbruck 2020b

Background

Over the last three decades tennis in general has become more and more dynamic and many new challenges have evolved with regard to a balanced and efficient technique. Research in neuroscience shows that it is possible to change and improve the structure of the brain by practicing new skills (Sherwood, 2000) and it has been reported that the neocortex of the brain continues to develop and grow into high age (Upledger, 1999). Further research reveals that the human brain can store information and recall it when required (Rose, 1993). However, only some of this information is selected and then stored to become available for recollection later when required (Rose, 1993). Supported by these arguments about the flexibility of brain capacity and the knowledge that it requires at least ten years of extensive work and dedication to become a skilled expert in any field (Bloom, 1985; Ericsson et al., 1993), it can be assumed that stored underlying factors may play a role in personal motor preferences. These facts may be an indication that the human locomotive system is able to prepare reactive patterns for incoming environmental stimuli. Certain traits like the personal hand-eye coordination (HEC) preference supposedly play a role in the development of technical details in the tennis serve. However, little is known about innate abilities and traits that can prepare and influence human biomechanical performance and locomotion. Therefore, this thesis investigates a new eye dominance test (Circular Test, CT) performed with the thumb and index finger similar to the grip used on a tennis racket (Figure 1b) and compares it with the conventional Porta Test (PT) as well as players’ preferred shoulder loading position (SLOP) in the tennis serve. The results of the SLOPs in the tennis serve are compared to the results of the PT and the CT and analysed for a possible association. The purpose is to better understand the influence of manually influenced eye dominance (MIED) on the SLOP in the tennis serve and to explain the structures and the underlying causes in order to optimize the SLOP and consequently avoid injury.

The tennis serve

The tennis serve has been the subject of much research. An efficient serve is an important component of success in tennis. In fact, the serve might be the most important shot of the tennis game (Johnson et al., 2006). The tennis serve requires good balance and an optimized energy flow in highly stressed biomechanical situations (Kovacs & Ellenbecker, 2011). The movement is complex and the many different individual variations in pattern are sometimes difficult to explain. The primary aim when developing the serve is to improve performance and prevent injury (Martin et al., 2013). Faster rackets and strings, better grips and more dynamic, better physical status of the players that promotes more powerful energy transfer through the legs, all these are factors that have enhanced the demands for an impeccable SLOP in the tennis serve. The serve motion seems to show personal variations and individual diversity. Independently of technique, skilled serve performance may be considered the precise timing of a balanced and adapted action within the kinetic chain. In fact, the serve performance of skilled players seems to be consistent, subtle and well coordinated in its execution while being interlinked with reproducible movement patterns, even under massive mental pressure. Thus, the feeling of being comfortable in the SLOP might be considered essential for successful serve performance.

Energy transfer and shoulder loading position

The need to maintain an accurate body position in the SLOP in order to support the energy transfer from bottom to top would seem to be obvious in the tennis serve. However, no studies have explained why players choose different SLOPs in the tennis serve. Therefore, a good understanding of the ED influences on the SLOP and its possible impact on spinal biomechanics and the pressure exerted on the disc by non-suitable and non-optimized personal body positions during the entire preparation phase can be helpful. However, as this thesis is limited to the research of MIEDs and the SLOP, only a short overview of the effect of the spine is presented.

Obviously, the spine and the shoulder are exposed to a greater load and more pressure in the tennis serve. Consequently, impeccable technique is a must (Martin et al., 2013; Martin et al., 2014). The kinetic chain interlinks the energy transfer from the legs, through the spine to the shoulder (Kovacs & Ellenbecker, 2011). The complexity of the serve makes it difficult to give straightforward answers about how personal traits like a coupled HEC preference impact the overall body position and the curvature of the spine. However, despite the extensive research conducted on the serve movement and the human spine, to the best of our knowledge no studies refer to or associate problems in movement or long-term injuries in the back and shoulder with eye dominance. Nevertheless, it is important to point out that this thesis exclusively investigates manual motor tasks that influence eye dominance in frontal gaze direction and the SLOP in the tennis serve. Nevertheless, the gap in knowledge about players‘ personal choice of SLOP and the association with MIEDs is evident. Therefore, further knowledge is important to support future studies on the spine and shoulder.

Tennis serve and injury

The tennis serve makes demands on the athlete’s musculoskeletal system (Elliott et al. 2003; Kibler, 1995; Fleisig et al., 2003). Strong compression and loading can cause problems in the back, shoulder, elbow, and wrist (Elliott et al., 2003; Martin et al., 2014; Fleisig et al., 2003; Campbell et al. 2013). Although studies report that a significant amount of the force exerted in the tennis serve is absorbed in the lumbar region as compared to other strokes (Campbell et al., 2013), the forces in the shoulder are considered to produce some of the most stressful movements in the tennis serve (Johansson, 2017). The consequence of biomechanical adaptations and adjustments of the serve motion can be various interruptions in the flow of energy and decreased power to the SLOP. In fact, it has been reported that an upright stance involves less spinal torque than forward and sidewards bending. The reason for this is found in the ground-reaction forces that act with a longer lever arm (Spörri et al., 2015). Specifically, the lateral flexion and the torque might be of concern (Spörri et al., 2016) during the SLOP (Figure 8). Therefore, the assumption of increased spinal disc loading because of the very short lever arms of the stabilizing muscles and/or passive structures seems justified (Spörri et al., 2015). Thus, as a direct result of the very short lever arms of the paraspinal muscles, the forces required to stabilize the trunk in such postures are relatively high (Peterhans et al., 2020). If the annulus fibrosus fibers are stretched, the axial loading increases the pressure on the nucleus pulposes (Haid & Fischler, 2013). The tension in some of the annulus fibrosus fibers can further increase as a result of vertebral bodies that twist against each other. The result will probably show higher pressure on the intervertebral discs (Spörri et al., 2015). Moreover, a combination of the previously mentioned factors (i.e. forward flexion, lateral bending and trunk torsion) is assumed to be associated with stronger spinal disc loading and to be a major mechanism leading to overuse injuries of the back (Spörri et al., 2015). Further studies have shown that forward flexion, lateral bending and trunk torsion as well as high peak loads are major risk factors for overuse injuries of the back in sports like golf, tennis, gymnastics or cricket (Spörri et al., 2016). A combined occurrence of lateral bending with either forward flexion or trunk torsion increases the spinal disc loading (Spörri et al., 2016). Obviously, all three factors may cause even greater problems. External stress and influence on the ligaments and on the muscles exert strong loading internal forces on the joints. This means that muscles and ligaments compensate the torque that is generated by a static weight load (Haid & Fischler, 2013). Investigations have shown that weight forces that act further away from the center of gravity have a greater acting torque (Haid & Fischler, 2013). Thus, it is reasonable to consider them important components of mechanisms leading to overuse injuries of the back in the tennis serve if players cannot direct the force from bottom to top without unnecessary deviations of the spine. Therefore, the immediate problem seems to be the understanding of a player‘s overall body position and its underlying mechanisms in order to optimize the postural curvature and minimize injury risk. To achieve total understanding of the impeccable biomechanical serve motion and the need for an individual SLOP the mechanics and the effect of appropriate energy transfer should be discussed. For this purpose, some basic pre-information on the mechanics of the complete serve from bottom to top as well as a knowledge of the hand-eye coordination phenomenon and hand-eye dominance are necessary.

Biomechanics and the energy flow through the kinetic chain in the tennis serve

The production of ground reaction forces is essential for the development of an effective serve (Kibler, 1995; Girard et al., 2005; Elliott et al., 1995). More than 50% of the force and the kinetic energy delivered to the shoulder and ultimately to the hand is built up through the legs and trunk work (Kibler, 1995; Toyoshima et al., 1974). Therefore, the kinetic chain is the key to a good and injury-free serve. The sequence of motions through the entire body starts in the lower limbs and passes through the trunk rotation that leads to upper limb rotation (Elliott et al. 2003). However, if the kinetic chain cannot work fluently without interruption it can limit performance and enhance the risk of injury. Skilled players show smooth use of the kinetic chain starting from the lower limbs, through trunk rotation to upper limb rotation. The biomechanics of the serve is commonly divided into three phases: 1) the preparation phase, 2) the acceleration phase, and 3) the follow-through phase (Kovacs & Ellenbecker, 2011). During the preparation phase of the serve the necessary energy is stored to support the release during the acceleration phase of the stroke (Kovacs & Ellenbecker, 2011). The first component to generate power that can be transferred to the trunk is the leg drive (Whiteside et al., 2015; Bahamonde, 2000). Consequently, a strong and stable base for the legs is important for complete loading. An efficient leg base position in the loading phase is considered a fundamental requirement for optimized transfer through the trunk and for an optimized cocking phase as a support for the acceleration phase (Kovacs & Ellenbecker, 2011) to the impact with the ball.

It is unquestionable that a stable base is important in the loading position. From current observations it can be concluded that players not only show a personal SLOP, but also a personal foot stance in the tennis serve. Typically, two types of foot positioning in the loading position are described: foot-back (FB) and foot-up (FU) (Elliott & Wood, 1983). The FU position is taken when the back foot (foot ipsilateral to the serving arm) is positioned up to the front foot during ball toss. The FB position leaves the back foot behind the front foot (Elliott & Wood, 1983). The different foot positions show different advantages. Research has shown that the FU position creates higher velocity. The foot-back is considered advantageous for serve and volley players. Compared to FU, FB can generate higher propulsive forces to the net (Elliott & Wood, 1983). Comparisons have revealed that both FU and FB produce similar joint loading at the shoulder (maximum knee flexion to maximum external shoulder rotation) (Reid et al., 2008). No research discusses the reasons behind the choice of FU or FB. However, a stable and strong loading position enables the pelvis and the shoulder to take a tilted position that puts the player in a better position to load the back leg. This is important for energy transfer through the trunk and up to the upper limb, which drives the shoulder upward (Reid et al., 2013). During the loading phase the pelvis and the shoulder tilt with lateral trunk flexion laterally away from the non-dominant arm (Reid et al., 2008) and interlink the cocking stage with the acceleration phase. This is important in order to produce speed in serve (Bahamonde, 2000). In fact, it is possible to detect differences in players with high and low serve speeds by evaluating this acceleration phase (back leg, trunk rotation and shoulder anterioposterior axis) (Bahamonde, 2000; Reid et al., 2013).

In order to understand the possible manual influence of eye dominance and its influence on the SLOP some information on the visual mechanisms is presented below.

The visual control mechanisms

The human visual system offers various control mechanisms that provide the motor system with all the information it needs for optimizing performance. Previous studies have shown that input through the visual system seems to dominate other sensory systems and that it is the sensory source, upon which we rely most (Lee, 1976; Cutting, 1986). Between Porta’s De Refractione (Porac & Coren, 1976), written in 1593 and perhaps the earliest reference to eye dominance, and 2003 more than 570 articles have discussed eye dominance (Mapp et al., 2003). Meanwhile, interest has grown rapidly and the number of publications on this topic now exceeds 2000. Different studies presented measurements and concepts ranging from ‘soldierliness’ (Banister, 1935) and marksmanship (Crider, 1944), reaction time (Minucci & Connors, 1964), the accuracy of line bisection (Mefferd & Wieland, 1969) and size distortion (Porac & Coren, 1976). However, most eye dominance research projects assumed a fixed coupling between eye and hand dominance, with right eye and hand dominance being the most common finding. This would mean that eye dominance does not change to correspond with hand use or gaze direction. Those studies argued that eye dominance when executing a task is consistent, which is similar to assumptions about handedness, where the hand preferably used does not change during a task. Moreover, it has been argued that the ocular motor system serves as a control mechanism and decides which eye should be activated, depending on perspective. This challenged the fixed eye dominance hypothesis (Khan & Crawford, 2001). The hypothesis behind the findings of Khan and Crawford was that eye dominance changes with gaze direction to optimize the field of view. The change in eye dominance was not considered until Khan and Crawford reported their findings. Subsequent studies based on the research conducted by Khan and Crawford (Banks et al., 2003; Khan & Crawford, 2003; Quartley & Firth, 2004) showed that the change in eye dominance depends on target direction and, in fact, the existence of different types of eye dominance had already been assumed (Walls, 1951). However, studies have shown that the world’s population is probably 90% right-handed (Frayer et al., 2012) and of all right-handers probably 65% show a right-eye dominance (McManus et al.,1999). Studies (Khan & Crawford, 2001) have reported that ED can shift depending on target direction, but that it cannot shift when in frontal gaze direction. However, to the best of our knowledge, neither a circular manually influenced eye dominance (C-MIED) shift performed in frontal gaze direction nor the influence of C-MIED on SLOP in the tennis serve has been investigated to date.

