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Design, model, prototype, test, analyse
and evaluate a mechanical human arm
(shoulder to wrist)
Year:
2006/07
Student: David Schroder
Course: BSc Engineering
Figure 1: assembly of mechanical human arm
(source: Solid Edge, self-made)
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FACULTY OF COMPUTING, ENGINEERING &
MATHEMATICAL SCIENCES
(CEMS)
UNIVERSITY OF THE WEST OF ENGLAND
(UWE)
Design, model, prototype, test, analyse and
evaluate a mechanical human arm
(shoulder to wrist)
Student: David
Schroder
Course: BSc
Engineering,
Final
Year
Year:
2006/07
Module: Individual
Project
Module
No.:
UFMEAY-30-3
Date:
02.05.2007
Heading Sheet
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I. Summary
This study sets out to investigate, model and analyse a mechanical human arm. The
study consists of four main steps: the literature research, modelling the mechanical
human arm, building the model and finally analysing it.
The mechanical human arm is the same size as the real human arm of a 20-year-old
male. The range of motion is also the same.
The investigations cover the functionality of real human arms, the history of
prostheses, and applications of mechanical human arms in robotics. Requirements
that are based on these information are defined and lead to the first model. This
model is tested, rapid-prototyped and evaluated. Weaknesses are shown and an
improved model is developed. Analyses of stresses and strains support the design
decisions.
The model is designed in such a way that it is possible to add in further investigations
components such as motors, pneumatic or hydraulic elements in order to allow the
model to be part of a humanoid robot.
"No human investigation can be termed true science if it is not capable of
mathematical demonstration. "
Leonardo da Vinci (1452 1519)
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II. Acknowledgements
Thanks to all those who have been involved in the investigations.
Thanks to Mr. Christopher Hart who helped me with the use of the Rapid-Prototyping
machines and other equipment in the CEMS labs.
Thanks to Neil Jones who helped me in producing some parts in the laboratory and
finding the right fasteners.
Thanks to Dr. Siamak Noroozi who explained how to use Visual Nastran 4D software.
Thanks to my friend Christian Abraham for his support related to this study.
Thanks to Mr. Rod Veazey whose CAD-course was very helpful to me in learning
how to use Solid Edge software.
Thanks to Mrs. Sue Scott for checking parts of the grammar.
Special thanks to my supervisor Dr. Gordon Smith for his help and guidance through
this study.
Additionally, I want to thank my brother Nicolas Schröder who gave me some good
technical advice for my work.
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III. Abbreviations, Terms, Symbols and Units
ABS
acrylonitrile
butadiene
styrene
Carpus
bones of the wrist
Glenohumeral
belonging to the shoulder joint
Humerus
the only bone of upper arm
Intercarpal
bones of the wrist towards the hand
Omoplate
bone of the shoulder (shoulder blade)
PVC
polyvinyl
chloride
Radius
the shorter bone of forearm
Ulna
the longer bone of forearm, lying at the side of the small fingers
Radiocarpal
bones of the wrist towards the forearm
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IV. Table of Figures
Page
Figure 1: assembly of mechanical human arm ...i
Figure 2: three-dimensional coordinate system ... 3
Figure 3: shoulder joint with omoplate and humerus ... 4
Figure 4: elbow joint with upper arm, radius and ulna bones... 6
Figure 5: radius and ulna ... 7
Figure 6: wrist joint with radius, ulna and digital bones... 8
Figure 7: prosthesis of arm of anonym wearer, 17
th
century ... 11
Figure 8: prosthesis of Götz von Berlichingen ... 12
Figure 9: "Sauerbruch-Arm" with bolt... 12
Figure 10: prosthesis of the elbow joint before and after operation ... 14
Figure 11: ball and socket joints of prostheses... 14
Figure 12: prostheses of the wrist joint ... 15
Figure 13: humanoid robot "Elektro" ... 17
Figure 14: humanoid robot "ASIMO"... 17
Figure 15: anthropomorphic muscle robot ZAR 4 ... 18
Figure 16: robotic arm of "ARMAR III", a robot built by students of Karlsruhe
University... 19
Figure 17: movements of joints of the arm (excluding finger joints)... 24
Figure 18: not yet finished shoulder joint of first version... 32
Figure 19: upper arm part and forearm part... 36
Figure 20: elbow joint (racked) ... 37
Figure 21: elbow joint (inflected)... 37
Figure 22: wrist joint ... 39
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Figure 23: redesigned CAD-model of shoulder joint (with fasteners)... 41
Figure 24: redesigned CAD-model of elbow joint (with fasteners) ... 42
Figure 25: redesigned CAD-model of wrist joint (with fasteners) ... 43
Figure 26: Fused Deposition Modelling machine during building process.