The role of visuality and the interactive hand-eye coordination

The role of HEC is to coordinate visual incoming information, calculate and determine the body position in the environment and consequently execute manual tasks like i.e. writing, catching, drawing and throwing. Athletes will always desire to gain an advantage over the opponent. Therefore, much research of the basic visual system and HEC has been conducted (e.g., Abernethy & Wood, 2001; Du Toit et al., 2006; Schwab & Memmert, 2012). Automaticity of the HEC is obvious and important for fast and optimized performance. To understand how to improve automaticity in sports many things must first be investigated. One main challenge seems to be the investigation of MIED processes. In fact, the knowledge of the manually influenced ED may lay the basis for a personal and comfortable energy transfer up to the shoulder loading phase during a tennis serve.

HEC is generally recognized as an important factor in different sports and its importance for good sports performance is undeniable. HEC is furthermore regarded as a key contributor to success in visual abilities (Ciuffreda, 2011; Cotti et al., 2011) and fundamental skills such as catching (Bennett et al., 1999) and postural balance (Prodea et al., 2013) as well as in specific interceptive aiming sports such as badminton (Yuan et al., 1995), tennis (Paul et al., 2011; Zetou et al., 2012) and table tennis (Paul et al., 2011; Faber et al., 2014). Nevertheless, studies show that the eyes are not merely passive recipients of visual images (Land et al., 1999; Johansson et al., 2001). Those studies are important for the understanding of how the neural basis of HEC may work and suggest that eye movement is intimately related to manual tasks and vice versa. However, conventional HEC research exclusively describes the visual influence of the hands (visuomotor control).

Hardware or software – the „abilities and the skills“

Technical skills and muscular power are important and necessary ingredients in sports. Most athletes would never go into a competition without having first prepared their skills like i.e. strength and conditioning (Wilson & Falkel, 2004). However, athletes and coaches should be aware that it is important to optimize performance of the entire body. Therefore, it is necessary to use a multifactorial approach for overall understanding of the vision and the assessment of skilled behavior in sports (Ward & Williams, 2000). Thus, the technical adaptation and adjustment with visual traits like personal MIED may be helpful for technical development. Vision is considered to be the dominant sensory feedback system (Atkins, 1998) and approximately 80% of brain input comes through the eyes (Hodge et al., 1999). Efficient visual skills are not only highly important for success in sports but also play a very important role in various activities (Wilson & Falkel, 2004). To optimize sport-specific tasks a player needs to move his eyes precisely and quickly (Wilson & Falkel, 2004). Vision involves two basic categories of function: visual motor skills and visual perceptual skills (Wilson & Falkel, 2004). Visual motor skills have the strongest relationship to sport-specific performance (Wilson & Falkel, 2004). While skill development is not dependent on age, it is influenced by age (Gallahue & Ozmun, 1997). Therefore, it seems logical to begin with hand-eye coordination assessment when attempting to support skill development at an early age. The visual system can be compared to a computer system. It can be divided into the hardware and the software visual systems (Abernethy, 1986). Research shows that eye-hand coordination is a skill (software) that can be improved and the vision is a critical source of information for the planning and execution of motor skills (Abernethy, 1996). More specifically, the hardware (visual abilities) system can be divided into static and dynamic visual acuity, depth perception, accommodation, fusion, colour vision, and contrast sensitivity, whereas the software system consists of eye-hand coordination, eye-body coordination, visual adjustability, visual concentration, central-peripheral awareness, visual reaction time, and visualisation (Ferreira, 2002). The hardware abilities are believed to be difficult or impossible to change and are described as the „mechanical and optometric properties“ of the visual system (Ludeke & Ferreira, 2003), whereas the software refers to the capacity of the visual system to process visual information. The scientific literature shows a clear lack of the impact of underlaying traits like MIED. Nevertheless, reports have shown differences in the visual abilities of athletes compared to non-athletes. The results suggest superior visual abilities among athletes compared to non-athletes (Stine et al., 1982; Christenson & Winkelstein, 1988). However, in order to provide a complete overview of the comparisons between the visual abilities of experts and novices some fundamental facts have to be considered (Ludeke & Ferreira, 2003):

1. A specific sport with specific requirements may attract individuals who have the superior visual abilities required to meet the specific visual demands of that sport. Players who do not have the sport-specific visual abilities may not reach top-level performance (Ludeke & Ferreira, 2003).
2. Practicing a sport and developing sport-relevant visual skills through experience are important to meet the demands of a specific sport and distinguish experts from novices (Ludeke & Ferreira, 2003).

It has been suggested that the differences between athletes and novices with regard to vision are software-related and have little to do with hardware performance once visual defects have been corrected (McLeod & Jenkins, 1991). That would mean that expert athletes have enhanced visual skills to meet the demands of their specific sport, as compared to those of novices (Abernethy, 1996). However, it is not known whether manual motor tasks coupled with eye coordination constitute an ability (hardware) and whether a motor task can influence eye-hand dominance. Obviously, knowledge of a player’s personal hand-eye coordination preference and fundamental knowledge about hardware (abilities) and software (skills) are critical for evaluating motor skills in tennis. Although studies have assumed that experts show better visual skills such as pattern recognition, visual search strategies, advanced cue utilisation and the ability to perceive situation probabilities than do less-skilled athletes (Williams et al., 2002), it seems, however, that skilled performers do not necessarily possess better visual hardware than their less skilled counterparts (Williams et al., 1999).

Knowledge of MIED may facilitate the development of software skills. However, the practicability of tests of general ability in the assessment and/or development of skills is reliant on evidence that the core general ability exists (Savelsbergh et al., 2003). To date, it seems that such evidence is missing for HEC. Nevertheless, practice on HEC tasks undeniably leads to improvement of HEC tasks (Zupan et al., 2006; Schwab & Memmert, 2012). It is questionable whether this improvement is specific to the practiced task, or whether it refers to body control adjustment systems.

The literature precisely describes decision-making in sport and its pronounced dependency on the level of attention, scanning for opportunities and then acting upon them in a specific situation (Greenwood, 1993). However, no information can be found about the initial moment of a movement before the eye receives information from the environment. From the sport science perspective, the emphasis should be on the design and implementation of practice activities to help athletes improve their capacity to use visual information in order to control motor skill performance (Erickson, 2007). However, the role of muscle preparation prior to muscle performance, i.e. a tennis serve, and the motor influence of the eye are still unknown. It thus seems important to clarify the possibility by assessing MIED. From a tennis perspective this would mean that a player’s personal MIED could be an innate ability (hardware) that enables the player to develop optimized HEC skills (software). Therefore, the investigation of MIED deserves special attention when analysing the development of sport performance.

Vision and sport

Normal balance relies on vision, proprioception and the peripheral vestibular system (Kanegaonkar et al. 2012). Consequently, balance is partly controlled by the eyes and the ears. Studies of the relationship between performance and visual skills training have claimed to show benefits of visual skills training in improving static balance and hand-eye coordination (McLeod & Hansen, 1989b; McLeod, 1991). Further studies show that visual motor skills can be improved through visual skills training and that the visual system performs more efficiently after loading or stress (Wilson & Falkel, 2004). Considerable literature has been published about vision and sport, although there is still no consensus about the potential to assist athletes by assessing their personal MIED preference. Obviously, it is necessary for tennis players to have a precise overview of the court and the upcoming situation. Therefore, knowledge of a personal MIED is important so that the player can keep his head steady and comfortable during the serving process. That could mean the difference between the success and the failure of a serve. In fact, knowledge of how the hand influences the eye seems to be as important as the visually-guided action itself. Thus, the immediate challenge seems to be to define the difference between visual abilities (hardware) and visual skills (software) and to understand how they impact the player’s SLOP in the tennis serve.

Eye dominance testing

Studies have shown that sighting dominance can shift in different gaze directions (Khan & Crawford, 2001). Different common and conventional tests are available for the assessment of eye dominance. Two popular types of sighting eye dominance tests are commonly used. The Miles Test and the Porta Test.

The Miles Test is a common standard eye dominance assessment test (Miles, 1930). Peeking through a hole, is the basis of this ABC test (Miles, 1930). In its original form, subjects held a truncated cone over their face, kept both eyes open and looked at a distant point. Observers aligned the cone with one of their eyes without realizing it. A more popular version of this test is to look through a hole in a card or through a circle formed with the fingers of both hands. When performing the Miles Test both arms should be extended, with the index fingers and the thumbs of both hands creating a circle/triangle. Both eyes should be open and the center of the circle/triangle should be focused on a middle-distance object. It is also possible to extend both arms to create the circle/triangle, centering it on a distant object and then slowly pulling the circle/triangle back towards your face (keeping the object centered) until you find the circle/triangle in front of one of your eyes. The eye that winds up with the circle in front of it is your dominant eye!

The Porta Test is another variation of the Miles Test (Roth, 2002). However, only one arm is extended. With both eyes open the test person aligns a thumb or an index finger with a distant object. It is important to be instinctive when the hands are positioned. Subconsciously one of the eyes takes precedence. If the left eye is closed and the object is still aligned with the thumb or index finger, the right eye is considered to be the dominant eye, and vice versa. Nevertheless, it is still unclear what sighting dominance assessment, like the Porta and the Miles Tests, is addressing.

Assessment of the shoulder loading position in the tennis serve

Tennis demands many abilities and skills. Assessment of the shoulder must incorporate biomechanical functional screening, a structural/anatomical investigation and a comprehensive battery of general and specific fitness tests (Johansson, 2017). However, for the purpose of this thesis the following assessments will be further discussed: 1. the manual influence of eye dominance and its ability to trigger an eye dominance switch in frontal gaze direction; 2. the role of the shoulder position in the tennis serve; 3. the association between the SLOP and the C-MIED and the P-MIED in the tennis serve.