The head with the nozzles at the right side builds the part (white material) and
supports (brown material) ... 46
Figure 27: fastener of the first model that connects the elbow to forearm part to
the medial elbow part ... 47
Figure 28: all 70 fasteners of the physical model of the mechanical human arm
grouped by joints (shoulder: left bottom; elbow: right top; wrist: left top) ... 48
Figure 29: rapid-prototyped parts of the first model for assembly of shoulder joint... 49
Figure 30: first model of shoulder joint with screw contacting shoulder connection
part to omoplate, error marked with red rectangle ... 49
Figure 31: first model of shoulder joint with screw contacting shoulder to upper
arm part, error marked with red rectangle... 50
Figure 32: rapid-prototyped parts for assembly of first model of elbow joint ... 50
Figure 33: first model of elbow joint with screw contacting elbow to forearm part,
error marked with red rectangle... 51
Figure 34: rapid-prototyped parts for assembly of first model of wrist joint... 51
Figure 35: first model of wrist joint with screw contacting wrist to hand part, error
marked with red rectangle ... 51
Figure 36: all rapid-prototyped parts of the redesigned model (all joints) ... 52
Figure 37: rapid-prototyped parts of the redesigned model for assembly of
shoulder joint ... 52
Figure 38: rapid-prototyped parts of the redesigned model for assembly of elbow
joint... 53
Figure 39: rapid-prototyped parts of the redesigned model for assembly of wrist
joint... 53
Figure 40: forearm part... 67
Figure 41: upper arm part ... 67
Figure 42: central shoulder part of first model ... 68
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Figure 43: central shoulder part of redesigned model ... 68
Figure 44: shoulder angle part of first model ... 68
Figure 45: shoulder angle part of redesigned model ... 69
Figure 46: shoulder connection part to omoplate of first model ... 69
Figure 47: shoulder connection part to omoplate of redesigned model ... 69
Figure 48: medial shoulder ring part ... 70
Figure 49: shoulder to upper arm part ... 70
Figure 50: elbow to upper arm part... 71
Figure 51: medial elbow part of first model ... 71
Figure 52: medial elbow part of redesigned model ... 71
Figure 53: elbow to forearm part of first model ... 72
Figure 54: elbow to forearm part of redesigned model ... 72
Figure 55: wrist to forearm part... 73
Figure 56: wrist angle part ... 73
Figure 57: medial wrist ring part ... 73
Figure 58: wrist to hand part of first model... 74
Figure 59: wrist to hand part of redesigned model... 74
Figure 60: shoulder joint: 0° extension, 60° lateral rotation, medial shoulder ring
part and shoulder to upper arm contact shoulder angle part, side view... 75
Figure 61: shoulder joint: 0° extension, 60° lateral rotation, medial shoulder ring
part and shoulder to upper arm contact shoulder angle part, top view ... 75
Figure 62: shoulder joint: 0° extension, 70° medial rotation, medial shoulder ring
part and shoulder to upper arm contact shoulder angle part, side view... 75
Figure 63: shoulder joint: 0° extension, 70° medial rotation, medial shoulder ring
part and shoulder to upper arm contact shoulder angle part, top view ... 76
Figure 64: shoulder joint: 0° flexion/extension, 0° lateral/medial rotation, neutral
position, side view ... 76
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Figure 65: shoulder joint: 65° extension, 0° lateral/medial rotation, shoulder to
upper arm part and shoulder connection part to omoplate contact each other,
side view... 76
Figure 66: shoulder joint: 90° flexion, 60° lateral rotation, shoulder to upper arm
part and shoulder connection part to omoplate contact each other, medial
shoulder ring part and shoulder angle part contact each other, side view ... 77
Figure 67: shoulder joint: 90° flexion, 60° lateral rotation, shoulder to upper arm