The manual influence of eye dominance and its ability to trigger an eye dominance switch in a frontal gaze direction

The eye dominance concept is still unclear (Mapp, et al. 2003). The existence of different types of eye dominance was reported more than six decades ago (Walls, 1951). However, most projects for the assessment of HEC and ED have involved laboratory-based tests that do not call for sport-specific abilities (Burns & Dobson, 1984). Reaction tests like pressing a button and touching a lamp may not be sufficient to investigate hand-eye associations and individual preference patterns. Such reaction tests may not be compatible with an ability to process individual movements (Burns & Dobson, 1984). Previous investigations have revealed that eye and arm movements for ipsilateral reaches to targets (on the same side of fixation as the reaching limb) differed from contralateral reaches (to targets across the body midline). The crucial finding was that eye movements associated with contralateral arm movements were initiated about 40–50 milliseconds later than those associated with ipsilateral arm movements (Fisk & Goodale, 1985). The results suggest that a common control mechanism links the two effectors and that eye movements are bound to hand movements, even though the eyes begin and complete their movements more rapidly than the does the hand (Fisk & Goodale, 1985). Furthermore, studies indicate an automatized relationship between the hand and the eye. The reports state that participants could not initiate saccades to a second target until the hand had reached the first target, even if they tried to do so (Neggers & Bekkering, 2000). Further experiments suggested that eye movements are bound to hand movements (Neggers & Bekkering, 2000). However, those studies reported a pathological link from the hand to the eye, rather than eye to hand, in patients with neurological defects (Snyder et al., 2000). Thus, it can be assumed that the connection from the hand to the eye may be available as innate „hardware“.

The role of the shoulder position in the tennis serve

The motion of the tennis serve is dependent on stability and consistency (Mendes et al., 2013). However, the tennis serve technique is difficult to measure and analyse. The three-dimensional aspect and the multidirectional movement pattern make it difficult to achieve a realistic, repetitive and reliable analysis. Therefore, the role of the SLOP was considered in order to obtain good reliability of measurments in the current serve study. The culmination of the energy flow through the kinetc chain is transferred through the serving shoulder and the dominant hand to the ball (Kovacs & Ellenbecker, 2011). Instantly before striking the ball, the player seems to choose a personal shoulder loading position (SLOP). This thesis describes two SLOPs, namely a. closed, clockwise more towards 1 o’clock (≥90°) in relation to the baseline, and b. open, clockwise more towards 11 o’clock (<90°) in relation to the baseline (see Methods and materials and Figures 5a, 5b, 7 and 8), and evaluated the most common SLOP with a P-MIED and a C-MIED (Dahlbo et al. 2020a; Dahlbo et al. 2020b).

The association between the SLOP and C-MIED and P-MIED in the tennis serve

The relationship between vision and sport performance has been thoroughly explored in previous research on visual abilities (hardware) and processing skills (software). Nevertheless, little is known about the influence of the hand-eye coordination patterns in the tennis serve. Eye dominance tests performed simultaneously with both hands, like the „Hole in the Card Test – Dolman Test and the Miles Test, have provided valuable information on eye dominance with sufficient accuracy. However, to the best of our knowledge, these methods have not yet developed further to permit investigation of e.g. one-handed motor tasks, similar to the manual grip on a tennis racket, and its specific influence on eye dominance. In fact, personal hand-eye traits have not devoted specific attention to serve development in tennis at all. This study is intended as a first step in that direction.

Experimental studies in baseball have shown that the pattern of hand-eye dominance appears to be related to athletic proficiency. Studies have shown that pitchers who were uncrossed hand-eye dominant (consistent) were distinctly more successful than crossed pitchers. Batters who were crossed hand-eye dominant were slightly more successful than uncrossed (Portal & Romano, 1998). In summary, it is of concern that the non-evaluated manual eye dominance may force the SLOP to a non-advantageous position and limit optimized flow through the kinetic chain.

Rationale of the thesis

Given the limited research on manual motor influence on the ED and its association with tennis serve performance, there is a huge demand for investigations. Currently, no conventional tests exist that assess ED from a manually influenced perspective. Moreover, the literature shows a lack of knowledge concerning different SLOPs and their association with ED. However, from current observations it can be concluded that tennis players seem to choose a personal SLOP in relation to the baseline, and MIED might presumedly play a role in the player’s personal choice of SLOP. Therefore, knowledge of a player’s personal MIED seems important for serve performance development.

With the expected further dynamic development in tennis serving technique, MIED should be investigated. The findings would guide assessment and coaching recommendations and improve the efficiency of instruction. The perspective of the presented results, with reference to background theories about conventional HEC testing, serving biomechanics of the kinetic chain and necessary hints and thoughts about the loading of the intervertebral discs, would stimulate future follow-up investigations. The possible association between the SLOP and MIED can play a role in the development of a player’s natural ability and optimize the stability and consistency of his serve performance, thus leading to an optimized injury-free process. Consequently, an extended and deeper understanding of the automaticity of HEC can be expected to increase acceptance of the MIED process in order to optimize a player’s performance in the tennis serve. This thesis makes an independent, significant and original contribution to such knowledge.

Aims

The general aim of the thesis is to further investigate the hand-eye relationship and explore the manual influence of eye dominance (MIED) and its association with the shoulder loading position (SLOP) in the tennis serve. In particular, this thesis tests the following hypotheses: firstly (H1), it is possible to identify a manual influence of eye dominance; secondly (H2), it is possible to identify an association between the choice of personal SLOP in the tennis serve and circular manually influenced eye dominance (C-MIED).

To achieve this overarching aim the thesis is divided into two studies (SI* and SII**), including three experiments (T1-SI/E1a and b, T2-SI/E2a and b, SII/E1) and two final comparative analyses (SII/A1 and SII/A2). SI/E1a compares the expected values according to general opinion and the observed PT values, while SI/E1b compares the current values of the PT (SI/E1a) and the result of the CT (only the right hand). SI/E2a compares the expected values based on general opinion and the observed PT values, namely that an ED switch in frontal gaze direction is not possible, while SI/E2b compares the PT values and the CT SI/E2a (right hand and left hand combined). SII/E1 investigates whether a closed SLOP is more common than an open SLOP. The final analyses compare the most common SLOP and a one-handed P-MIED (SII/A1). The SII/A1 results are then compared with the results for the most common SLOP and one-handed C-MIED (SII/A2). This thesis has three specific aims:

The first specific aim* is to validate the newly developed one-handed CT in comparison to the conventional PT in order to address critical gaps in the existing literature.

The second specific aim** is to increase the understanding of a player’s personal SLOP shortly before impact with the ball.

The third specific aim** is to contribute to the understanding of one-handed P-MIED in comparison to C-MIED, differences or similarities and their association with the SLOP in the tennis serve. Indeed, the focus will be on the circular motor task (C-MIED) because of its similarity with the manual grip used on a tennis racket.

Methods and materials

In order to fulfill the aims of this thesis and address the functional research problem the first study (SI) focused particularly on test reliability and validity by evaluating SI/E1a and SI/E1b (only the right hand) and SI/E2a and SI/E2b (right and left hand combined). To evaluate the first hypothesis (H1, see aims) the SI/E1a first evaluated the one-handed PT and the values expected according to general opinion; then SI/E1b compared the one-handed CT and the values of the PT; next, SI/E2a and SI/E2b investigated the ability of the PT and the CT to trigger an ED switch in frontal gaze direction depending on which hand is used. The second study (SII) investigated the SLOP and focused on analysing the most common SLOP in comparison to SI/E1a and SI/E1b in both the PT and the CT. SII explored the second hypothesis (H2, see aims) and investigated whether a closed SLOP was more common than an open SLOP (SII/E1). SII/A1 analysed the most common SLOP with one-right-handed P-MIEDs (crossed and consistent). SII/A2 analysed the SII/A1 results with a closed SLOP and one-right-handed C-MIEDs (crossed and consistent).

The experiments SI/E1a and SI/E1b as well as SI/E2a and SI/E2b investigate the player’s C-MIED in frontal gaze direction (straight forward without rotational head movements, right or left, Figure 2a and 2b) in comparison to the common conservative one-handed P-MIED and evaluated whether C-MIED shows a different result than does P-MIED and whether a two-handed (performed with each hand separately) CT can trigger an ED switch in frontal gaze direction. The two-handed PT was compared with the two-handed CT. Both tests are developments based on the „Hole in the Card Test - Dolman Test,“ where the test subjects sight a target through a hole in the middle of a card (Durand & Gould, 1910) and the Miles Test, where the test subjects look at a distant target through a hole (a circle/triangle created by overlapping both hands) (Miles, 1930).

Overview Study I and Study II

Study I - Experiment T1 (SI/E1a; Table 1a and SI/E1b; Table 1b) investigated the conventional and common eye dominance one-handed PT in comparison with the newly presented one-handed CT in order to provide acceptable reliability and validity for MIED measurement. Both tests performed a manual task (the PT with an index finger task and the CT with a circle formed with the index finger and the thumb; Figure 2a and 2b). The tests were performed exclusively one-handed with the right hand (T1-SI/E1a and SI/E1b).

Study I - Experiment T2 (SI/E2a; Table 1c and SI/E2b; Table 1d) compared the PT and the CT with both hands (single one-handed performance) as an additional validation of C-MIED as a requirement for further investigations. The general opinion and previous research claim that eye dominance cannot switch in frontal gaze direction. Therefore, the aim was to investigate the effectiveness of the CT with an additional single left-handed CT to determine whether a switch in frontal gaze direction in comparison with the PT took place and whether the CT showed different results than the PT. T2-SI/E2a and T2-SI/E2b are specifically important for future perspectives, studies and comparisons.

Study II - Experiment 1 (E1) and analyses 1 (A1) and (A2): The second study (SII) investigated the same sample as SI concerning the most common shoulder loading position (SLOP) in the tennis serve and analysed the association between a personal SLOP and the one-right-handed P-MIED and C-MIED. First, Experiment 1 (SII/E1) evaluated whether a closed (≥90°) or an open (<90°) SLOP in relation to the baseline was more common than an open loading position (Figure 5a and 5b; Table 1e); then, the most common SLOP result was compared with P-MIED (SII/A1; Table 1f). Finally, the result of SII/A1 was compared with the C-MIED results (SII/A2; Table 1g).

Study population

All three experiments and the two analyses in the two studies (SI and SII) were performed with the same group of players. This was necessary in order to be able to compare the test results of the PT and the CT and to also compare the SLOP results with the P-MIED and the C-MIED results. All players in the sample were trained competitive tennis players coached by a qualified certified coach for more than six years. The players were Austrian, German, Italian, Romanian, Russian and Swedish citizens.

Data collection

The study exclusively investigated right-handers. Data were collected from a total of 31 healthy players, nine females (mean ± SD: age 18 ± 4 years, height 171 ± 4 cm, weight 59 ± 5 kg) and 22 males (mean ± SD: age 23 ± 10 years, height 178 ± 10 cm, weight 70 ± 14 kg), who were free of any known eye problems and who did not use prescription glasses or lenses. This convenience sample size of 31 subjects was chosen because there were no preliminary data available for formal calculation of sample size. Before commencement of the experiment, the participants were fully informed about the procedures and provided written informed consent. The participants had no previous experience with hand-eye dominance testing. The tests were performed at the Department of Sport Science, University of Innsbruck, Austria.

Data analysis

Study I – T1 (SI/E1a and SI/E1b).

Previous research data and the general opinion on eye dominance distribution indicate that 35% of the population have crossed eye dominance, while 65% are considered consistent eye dominant. Therefore, the expected value of the PT statistic analysis (SI/E1a) was set at 35% crossed and 65% consistent for this experiment. Since the literature does not describe any difference or change in eye dominance in frontal gaze direction associated with manual motor tasks, the expected value of the CT (SI/E1b) was compared with the observed value of the PT to obtain a fair comparative value (Table 1f).

Study I – T2 (SI/E2a and SI/E2b).

T2 additionally added the left hand to the PT/CT experiments SI/E2a and SI/E2b (right hand/both eyes; left hand/both eyes). The test investigated whether ED can switch in frontal gaze direction depending on the test (PT or CT) and the manual motor task. According to previous reports that eye dominance cannot switch in frontal gaze direction, the expected value is set at 0% switch and 100% consistent for PT evaluation (Table 1c). The PT result was taken as the expected value in the CT evaluation (Table 1d).

Statistical methods

To evaluate all three experiments and two analyses (SI/E1a and SI/E1b, SI/E2a and SI/E2b) chi-squared calculation with one-sample test was used. The observed values were compared with expected values. The expected values for the PT experiments were based on general opinion (see data analysis, Study I), whereas the expected values of the CT experiments were based on the PT results.

Ethical considerations

The experiments were approved by the Board of Ethical Questions in Science of the University of Innsbruck (Certificate 10/2018), Innsbruck, Austria. Detailed information on background, purpose, study procedure, data and data processing for the experiments in studies I and II was given to all potential participants before they entered the trials. They were informed verbally that participation was voluntary and that they were free to leave the study at any time. Potential risks from participating in the studies were likely to be small, but minor injuries could not be completely ruled out. All data were cleaned and anonymized before any data analysis was performed. The raw data were never transmitted electronically. The data are stored with the University of Innsbruck.

Study I - Specific procedure

A circular manual motor task (CT) involving the thumb and the index finger, similar to the grip used on a tennis racket (Figure 1 b), seems to influence eye dominance (ED). The procedure investigates and compares the results of the PT and the CT. The PT involves the pointing index finger and the CT is the circular manual task (Figure 2a and 2b). Both tests investigate the manual influence of the eye. Two tests were performed; T1 one-handed, right hand); T2 (each hand separately).

T1one-handedvalidity test(SI/E1a and SI/E1b)

T1 tested the player’s CT eye dominance (C-MIED) in frontal gaze direction in comparison to the conventional PT (straight forward without rotational head movements, right or left, Figure 2a and 2b) and evaluated whether a circular manual motor task (CT) can trigger a change in eye dominance in frontal gaze direction. T1 (C-MIED and P-MIED) compared the players’ one-handed ED. The definitions of CT and PT in T1 were: I. crossed – RH/left eye; II. consistent – RH/right eye.

T2two-handedvalidity test(SI/E2a and SI/E2b)

T2 (SI/E2a and SI/E2b) is also a procedure for investigating the results of the PT and the CT. However, each hand is investigated with each eye. The PT is the pointing index finger and the CT is the circular manual task. Both tests investigate the manual influence of the eye on each hand and eye: right hand, right eye/left eye and left hand, right eye/left eye (Figure 4a and 4b, Table 1). The test starts with the right hand. The focus of this experiment is to explore whether the CT (depending on which hand is used) can trigger a switch in eye dominance.

T2 (performed with each hand separately) defined the CT and the PT as: a. switched (eye dominance switches with the test hand, right-hand/left-eye and left-hand/right-eye); b. consistent (same dominance in eyes and hands; left eye-consistent, right eye-consistent). The observed values and the expected values were compared. It is important that each and every test be conducted separately and each time from the beginning. Changing the eye during the manual motor task will give non-reproducible results.

Description of PT performance

The subject uses the index finger of the right hand to cover the distant subject (Figure 2a) by holding the index finger in front of both wide-open eyes. The right arm is bent approximately 45 degrees. With both eyes open the subject looks at a specific target (at least five meters away). When instructed, the right eye is closed. If the target cannot be seen behind the index finger, a crossed PT is indicated (right hand/left eye). A consistent P-MIED is given when a right hand/right eye is detected.

Description of CT performance

The CT is a procedure for investigating the manual influence of the eye by means of a specific circular finger motion (Figure 2b). The subject forms a circle (a ring, Figure 2b) with the index finger and the thumb of the right hand and holds it in front of both wide-open eyes. The right arm is bent approximately 45 degrees. With both eyes open the subject looks through the circle at a specific target (at least five meters away). When instructed, the right eye is closed. If the target can still be seen with the left eye through the finger circle, a crossed C-MIED is indicated (right hand/left eye). A consistent C-MIED is given when a right hand/right eye is detected.

Validity and reliability (SI/E1a, SI/E1b, SI/E2a, SI/E2b)

Three independent testers evaluated the reliability of the HED test. The tests (T1 and T2) were repeated three times each per subject SI/E1a, SI/E1b (T1), SI/E2a, SI/E2b (T2). All 31 subjects were tested by all three testers on the same day at 20-minute intervals so that the players would not have time to interpret or learn the test and the testers would not be able to learn the players’ dominance. The players did not learn any results before all HED tests were finalized. The results of the three tests per subject did not reveal any deviations. Thus, the bias can be considered very low. The HED test showed 100% consistency, high reliability, strong accuracy of measure and is reproducible (see Strengths and limitations).

Figure 1a Figure 1b Figure 1c

Distance to the target Circular grp of the racket Circular grip of the ball

Abbildung in dieser Leseprobe nicht enthalten

Figure 2a Figure 2b

One-handed PT One-handed CT

Abbildung in dieser Leseprobe nicht enthalten

Figure 3

Comparison; One-handed PT vs. CT

Abbildung in dieser Leseprobe nicht enthalten

Figure 4a Figure 4b

Two-handed PT Two-handed CT

Abbildung in dieser Leseprobe nicht enthalten

Interlude

The first experiments with the four trials SI/E1a, SI/E1b (T1), SI/E2a, SI/E2b (T2) that make up Study I are important for exploring the validity of the CT in comparison with the conventional and common standard PT. SI/E1a, SI/E1b (T1), SI/E2a, SI/E2b (T2) increase our knowledge of the one-handed (T1) and the two-handed (T2) CT and its influence on eye dominance. Furthermore, Study I underlines the assumption that a complete eye dominance assessment seems to be important before being instructed how to serve. Study I is a necessary foundation for Study II. Study II explores the player’s personal SLOP and investigates the differences between P-MIED and C-MIED in association with the SLOP and the association between the one-handed crossed C-MIED and the closed SLOP.

Study II - Specific procedure of the SLOP in the tennis serve

The biomechanical serve study is based on the 8-stage model by Kovacs and Ellenbecker for tennis serve analysis and focuses on the loading stage of the preparation phase (the shoulder loading position; SLOP). Specifically, the study investigated closed and open SLOP frequency:

„The components usually seen in the traditional throwing analysis 30, 35 have been altered in this proposed 8-stage tennis-specific serve model. The 8-stage model has three distinct phases: preparation, acceleration, and follow-through. Each stage is a direct result of muscle activation and technical adjustments made in the previous stage. When a serve is evaluated, the total body perspective is just as important as the individual segments alone“ (Kovacs & Ellenbecker, 2011).

Three-dimensional (3D) analysis is considered the gold standard in movement analysis (Ford et al., 2007) and is the most recognized and preferred method for investigating the biomechanics of the serve. The use of 3D motion analysis made it possible for researchers to investigate the biomechanical and kinetic demands on upper limb loads that contribute to upper extremity injury in the tennis serve (Martin et al., 2013; Elliott et al., 2003; Girard et al., 2007; Martin et al. 2014; Elliott & Wood, 1983; Campbell et al., 2013). The 8-stage descriptive model of normal serve mechanics was derived from the 3D literature and provided readers with a detailed breakdown of proper mechanics (Kovacs & Ellenbecker, 2011). It described specific body positions and motions and presented the accompanying joint forces and rotational velocities during all the different phases of motion (Kovacs & Ellenbecker, 2011).

The current serve study involved one distinct phase, namely the SLOP. A detailed framework of the specific SLOP is described on pages 20-25. This framework explains the specific SLOP and describes how to support optimized flow up and through the kinetic chain to the shoulder and consequently transmission to impact with the ball.

Study population

All players in the sample are/were trained competitive tennis players coached by a qualified certified coach for more than six years (see Study I). The participants had no previous experience with Vicon testing.

Data collection

All three experiments of this thesis (SI/T1, SI/T2 and SII/E1) and the final analyses were performed with the same group of players (see Study I).

Data analysis

To analyse the second study (SII/E1, SII/A1 and SII/A2), namely whether the closed SLOP or the open SLOP is more common (Figure 5a and 5b), chi-squared calculations with a one-sample test were used. The observed values and the expected values were compared. For the analysis (SII/A1 and SII/A2) stratified sampling (n=22) was performed (to ensure a suitable subgroup) to compare the frequency of the most common SLOP, closed or open, with the frequency of the most common one-handed P-MIED or C-MIED group, crossed or consistent (Figure 9, Table 1f and 1g). Due to the fact that no studies have shown that players choose different positions in the SLOP, the values expected for experiment SII/E1 were equally distributed (50% closed SLOP and 50% open SLOP; Table 1e).

In situ capture

The experiment was conducted in an indoor laboratory at the Department of Sport Science, University of Innsbruck, Austria. The Vicon tests were performed in a standardized setting with eight cameras. The SLOP (shoulder orientation line in relation to the baseline; Figures 5a, 5b, 7 and 8) was recorded with eight cameras (Vicon Bonita 10, 200 Hz). The SLOP in the serve was defined as a „closed“ position (≥90°; more clockwise towards 1 o’clock) or an „open“ position (<90°; more clockwise towards 11 o’clock) in relation to the baseline. The players wore their own shoes and sports underwear. All players used the same racket (Head Radical MP™). Thirty-one digitalized standard body location landmarks, four markers on the head band and two markers on each wrist, five racket landmarks, two baseline landmarks and soft tennis balls approved by the International Tennis Federation (ITF) were used (Figures 6, 7 and 8).

Protocol

A virtual baseline was drawn on the floor five meters away from the target. Sport tape (white) was used to mark the baseline (Figures 7 and 8). After warming up (personal program as in usual training program, at least fifteen minutes), each athlete took twenty imaginary swings (serve movements) without a ball in order to familiarize himself with the test equipment. These movements included set-up, loading, cocking/jumping and ending. Each athlete also took some swings with a ball to adjust to the soft ball. Each measurement followed a particular routine. The subject had to execute calibration preparation movements to avoid recording errors. Recording took place before and during the full swing and at ball impact. Biomechanical analysis concentrated on the shoulder position in relation to the baseline during the loading position (Figures 7 and 8). The players were told to hit the ball as hard as possible (the way they always hit) into a protected plywood wall with a red target circle (0.5 x 0.5 m) approximately five (5) meters away from the test lab baseline (Figure 7).

Each athlete executed ten serves. Recording errors by some players (marker loss or inability to detect markers during the labeling process) meant that only the first five measurable (out of ten) recorded trials were chosen as parameter value. Consequently, the mean of the SLOP was calculated from each player’s first five recorded trials.

Figure 5a

Closed (5a.i and iv) open (5a.ii and 5b) and shoulder-loading positions (SLOP)

i. Closed SLOP ii. Open SLOP iii. Axis of SLOP iv. Closed SLOP

Acromion (marker)

Abbildung in dieser Leseprobe nicht enthalten

Figure 5b

Shoulder orientation lines description loading position

Baseline: line indicating the boundary of the area of play. (See: In Situ Capture)

Abbildung in dieser Leseprobe nicht enthalten

Figure 6

Lab documentation

Body marker positions

Abbildung in dieser Leseprobe nicht enthalten

Figure 7 Figure 8

Lab documentation Lab documentation

Aligned position - no dic pressure Not aligned position – possible disc pressure

Abbildung in dieser Leseprobe nicht enthalten

Kinetic values

SLOP is defined as the toss movement directly before the hitting hand and the racket head cock downwards (Figures 7 und 8). At this point, the athlete chooses to position his shoulders in a particular preferred orientation line in relation to the baseline (see In Situ Capture: Figures 5a, 5b, 7 and 8). The SLOP instantly before ball impact is explained as: a. both feet are on the ground, b. the knees are bent, c. the hitting elbow points backwards, and d. the tossing non-serving arm aims (with arm extended) the ball at the highest point (after toss, Figure 5a). The SLOP of the serve was chosen as a measurement value, because no external influences impact the consistency or reproducibility of the position. Therefore, the Vicon SLOP test is considered to be reliable and reproducible (see Statistical methods).