part and shoulder connection part to omoplate contact each other, medial
shoulder ring part and shoulder angle part contact each other, top view... 77
Figure 68: shoulder joint: 90° flexion, 70° medial rotation, shoulder to upper arm
part and shoulder connection part to omoplate contact each other, medial
shoulder ring part and shoulder angle part contact each other, side view ... 77
Figure 69: shoulder joint: 90° flexion, 70° medial rotation, shoulder to upper arm
part and shoulder connection part to omoplate contact each other, medial
shoulder ring part and shoulder angle part contact each other, top view... 78
Figure 70: shoulder joint: 165° flexion, 0° lateral/medial rotation, shoulder to upper
arm part and shoulder connection part to omoplate contact each other, side view .. 78
Figure 71: elbow joint: 0° extension, 0° pronation/supination, neutral position,
elbow to upper arm part and medial elbow part contact each other, side view... 79
Figure 72: elbow joint: 0° extension, 0° pronation/supination, neutral position,
elbow to upper arm part and medial elbow part contact each other, top view ... 79
Figure 73: elbow joint: 0° extension, 95° pronation, elbow to upper arm part and
medial elbow part contact each other, elbow to forearm part and medial elbow
part contact each other, side view ... 79
Figure 74: elbow joint0° extension, 95° pronation, elbow to upper arm part and
medial elbow part contact each other, elbow to forearm part and medial elbow
part contact each other, top view... 80
Figure 75: elbow joint: 150° flexion, 0° pronation, elbow to upper arm part and
medial elbow part contact each other, side view ... 80
Figure 76: elbow joint: 150° flexion, 95° pronation, elbow to upper arm part and
medial elbow part contact each other, elbow to forearm part and medial elbow
part contact each other, side view ... 80
Figure 77: elbow joint: 150° flexion, 95° pronation, elbow to upper arm part and
medial elbow part contact each other, elbow to forearm part and medial elbow
part contact each other, top view... 81
Figure 78: wrist joint: 0° flexion/extension, 0° abduction/adduction, neutral
position, side view ... 82
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Figure 79: wrist joint: 0° flexion/extension, 0° abduction/adduction, neutral
position, top view ... 82
Figure 80: wrist joint: 0° flexion/extension, 20° abduction, medial wrist ring part
and wrist angle parts contact each other, side view ... 82
Figure 81: wrist joint: 0° flexion/extension, 20° abduction, medial wrist ring part
and wrist angle parts contact each other, top view ... 83
Figure 82: wrist joint: 0° flexion/extension, 36° adduction, medial wrist ring part
and wrist angle parts contact each other, side view ... 83
Figure 83: wrist joint: 0° flexion/extension, 36° adduction, medial wrist ring part
and wrist angle parts contact each other, top view ... 83
Figure 84: wrist joint: 73° extension, 0° abduction/adduction, wrist to hand part
and medial wrist ring part contact each other, side view ... 84
Figure 85: wrist joint: 73° extension, 0° abduction/adduction, wrist to hand part
and medial wrist ring part contact each other, top view ... 84
Figure 86: wrist joint: 75° flexion, 0° abduction/adduction, wrist to hand part and
medial wrist ring part contact each other, bottom view ... 84
Figure 87: wrist joint: 75° flexion, 0° abduction/adduction, wrist to hand part and
medial wrist ring part contact each other, side view ... 85
Figure 88: central shoulder part, both shoulder angle parts and shoulder
connection part to omoplate; force (50N, z-direction) applying to the surfaces of
the shoulder angle parts that limit lateral rotation ... 86
Figure 89: elbow to forearm part; force (100N, y-direction) applying to the surface
that limits supination/pronation ... 86