Statistical methods – experiment SII/E1

The analyses in the second experiment (SII/E1) were calculated using SPSS (version 1.0.0.1213). Mean and SD values for the serve trials were computed for 31 recorded players in SLOP (Table 2). As a reproducibility value, the mean of five recorded trials per player was used in the statistics (see In Situ Capture). The chi-squared goodness of fit test based on a one-sample test was used. The expected values were calculated to 50%. Level of significance was established at p <0.05. To investigate the hypothesis chi-squared calculation with one-sample test was used. The observed values and the expected values were compared. The expected value for SII/A1 was set at 35% crossed and 65% consistent (general opinion, see Study I). The expected value for A2 was based on the SII/A1 results (45% crossed, 55% consistent). Level of significance was established at p <0.05.

Figure 9

Study overview - research chart

Abbildung in dieser Leseprobe nicht enthalten

Result

Study I (SI)

T1 -one-handedvalidity test;PT(SI/E1a)

The results of the one-handed PT/T1 (SI/E1a) showed that 14 (45%) of 31 players preferred crossed ED (right hand/left eye) and 17 (55%) players showed constant ED (right hand/right eye) (Table 1a). Studies have shown that approximately 90% of the general population is right-handed and that 65% of those right-handers are right eye-dominant (consistent). The result of the conventional PT experiment approximately matches previous results for eye dominance. Therefore, the result is not significant.

T1 -one-handedvalidity test;CT(SI/E1b)

The sample performed the new developed CT/T1 (SI/E1b) and it was seen that 22 players showed crossed ED, while nine players showed consistent ED (Table 1b).

T1 – SI/E1a and SI/E1b; comparative analysis

The T1 comparative analysis of the PT (n=14/n=17) in comparison to the CT (n=22/n=9) revealed a significant (p =0.004) difference in crossed and consistent EDs (Table 1b). SI/E1a and SI/E1b showed different results within the same sample (PT or CT) and revealed that the circular manual motor task can influence eye dominance in frontal gaze direction.

T2 - two-handed validity test; both eyes CT and PT (SI/E2a and SI/E2b)

According to general opinion and previous research, eye dominance cannot switch in frontal gaze direction. The findings from experiment SI/E2a confirmed previous studies and found no switch when the PT was performed (Table 1c). On the contrary, experiment SI/E2b showed that 19 (61%) of the 31 players switched eyes in frontal gaze direction, depending on the active hand when CT was performed. This result, additionally to T1, significantly contradicts the general opinion and shows that a circular manual motor task can influence eye dominance in frontal gaze direction. The PT turned out to be 100% consistent as no players shifted eye dominance (Table 1d). SI/E2a and SI/E2b (Table 1c and 1d) compared the CT and the PT as a validation of manually influenced eye dominance as a requirement for the further investigation.

Study II (SII)

Experiment SII/E1; SLOP test

The chi-squared results in SII/E1 (Tables 1e and 3) demonstrate that 22 (71%) of the 31 players preferred a closed SLOP, whereas nine (29%) of the 31 players preferred an open SLOP, p =0.02. To the best of our knowledge there are no studies that can be used for comparison purposes. Therefore, the expected value of SII/E1 was set at 50%. The experiment demonstrates that more players chose a closed SLOP rather than an open SLOP. Therefore, the closed SLOP/P-MIED (SII/A1) result was compared with SLOP/C-MIED (SII/A2) results in the analysis.

Analysis 1 - SII/A1; comparison of the closed SLOP with one-handed crossed and consistent P-MIED

Analysis SII/A1 (Table 1f) focused on the closed SLOP in comparison to crossed or consistent P-MIED. The chi-squared result revealed that ten (45%) of the 22 players in the closed SLOP group showed a combination of closed SLOP and crossed eye dominance (P-MIED) and 12 (55%) of the 22 players showed consistent P-MIED. The expected value was set at crossed n=8 (35%) and consistent n=14 (65%) in accordance with the general opinion about ED.

Analysis 2 - SII/A2; comparison of CSCr1PM and CSCr1CM and of CSCn1PM and CSCn1CM

The final analysis (SII/A2) compared the results of CSCr1PM and CSCr1CM (A1) and the results of CSCn1PM and CSCn1CM (A2). The result of A1(P-MIED) was used as the expected value for A2. A1 revealed that ten of the 22 closed SLOPs showed a crossed PT (CrP-MIED) result (Table 1f). A2 showed that 16 (73%) of the 22 players with a closed SLOP exhibited crossed C-MIED, whereas six (27%) players with a closed SLOP showed consistent C-MIED (n=6) (Table 1g). Thus, comparison of the results of CSCr1PM and CSCr1CM and of the results of CSCn1PM and CSCn1CM demonstrates that six (50%) of the 12 players with a closed SLOP/consistent PT result changed their ED from consistent to crossed in the one-handed CT. This is a significant difference (p=0.01; Table 1g) that shows that manual motor tasks may have an effect on ED and consequently be important for an optimized SLOP.

Table 1a-g

Results of chi-squared tests of study I (Experiments 1a and 1b and Experiment 2a and 2b) and Study II (Experiment 1 and analyses 1 and 2) observed and expected values, their proportions and distribution.

Table 1a

Study I / Experiment 1a One-sample test; Goodness of Fit; SI/E1 (Validity Test 1)

One-handed Porta Test (PT)

One-handed ; 1Cr1PM: Crossed P-MIED; 2Cn1PM: Consistent P-MIED

Abbildung in dieser Leseprobe nicht enthalten

Table 1b

Study I / Experiment 1b One-sample test; Goodness of Fit; SI/E1 (Validity Test 1)

One-handed Circular Test (CT)

One-handed ; 3Cr1CM: Crossed C-MIED; 4Cn1CM: Consistent C-MIED

Abbildung in dieser Leseprobe nicht enthalten

Table 1c

Study I/Experiment 2a One-sample test; Goodness of Fit; SI/E1 (Validity Test 1)

Two-handed Porta Test (PT)

Two-handed; 5Sw2PM: Switched P-MIED; 6Cn2PM: Consistent P-MIED

Abbildung in dieser Leseprobe nicht enthalten

Table 1d

Study I / Experiment 2b (SI/E2b); One-sample test - Goodness of Fit

Two-handed Circular Test (CT)

Two-handed; 7Sw2CM: Switched C-MIED; 8Cn2CM: Consistent C-MIED

Abbildung in dieser Leseprobe nicht enthalten

Table 1e

Study II / Experiment 1; One-sample test - Goodness of Fit

Closed and Open SLOP; 9 CS: Closed SLOP; 10OS: Open SLOP

Abbildung in dieser Leseprobe nicht enthalten

Table 1f

Study II /Analysis 1

Only closed SLOP with one-handed crossed or consistent P-MIED; 11 CSCr1PM: Closed

SLOP/Crossed P-MIED; 12CSCn1PM: Closed SLOP/Consistent C-MIED

Abbildung in dieser Leseprobe nicht enthalten

Table 1g

Study II / Analysis 2

SLOP/C-MIED Comparison

Only one-handed crossed with closed or open; 13 CSCr1CM: Closed SLOP/Crossed C-MIED; 14CSCn1CM: Closed SLOP/Consistent C-MIED

Abbildung in dieser Leseprobe nicht enthalten

Discussion

The outspoken overarching aim of the study was to identify different classifications of manually influenced eye dominance (MIED) traits (H1-SI) and to investigate the association between MIEDs and theshoulder loading position (SLOP) in the tennis serve (H2-SII). Knowledge of such an association can be considered a first step to supporting coaches and players in their attempts to enhance serve performance.

The main findings of this thesis confirm the manual influence of the ED in frontal gaze direction as well as an association between MIED and the SLOP.The most important findings of the investigated sample in SI were: first, the results of the conventional PT differed from the results of the CT in both one-handed (T1) and two-handed tests (T2); secondly, eye dominance can switch, depending on which hand is used (T2). These important findings of SI contradict previous studies claiming that the general population cannot change eye dominance when gazing in frontal direction (Chaurasia & Mathur, 1976).The most important findings of SII were: first, that the personal SLOP in the tennis serve was documented and showed a significantly higher frequency of closed SLOPs than of open SLOPs; secondly, the results of the closed SLOP combined with one-handed P-MIED (CSCr1PM and CSCn1PM) differed from the results for closed SLOP combined with one-handed C-MIED (CSCr1CM and CSCn1CM) and revealed a significant difference.Thus, a player conventionally assessed with consistent (PT) eye dominance may have crossed (CT) eye dominance when holding his racket and folding his thumb and index finger around the grip. Therefore,these findings are important, because they show that screening results for MIED and SLOP can contribute to our understanding of a player’s personal choice of serving technique in tennis.

Although the final analysis of the thesis exclusively compared the closed SLOP and one-handed crossed and consistent P-MIED and C-MIED, the results of the two-handed PT and CT can be considered highly important for future studies. Therefore, the CT results concerning switched ED will be incorporated in the following discussion to facilitate understanding and support further research projects.

The results of the thesis are partly in agreement with the results of previous studies (Khan & Crawford, 2001;Chaurasia & Mathur, 1976). To our knowledge, this thesis is the first to show that in frontal gaze direction eye dominance can shift as a function of right or left manual motor tasks. Therefore, the suggestion that the ocular motor system determines eye dominance at a specific angle (Khan & Crawford, 2001) might support our study’s finding that also the manual motor system contributes to a player’s C-MIED classification, which, in turn, permits C-MIED and SLOP results to be evaluated for serve assessment in tennis.

The results neither reveal why a circular hand-finger movement causes a shift in eye dominance, nor do they explain why crossed C-MIED players seem to be more common in a closed SLOP than in the consistent group of the study sample. Nevertheless, since this is the first study to describe and evaluate C-MIED in comparison with P-MIED and SLOP assessment, no other studies are available for the purpose of comparison. The research findings presented in this thesis will be considered on the basis of the original hypothesis with detailed discussions from an on-court coaching perspective. With regard to practical implications, some examples below clarify the possible advantages and disadvantages for players who show a consistent versus a crossed or crossed-consistent, ipsilateral-consistent and crossed-switched C-MIED. In keeping with the hypotheses, the significant result reveals that crossed-switch players are more common in the study (SI, T2), which indicates that initially, as a right-hander, it might be beneficial to be a crossed-switch C-MIED player (right hand/left eye and left hand/right eye) during a first learning process in a sport like tennis. Hence, it must be considered that the handling of complicated movements might be easier for crossed C-MIED players.The experiments in SI show that the consistent right-handed players use the same eye in both finger circle tests T1 and T2 (crossed-consistent; right hand/left eye, left hand/left eye, ipsilateral-consistent; right hand/right eye, left hand/right eye). Thus, it may be argued that crossed-consistent right-handers prefer a more closed SLOP, whereas ipsilateral-consistent right-handers favor an open SLOP. It can also be argued that crossed-switch (two-handed; right hand/left eye, left hand/right eye) C-MIED tennis players use both eyes and hands interchangeably as required in order to themselves solve and optimize immediate motor challenges. Crossed-consistent C-MIED players might have abilities that are similar to those of ipsilateral-consistent players. However, they may differ in the SLOP. A crossed-consistent C-MIED player and an ipsilateral-consistent C-MIED player use the same eye, regardless of which hand they are using. However, both crossed-consistent and ipsilateral-consistent C-MIEDs can be more consistent when assuming the loading position (fewer choices and thus less need for adjustment) than can crossed-switch players.

The fact that players with a crossed (right hand/left eye) C-MIED are more frequent in the closed group, but are found in both the closed and the open SLOP sample group can be explained as an effect of the hand/finger activation sequence at the beginning of a swing (Figure 1b). It may be argued that, in this case, right-handed players unconsciously choose a hand activation sequence (instinctively or as taught to them) and thus activate a preferred eye, which might favor a particular SLOP. Hence, crossed-switch C-MIED tennis players might allow the left eye to take command by activating the right hand first (firm grip) and vice versa. The left-consistent player always activates the left eye and the right-consistent player always activates the right eye, regardless of which hand is being used. Ipsilateral consistent C-MIED (right hand/right-eyed players and vice versa for left-handers) and crossed-consistent C-MIED players (T2) might not easily adjust to a position that is not ideal for them and may need optimized and clear instructions, thus suggesting that they might be forced to stay in the learned SLOP. A centrally placed toss (frontal in relation to the body and placed frontal between the legs, player sideways to the net) may be more beneficial for an ipsilateral-consistent player (RH/RE-LH/RE). Thus, the shoulder does not turn forward too early as a result of the personal C-MIED. A toss sideways towards the net in a sideways position might be very difficult for a right-handed ipsilateral-consistent C-MIED player, whereas a toss by a crossed-switch C-MIED player may be more flexible thanks to the ability to adapt and instantly interchange the sequence between both hands and eyes. Left-consistent C-MIED players can also toss the ball forward towards the net and can show consistent movement once it is properly learned. Moreover, the crossed-switch C-MIED player might show a greater ability to hit the ball on the way up or play serve and volley, where the ability to toss the ball forward (towards the net) is important and the hands and eyes need to interlink rapidly. Further explanations why the CT seems to be effective as a two-handed standard test might be that the C-MIED screening test is not only similar to the manual grip of the hitting hand (racket hand), but also similar to a circle-like manual motion during the toss of the ball (left hand; Figure 1c), instants before the SLOP. The player uses his thumb and index finger, supported by two other fingers as a manual platform, to keep the ball in his hand (Figure 1c). However, the circle-like grip on the ball must be investigated in further studies.

The presented results provide new insights into the relationship between hand-eye coordination and the SLOP, with specific focus on the one-handed C-MIED. The results appear to be important for the assessment of skill development in the tennis serve and contribute to a clearer understanding of visuomotor coaching. However, further studies are needed to strengthen, extend and diversify the presented hypotheses. Consequently, instructions aimed at having a student copy a successful top player, who might have a completely different C-MIED than the student, could indeed be counterproductive and should be reconsidered. Nevertheless, it is important to understand that a consistent C-MIED might be at least as beneficial in the long-term once an appropriate technique has been learned. Moreover, it deserves to be mentioned that the study’s results do not say anything about a player’s success. This remains to be examined in future studies. Nonetheless, the C-MIED screening results contribute to our understanding of a player’s manually influenced hand-eye dominance and his ability to implement environmental information into the biomechanical process.

All in all, the purpose of this study was also to create a screening procedure that is instantly applicable for on-court coaching assessments. The user is advised to develop a personal strategy for implementing the C-MIED procedure and to interpret results for the specific player. Without a doubt, the C-MIED assessment procedure is not yet optimized, but it is simple and serves as a tool for measuring C-MIED and SLOP screening results, which can play a supportive role in tennis serve assessment in future.

Strengths and limitations

One of the strengths in Study I (SI/E1a and SI/E1b, SI/E2a and SI/E2b) is the fact that neither the PT nor the CT showed any deviations during the three evaluations and can thus be considered 100% reliable, which strengthens the validity of our findings. Furthermore, the various nationalities of the studied players with different backgrounds increase the generalizability of the results (see Methods and materials), and the functional design of the study is strongly linked to the practical testing procedure that can be used on an everyday basis. New hypotheses can be derived from the presented results and may be useful and adaptable for further studies concerning tennis serving technique. Some limitations of this study should be addressed. The time interval between the manual motor tasks may be a limitation of the study because of the memorizing effect. The player may remember previous results and unintentionally repeat the same result.

The second study (SII/E1) with the Vicon test can be considered 100% reliable, which strengthens the validity of our recordings. The investigators’ specific tennis experience is another strength because of the challenge to decide the exact SLOP and the potential risk of misclassification. Tennis is a three-dimensional sport and involves many different strokes. Therefore, extracting one single stroke and one position of that stroke (SLOP) may be a limitation. Recordings with the eight-camera Vicon system may act as a limitation due to the fast and complex three-dimensional motion in the tennis serve. Therefore, more cameras might be beneficial in order to capture more specific details (markers) during the fast serve motion. Finally, the study did not evaluate the quality (rhythm, coordination, equilibrium, balance, power, accuracy) of the serve, which may be a limitation to fully analyzing the success (ranking, tournament victories) of a player’s serving ability.

Future perspective

Future research on a larger scale should involve more tennis players from different countries. Left-handers should also be examined.

As an additional reliability tool, we suggest using red/green duochrome lenses during the CT in order to document the ocular shift during the finger circle test. This would eliminate the need for the player to close his eyes. The use of such red/green duochrome lenses would facilitate the CT, which would make it possible to test young adolescents for their C-MIED, follow them for some years and evaluate possible changes in their C-MIED.

More Vicon cameras should be involved for excellent and totally flawless biomechanical high-speed three-dimensional recordings. Evaluations using electroencephalography (EEG) when executing the HED test with the finger muscles and during active serving as well as electromyography ( EMG ) during assessement of the shoulder loading position are suggested for additional studies. Fingertip grip sensors can be applied to determine which hand a player primarily uses (the first activity, right or left hand) when delivering a serve.

Finally, we recommend that the results showing the quality of the serve (placement and power) also be evaluated in future studies. The complexity of three-dimensional tennis biomechanics and manually influenced dominance requires further research to investigate the challenging mechanisms behind this C-MIED/SLOP phenomenon. Therefore, this study should be viewed as a door-opener and should support future studies of C-MIED and SLOP screening results and their role in the assessment of the tennis serve.

Additional perspectives and considerations

This thesis shows for the first time that manually influenced eye dominance can influence an ED shift in frontal gaze direction and that a circular MIED is associated with the SLOP in the tennis serve. Traits like manually influenced eye dominance (MIED) can influence the shoulder loading position (SLOP) and consequently also influence the postural curvature (Figure 8) of the non-adapted spine, thus possibly leading to injuries. The SLOP should preferably be aligned and adjusted with the rest of the body without unnecessary adaptions or adjustments to avoid unfavourable spine curvature during the serving process. In fact, the imminent problem seems to be comprehension of a player’s complete body position and its underlying mechanisms in order to optimize the postural curvature and minimize injury risk. The fact that players choose a personal SLOP shortly before the ball is impacted enhances the importance of optimized postural curvature during the SLOP that can smoothly interlink the personal SLOP with the legs and trunk. Future studies should investigate the personal SLOP in combination with the legs and trunk to avoid unnecessary vertebral disc pressure. A server that has an excessive toss to clockwise more towards 11 o’clock must bend his spine backwards in a lumbar flexion and then hit the ball forward upwards with lateral flexion. Such motion might be an adaptation and a reaction to a previously instructed toss placement by a player with right hand/right eye dominance. Further analysis of the serve motion can involve a focus on the overall body position. If the player has a frontal knee position (Figure 8) and a closed SLOP (≥90°) in the loading phase when serving, the back shoulder will be much further away from the knee and may stress the lumbar spine more than if the knee is under the back shoulder (Figure 7). Therefore, comprehension of the C-MIED could be an advantage, not only for performance but also for minimizing the risk of injury.

Practical applications

In view of the presented results the benefits and possibilities of the C-MIED and SLOP screening procedure (see C-MIED procedure) for assessment of the tennis serve can be presented in coach training programs to raise the sensitivity and awareness for hand-eye dominance in coaching. Furthermore, inclusion of the HED procedure (see below) in daily skill training should be considered with the aim to avoid unnecessary basic technical errors and mandatory assessment routine.

The study suggests that the C-MIED procedure be taken into consideration when:

-the coach notices an asymmetric (rhythm, coordination) movement during the serve process
-the coach notices that the player cannot follow technical instructions
-the player lacks power in his serve
-the coach observes unnecessary movements in the preparation process like „dancing“ (back and forth or up and down), knee or elbow and wrist deviations (waiter’s serve)
-the coach cannot decide if the player is using „non-fit“ taught movements or if a deviation/adjustment occurs because of an unintentional „non-fit“ sequence action of the hand.

The evaluation process works step by step. For optimal assessment of a tennis serve, we recommend that the process be administered in exactly the form suggested below.

C-MIED procedure

-First, explore the player’s closed or open SLOP (≥90° or <90° in relation to the baseline; see Introduction and Methods and materials).
-Then, perform the CT to assess the athlete’s hand-eye dominance.
-Next, match the one-handed crossed CT result and the closed versus the open SLOP (because crossed C-MIED appears to be more common in the study sample).
-Finally, if no explanations are found, extend the test process and match right hand/right eye C-MIED (consistent C-MIED) versus closed and open SLOP.
-The CT is intended only as a screening tool and should not be used for diagnosis. This means that the results do not confirm a clear serve diagnosis plan, but identify the player’s personal hand-eye dominance. Therefore, the CT procedure should be only one part of an extensive evaluation process including various other methods.

The CT process tool is used as the basis for developing an observational tool for the purpose of evaluating a player’s personal hand-eye coordination preference. The study suggests that the CT tool could facilitate evaluation of tennis serve mechanics without expensive equipment. There are several advantages to this type of analysis. First, it is mobile and thus available for practice or tournament sites and can be implemented on court. With the CT tool, coaches and health care professionals are able to easily identify mechanical flaws in the service motion to improve performance and diminish the risk of injury. However, there is no direct correlation between the findings of this analysis and either the incidence or prevention of injuries or performance. Future studies need to investigate this procedure as one of the primary outcomes in both serve performance and injury-related studies.

The study investigated the reliability of a field-based tool by using a personal hand-eye preference (C-MIED) and a preferred SLOP to grade the preferences of the tennis serve. It was hypothesized that it is possible to find a manual influence on eye dominance and its association with the SLOP in the tennis serve. The thesis describes a statistically significant difference between the crossed C-MIED and P-MIED in association with the SLOP in the tennis serve.

This study is limited to hand-eye coordination in its attempt to describe the possible mechanisms underlying a particular SLOP in the tennis serve. Needless to say, this study limitation does not mean that other information sources are less important. More research has to follow this thesis, which can only be considered one small step on the road to exploring a world of extraordinary possibilities of serve performance and development. Measuring, defining and establishing values in three-dimensional movements like the tennis serve is a huge challenge. Most visual research has been performed in sports like archery and shooting. Hence, it may therefore be fundamental to conduct the research in a combination of a phenomenon and the performance of a specific action like the tennis serve.

The thesis describes a new theoretical approach to study of HEC and MIED and thus makes a substantial contribution to the assessment of eye dominance and serve technique. The three experiments fundamentally discuss the mechanisms behind the hand-eye phenomenon and how the sequence of the hand-eye relationship may influence a biomechanical shoulder position. Moreover, this thesis brings together some ideas and theories on the implications for teaching the tennis serve.

The author is well aware that possible interlinks between hand-eye coordination and eye dominance as well as the association with the shoulder loading position have to be investigated in future projects and also appreciates the difficulty involved in attempting to solve the complicated question of how the human motor system obtains information from the environment and how the musculoskeletal system reacts to it.

Conclusions

This thesis reveals previously unknown mechanisms in the hand-eye relationship and the association with the shoulder loading position in the tennis serve. It can be concluded that the CT procedure is significantly changing the proportion of left and right dominants in contrast to the usual right dominance prevalence found with the PT and also that the CT procedure is superior to the PT eye dominance rigidity, where only the visual influence of the hand is measured and not vice versa. That provides an additional objective measure for the manual influence of eye dominance. Finally, without considering personal traits such as hand-eye coordination (HEC) and eye dominance (ED), a player’s natural serving ability cannot be defined, supported or optimized. This thesis presents a new eye dominance testing procedure and describes that C-MIED may be associated with a player’s choice of SLOP in the tennis serve. These findings are important, because they show that screening results for MIED and SLOP can contribute to our understanding of a player’s personal choice of serving technique in tennis.

Epilogue

The fast and exciting developments in tennis demand new assessment tools and strategies in order to master the evaluation of innate personal abilities as a support for skill development. One of the biggest challenges in tennis is generally to overcome the high incidence of injuries (Johansson, 2017). The author’s more than three decades of experience in professional tennis lead him to hold the strong conviction that personal and impeccable shoulder position in the tennis serve can reduce the risk of injury.

Despite the considerable body of knowledge on biomechanics in the tennis serve, still little is known about the underlying mechanisms for injury and performance. This thesis investigates and discusses the necessity for MIED assessment and its association with the SLOP. The fact that a manual motor task can influence eye dominance increases the importance of highlighting the sequence of the hand-eye coordination relationship at the start of the swing, which may be the first trigger for a player’s personal ED and can influence his personal SLOP. This knowledge cannot be overestimated and is important as a support to the smooth flow of the kinetic chain that can decrease the load exerted on the shoulder. Combining the information from all three experiments (SI/T1, SI/T2 and SII/E1) and the two following analyses (SII/A1 and SII/A2), this thesis presents a new additional procedure for measuring eye dominance. The thesis also discusses additional thoughts about future research on the kinetic chain and the postural curvature during the serve to put the findings in perspective. The endeavors to achieve smooth movement from bottom to top are essential for the injury-free serve motion. Therefore, the C-MIED knowledge can be of great help in avoiding an involuntary SLOP short before the ball is impacted.

The final message of this thesis is to focus on individual and personal eye dominance traits and personal biomechanical shoulder positions in order to optimize performance in the tennis serve. Subsequently, knowledge of the player’s C-MIED can help him achieve a more personal and efficient serving technique that can also aid him in optimizing technical performance and consequently avoiding injury.

Summary

Although the biomechanics of the tennis serve has been extensively researched, many players continue to struggle with the tennis serve technique. Well taught biomechanical movement patterns enhance performance in tennis and prevent injury (Martin et al., 2013; Martin et al., 2014). Specifically, the forces at work in the shoulder produce some of the most stressful movements in the tennis serve (Johansson et al., 2017). The overarching aim of this thesis was to explore manually influenced eye dominance (MIED) and investigate its possible association with the shoulder loading position (SLOP) in the tennis serve. In particular, the thesis tests the following hypotheses: firstly, it is possible to identify a manual influence on eye dominance; secondly, it is possible to identify an association between the choice of a personal SLOP in the tennis serve and manually influenced eye dominance (MIED).

The first aim of this thesis was to evaluate whether the outcome of the newly developed circular manual motor test (CT) differs from the result of the common and conventional Porta (PT) eye dominance assessment test (SI/E1a and SI/E1b) and whether eye dominance can switch in frontal gaze direction, depending on which hand is used, as a result of CT performance (SI/E2a and SI/E2b). While much research has studied the visual influence of the hand movements, there is still a lack of information about the manually influenced eye. For this reason, we examined one-handed and two-handed PT and CT and found significant differences in the CT by comparison with the PT. Moreover, a significant number of players with switching eye dominance were identified when performing the CT.

The second aim of this research project was to investigate whether a closed loading position is more common than an open loading position (SII/E1). No previous studies have investigated the association between ED and the SLOP and there is a large gap in the existing literature with regard to the SLOP in the serve. Therefore, we explored the SLOP in comparison to the baseline and found a significantly higher frequency of players with a closed SLOP than players with an open SLOP.

The third specific aim was to contribute to comprehension of one-handed P-MIED in comparison to C-MIED, differences or similarities and their association with the SLOP in the tennis serve. However, the main focus was on C-MIED. C-MIED was chosen because of its similarity to the manual grip used on the tennis racket. Tennis is a three-dimensional sport with a lot of fast changes of direction. Therefore, it is important to explore innate traits and personal preferences in order to develop a personal and optimized technique.

The most important finding of our study was that the results of the closed SLOP combined with one-handed P-MIED significantly differed from the results for closed SLOP combined with one-handed C-MIED. The experiments demonstrate that the CT can influence ED in frontal gaze direction and that C-MIED may be associated with a player’s choice of SLOP in the tennis serve. These findings are important, because they show that screening results for MIED and SLOP can contribute to our understanding of a player’s personal choice of serving technique in tennis.

Table 2

Eye dominance Porta´s Test vs. Circular Test One-handed and Two-handed (Right-handers)

Abbildung in dieser Leseprobe nicht enthalten

Table 3

Shoulder position in relation to the baseline - CT

Abbildung in dieser Leseprobe nicht enthalten

Table 4

Shoulder position in relation to the baseline - PT

Abbildung in dieser Leseprobe nicht enthalten

References

Abrams, G. D., Sheets, A. L., Andriacchi, T. P., & Safran, M. R. (2011). Review of tennis serve motion analysis and the biomechanics of three serve types with implications for injury. Sports Biomechanic, 10(4), 378–390.

Abernethy, B. (1986), Enhancing sports performance through clinical and experimental optometry. Clinical and Experimental Optometry, 69: 189-196.

Abernethy, B. (1996). Training the visual-perceptual skills of athletes.

The American journal of Sports Medicine, 24(6), 89-92.

Abernethy, B., & Wood, J.M. (2001). Do generalized visual training programmes for sport really work? An experimental investigation. Journal of Sports Sciences, 19, 203-222.

Atkins, D.L. (1998). The eye and sense of vision. Journal of Science and Medicine in Sport, 1(1), 3 - 17.

Bahamonde, RE. (2000). Changes in angular momentum during the tennis serve. Journal of Sports Sci ence, 18(8), 579-592.

Banister, H. (1935). A study in eye dominance. British journal of Psychology, 26, 32-48.

Banks, M.S., Ghose, T., & Hillis, J.M. (2003). Relative image size, not eye position, determines dominance switches. Vision Research, 44, 229-234.

Bennett, S., Button, S., Kingsbury, D., & Davids, K. (1999). Manipulating Visual Informational Constraints during Practice Enhances the Acquisition of Catching Skill in Children. Research Quarterly for Exercise and Sport, 70(3), 220-232

Bloom, B. (1985). Developing talent in young people. New York, Ballantine. ISBN 9780345315090

Burns R.B., & Dobson C.B., (1984). The self-concept. In: Introductory Psychology. Springer, Dordrecht. https://doi.org/10.1007/978-94-011-6279-1_13 ISBN : 978-0-85200-491-3

Campbell, A., Straker, L., O'Sullivan, P., Elliott, B., & Reid, M. (2013). Lumbar loading in the elite adolescent tennis serve: link to low back pain. Medical Science of Sports Exercise, 45(8), 1562-1568.

Campbell, A., O'Sullivan, P., Straker, L., Elliott, B., & Reid, M. (2013). Back Pain in Tennis Players: A Link with Lumbar Serve Kinematics and Range of Motion. Medicine and Science in Sports and Exercise, 46(2), 351-357

Chaurasia, B.D., & Mathur, B.B. (1976). Eyedness.Acta Anatomica,96(2), 301–305.

Christenson, G.N., & Winkelstein, A.M. (1988a). Visual skills of athletes versus non-athletes: development of a sports vision testing battery. Journal of American Optometry Association, 59(4), 666 – 675.

Christenson, G.N., & Winkelstein, A.M. (1988b). Sports vision testing battery. Journal of American Optometry Association, 59(9), 666 – 675.

Ciuffreda, K.J. (2011). Simple hand-eye reaction time in the retinal periphery can be reduced with training. Eye and Contact Lens, 37(3), 145-146.

Cotti, J., Vercher, J.L., & Guillaume, A. (2011). Hand–eye coordination relies on extra-retinal signals. Evidence from reactive saccade adaptation. Behavioural Brain Research, 218(1), 248-252

Crider, B. A. (1944). A battery of tests for the dominant eye. The Journal of General Psychology , 31(2), 179–190.

Cutting, J. E. (1986). Perception with an eye for motion. Four ways to reject directed perception. Ecological Psychology, 3, 25–34.

Dahlbo, H., Flatz, M., & Federolf, P. (2020). Eye dominance

testing: An exploration of the conventional standard eye dominance Porta Test in comparison with a circular manually influenced eye dominance Test. Department of Orthopedics - Medical University Innsbruck / Department of Sports Science - University of Innsbruck 2020a

Dahlbo, H., Flatz, M., Haid T., Federolf, P., & Krismer, M. (2020)

Eye dominance in tennis serve: An exploration of shoulder loading position in the tennis serve and its association with manually influenced eye dominance. Department of Orthopedics - Medical University Innsbruck / Department of Sports Science - University of Innsbruck 2020b

Durand, A.C. & Gould, M. (1910). A Method of Determing Ocular Dominance. Journal of the American Medical Association , 55(5), 369-370.

Du Toit, P.J., Kruger, P.E., de Wet, K.B., van Vuuren, B., Joubert, A., Lottering, M.L., & van Wyk, G.J. (2006). Transfer effects of hand-eye coordination skills from the right to the left cerebral hemispheres in South African schoolboy rugby players. African Journal for Physical Health Education, Recreation and Dance, 12(1), 41-49.

Elliott, B., & Wood, G. (1983) The biomechanics of the foot-up and foot-back tennis service techniques. The Australian Journal of Sports Sciences, 3(2), 3-6.

Elliott, B.C., Marshall, R.N., & Noffal, G.J. (1995). Contributions of upper limb segment rotations during the power serve in tennis. Journal of Applied Biomechanics, 11, 433-442.

Elliott, B., Fleisig, G., Nicholls, R., & Escamilia R. (2003). Technique effects on upper limb loading in the tennis serve. Journal of Science and Medicine in Sport / Sports Medicine Australia, 6(1), 76-87.

Ericsson, K., Krampe, R., & Tesch-Römer, C. (1993). The role of deliberate practice in the acquisition of expert performance, Psychological Review, 100, 363–406.

Erickson, G. (2007). Sports vision: Vision care for the enhancement of sports performance. St. Louis, MO, Butterworth Heineman Elsevier. ISBN-13:978-0750675772, ISBN-10:0750675772

Faber, I. R., Oosterveld, F.G.J., Van der Sanden, N., & Maria, W.G. (2014). Does an hand-eye coordination test have added value as part of talent identification in table tennis? A validity and reproducibility study. Plos one, 9(1), e85657-e85657.

Ferreira, J.T. (2002). Sports vision as a hardware and software system. Eyesight, 40.

Fisk, J.D., & Goodale, M.A. (1985). The organization of eye and limb movements during unrestricted reaching to targets in contralateral and ipsilateral space. Experimental Brain Research, 60, 159-178.

Frayer, D.W., Lozano, M., Bermúdez de Castro, J.M., Carbonell, E., Arsuaga J.L., Radovčić, J., Fiore, I., & Bondioli, L. (2012). More than 500,000 years of right-handedness in Europe. Laterality, 17(1), 51–69.

Fleisig, GS., Nicholls, R., Elliot, BC., & Escamilla, RF. (2003). Kinematics used by world classtennis players to produce high-velocity serves. Sports Biomechanics, 2(1), 51-64.

Ford, K., & Myer, G., & Hewett, T. (2007). Reliability of Landing 3D Motion Analysis. Medicine and science in sports and exercise. 39. 2021-8. 10.1249/mss.0b013e318149332d.

Gallahue, D.L., & Ozmun, J.C. (1997). Understanding motor development: Infants, children, adolescents, adults, (4th edition). WCM McGraw-Hill, Boston.

Girard, O., Micallef, JP., & Millet, GP. (2005). Lower-limb activity during the power serve intennis: effects of performance level. Medicine and Science in Sports and Exercise, 37(6), 1021-1029.

Girard, O., Micallef, JP., & Millet, GP. (2007). Influence of restricted knee motion during the flat first serve in tennis. Journal of Strength and Conditioning research, 21(3), 950-957.

Greenwood, J. (1993). Think Rugby. London, Bedford Row, 11 – 16.

Haid, C., & Fischler, S. (2013). Biomechanische Belastungsaspekte der Wirbelsäule beim Golfschwung. Sport-Orthopädie - Sport-Traumatologie - Sports Orthopaedics and Traumatology, 29, 89–95.

Hodge, R.D., Atkinson, J., Gill, B., Crelier, G.R., Marrett, S., & Pike, G.B. (1999). Linear coupling between cerebral blood flow and oxygen consumption in activated human cortex. Proclaimed National Academy of Science in USA, 96(16), 9403 – 9408.

Johansson, F.(2017). The shoulder in the elite adoloscent tennis player: Exploration of structural and functional sportspecific adaptations, Ghent University Faculty of Medicine and Health Sciences, Ghent, Belgium (PhD-Thesis), 15-17.

Johansson, R. S., Westling, G., Bäckström, A., & Flanagan, J. R. (2001). Eye-hand co-ordination in object manipulation. Journal of Neuroscience, 21(17), 6917–6932.

Johnson, C. D., McHugh, M. P., Wood, T., & Kibler, B. (2006). Performance demands of professional male tennis players. British Journal of Sports Medicine, 40(8), 696–699.

Kanegaonkar, R., Amin, K., & Clarke, M. (2012). The contribution of hearing to normal balance. The Journal of Laryngology and Otology, 126, 984-8.

Khan, A.Z., & Crawford, J.D. (2001). Ocular dominance reverses as a function of horizontal gaze angle. Vision Research, 41, 1743–1748.

Khan, A.Z., & Crawford, J.D. (2003). Coordinating one hand with two eyes: optimizing for field of view in a pointing task. Vision Research, 43, 409-417.

Kibler, W.B. (1995). Biomechanical analysis of the shoulder during tennis activities. Clinical Sports Medicine, 14, 79-85.

Kovacs, M., & Ellenbecker, T. (2011). An 8-stage model for evaluating the tennis serve, implications for performance enhancement and injury prevention. Sports Health, 3(6), 504–513.

Land, M., Mennie, N., & Rusted, J. (1999). The roles of vision and eye movements in the control of activities of daily living. Perception, 28 (11), 1311–1328.

Lee, D. N. (1976). A theory of visual control of braking based on information about time-to collision. Perception, 5, 437–459.

Ludeke, A., & Ferreira, J.T. (2003). The difference in visual skills between professional versus non-professional rugby players. The South African Optometrist, 62(4), 150-158.

Mapp, A.P., Ono, H., & Barbeito, R. (2003). What does the dominant eye dominate? A brief and somewhat contentious review. Perception and Psychophysics, 65(2), 310-317.

McLeod, B., & Hansen, E. (1989b). The effects of the eyerobics visual skills training programme on hand-eye coordination. Canadian Journal of Sport Sciences, 14, 127.

McLeod, B. (1991). Effects of eyerobics visual skills training on selected performance measures of female varsity soccer players. Perceptual and Motor Skills, 72, 863 - 866.

McLeod, P., & Jenkins, S. (1991). Timing accuracy and decision time in high speed ball games. International Journal of Sport Psychology, 22(3-4), 279-295.

McManus, I. C., Porac, C. K., Bryden, M. P., & Boucher, R. (1999).Eye-dominance, writing hand, and throwing hand. Laterality. 4(2), 173-192 .

Mefferd, R.B.J., & Wieland, B.A. (1969). Influence of eye dominance on the apparent centers of simple horizontal lines. Perceptual and Motor Skills, 28(3), 847-850.

Mendes, P. C., Fuentes, J. P., Mendes, R., Martins, F. M., Clemente, F. M., & Couceiro, M. S. (2013). The variability of the serve toss in tennis under the influence of artificial crosswind. Journal of Sports Science and Medicine, 12(2), 309–315.

Miles, W. R. (1930). Ocular dominance in human adults. Journal of Generel Psychology, 3(3), 412–430.

Minucci, P.K., & Connors, M.M. (1964). Reaction time under three viewing conditions: Binocular, dominant Eye, and nondominant Eye. Journal of Experimental Psychology, 67, 268-275.

Martin C., Kulpa R., Ropars M., Delamarche P., & Bideau B. (2013). Identification of temporal pathomechanical factors during the tennis serve. Medical Science of Sports Exercise, 45(11), 2113-2119.

Martin, C., Bideau, B., Bideau, N., Nicolas, G., Delamarche, P., & Kulpa, R. (2014). Energy Flow Analysis During the Tennis Serve: Comparison Between Injured and Noninjured Tennis Players. American Journal of Sports Medicine, 42(11), 2751–2760.

Martin C., Bideau B., Ropars M., Delamarche P., & Kulpa R. (2014). Upper limb joint kineticanalysis during tennis serve: Assessment of competitive level on efficiency andinjury risks. Scandinavian Journal of Medical Science in Sports, 24(4), 700-707.

Neggers, S.F., & Bekkering, H. (2000). Ocular gaze is anchored to the target of an ongoing pointing movement. Journal of Neurophysiology, 83(2), 639-651

Porac, C., & Coren, S. (1976). The dominant eye. Psychological Bulletin , 83(5), 880–897.

Portal, J.M., & Romano, P. (1998). Major review: ocular sighting dominance: a review and a study of athletic proficiency and hand-eye dominance in a collegiate baseball team. Binocular Vision and Strabismus Quart erly, 13(2), 125-132

Paul, M., Shukla, G., & Sandhu, J. S. (2011). The effect of vision training on performance in tennis players. Serbian Journal of Sports Sciences, 5(1), 11-16.

Peterhans, L., Fröhlich, S., Stern, C., Frey, W., Farshad, M., Sutter, R., & Spörri, J. (2020). High rates of overuse-related structural abnormalities in the lumbar spine of youth competitive alpine skiers: A cross-sectional MRI study in 108 athletes. The Orthopaedic Journal of Sports Medicine, 8(5).

Prodea, C., Pătraşcu, A., & Stanciu, L.A. (2013). The effects of hand-eye coordination over postural balance. Studia Universitatis Babes-Bolyai, Educatio Artis Gymnasticae, 58(3), 31.

Quartley, J., & Firth, A.Y. (2004). Binocular sighting ocular dominance changes with different angles of horizontal gaze. Binocular Vision and Strabismus Quarterly,19(1), 25–30

Reid, M., Elliott, B., & Alderson, J. (2008). Lower-limb coordination and shoulder jointmechanics in the tennis serve. Medicine and Science in Sports and Exercise, 40(2), 308-315.

Reid, M., Whiteside, D., Gilbin, G., & Elliott, B. (2013). Effect of a common task constraint on the body, racket, and ball kinematics of the elite junior tennis serve. Sports Biomechanics, 12(1), 15-22.

Rose, S. (1993). The Making of Memory , Bantam Books. ISBN 0553407481

Roth, H.L., Lora, A.N., & Heilman, K.M. (2002). Effects of monocular viewing and eye dominance on spatial attention, Brain, 125, (9), pp. 2023–2035,

Savelsbergh, G.J., Rosengren, K., Van Der Kamp, J., & Verheul, M. (2003). Catching action development. In Savelsbergh, G., Davids, K., Van Der Kamp, J., and Bennett, S.J., eds. Development of movement coordination in children. Applications in the Fields of Ergonomics, Health Sciences and Sport, London: Routledge , 191-212.

Schwab, S., & Memmert, D. (2012). The impact of a sports vision training program in youth field hockey players. Journal of Sports Science and Medicine, 11(4), 624-631.

Sherwood, P.W. (2000). Brain neurons change shape. www.csh.org, retrieved on 2002/05/15.

Stine, C.D., Arterbrun, M.R., & Stern, N.S. (1982). Vision and sports: A review of the literature. Journal of the American Optometric Association, 53(8), 627 - 633.

Snyder, L.H., Batista, A.O., & Andersen, R.A. (2000). Saccade-related activity in the parietal reach region. Journal of Neurophysiology, 83(2), 1099-1102

Spörri, J., Kröll, J., Haid, C., Fasel, B., & Müller, E. (2015). Potential mechanisms leading to overuse injuries of the back in alpine ski racing - A descriptive biomechanical study. The American Journal of Sports Medicine, 43, 2042-2048.

Spörri, J., Kröll, J., Fasel, B., Aminian, K., & Müller, E. (2016). Course setting as a prevention measure for overuse injuries of the back in alpine ski racing. A kinematic and kinetic study of giant slalom and slalom. Orthopaedic Journal of Sports Medicine,4(2), 2325967116630719.

Toyoshima, S., Hoshikawa, T., & Miyashita, M. (1974). Contributions of body parts to throwing performance. Biomechanics IV. Baltimore: University Park Press;169-174.

Walls, G. L. (1951). A theory of ocular dominance. American Medical Association Archive of Ophthalmology. , 45(4), 387–412.

Ward, P., & Williams, A.M. (2000). Development of visual function in expert and novice soccer players. International Journal of Sports Vision, 6(1), 1 – 11.

Whiteside, D., Elliott, BC., Lay, B., & Reid, M. (2015). Coordination and variability in the elite female tennis serve. Journal of Sports Sciences, 33(7), 675-686.

Williams, A.M., Davids, K., & Williams, J.G. (1999). Visual Perception and Action in Sport. New York: Routledge, 60 – 95.

Williams, A.M., Ward, P., Knowles, J.M., & Smeeton, N.J. (2002). Anticipation skill in a real-world task: Measurement, training and transfer in tennis. Journal of Experimental Psychology, 8(4), 259-270.

Wilson, T.A., & Falkel, J. (2004). Sports Vision: Training for better

performance. USA: Human Kinetics Publishers (Pty) Ltd.

Yuan, Y. W .Y., Fan, X., Chin, M., & So, R. C. H. (1995). Hand-eye co-ordination and visual reaction time in elite badminton players and gymnasts. New Zealand Journal of Sports Medicine, 23(3), 19-22

Upledger, J. E. (1999). SomatoEmotionale Praxis der CranioSacralen Therapie. SomatoEmotional Release. Haug Verlag, Heidelberg, ISBN 3-8304-7069-X.

Zetou, E., Vernadakis, N., Tsetseli, M., Kampas, A., & Michalopoulou, M. (2012). The Effect of Coordination Training Program on Learning Tennis Skills. Sport Journal, 15(1), 1-1.

Zupan, M.F., Arate, A.W., Wile, A., & Parker, R. (2006). Visual adaptations to sports vision enhancement training: A study of collegiate athletes at the US Airforce Academy. Clinical Sports Vision, 43-48.

[...]