Figure 90: elbow to upper arm part; force (100N, -y-direction) applying to the
surfaces that limit flexion ... 87
Figure 91: medial elbow part; force (100N, y-direction) applying to the surface that
limits supination/pronation ... 87
Figure 92: medial shoulder ring part; force (100N, -x-direction) applying to the
surfaces that limit lateral/medial rotation... 88
Figure 93: medial wrist ring part; force (25N, y-direction) applying to the surfaces
that limit flexion... 88
Figure 94: shoulder connection part to omoplate; force (300N, -y-direction)
applying to the surfaces that limit flexion ... 89
Figure 95: shoulder to upper arm part; force (100N, x-direction) applying to the
surfaces that limit flexion/extension ... 89
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Figure 96: wrist to fore arm part and both wrist angle parts; force (100N,
-y-direction) applying to the surfaces that limit adduction ... 90
Figure 97: wrist to hand part; force (100N, -z-direction) applying to surfaces that
limit flexion/extension ... 90
Figure 98: shoulder joint with central shoulder part and medial shoulder ring part.
The central shoulder part already has an angled surface for limiting flexion and
extension ... 93
Figure 99: central shoulder part with lugs in order to limit flexion, extension,
abduction and adduction all in one. This solution is not feasible ... 93
Figure 100: central shoulder part close to the final chosen design. The large lug
should limit flexion, but this solution is also not feasible ... 93
Figure 101: shoulder joint with central shoulder part, medial shoulder ring part
and shoulder to upper arm part ... 94
Figure 102: elbow joint with all three main parts (inflected) ... 94
Figure 103: medial wrist ring part in early design stage... 94
Figure 104: Gantt Chart of Individual Project...106
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V. Table of Charts
Page
Chart 1: movements of the joints of the arm and their explanations ... 9
Chart 2: ranges of motion of all joints of the human arm with minimum and
maximum values... 9
Chart 3: lengths, circumferences and diameter of the human arm (50 percentile) ... 10
Chart 4: design requirements in order of importance... 20
Chart 5: decision matrix between ball and socket joint and cardan joint... 23
Chart 6: range of motion of the mechanical human arm, based on Chart 2
(Chapter 2.1.6.) ... 25
Chart 7: parts list with main parts, their file names, and joints where they belong to 30
Chart 8: assembly files with descriptions ... 30
Chart 9: parts that limit the range of motion... 31
Chart 10: calculation of distances and lengths of upper arm part and forearm part.. 36
Chart 11: available Rapid-Prototyping methods at UWE with their advantages and
disadvantages ... 45
Chart 12: parts of the physical model that are produced with Rapid-Prototyping ... 46
Chart 13: fasteners of CAD-model... 47
Chart 14: parts list ... 92
Chart 15: series of tasks of Individual Project. The bold and italic events are
accumulative events. ...105
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VI. Table of Contents
Page
Heading Sheet...i
I. Summary ...ii
II. Acknowledgements ... iii
III. Abbreviations, Terms, Symbols and Units ...iv
IV. Table of Figures ...v
V. Table of Charts... xi
V. Table of Charts... xi
VI. Table of Contents... xii
1. Introduction... 1
Aims and objectives ... 1
Managing the Individual Project ... 1
2. Literature Review... 2
2.1. The human arm... 2
2.1.1. General overview... 2
2.1.2. Shoulder joint... 4
2.1.3. Elbow joint ... 6
2.1.4. Radioulnar joint... 7
2.1.5. Wrist joint... 8
2.1.6. Range of motion ... 9
2.1.7. Size, weight, and force ... 10
2.2. History of prostheses ... 11
2.2.1. General overview... 11
2.2.2. Prostheses of the arm ... 12
2.3. Joints in prostheses ... 14
2.4. Robotic human arms ... 16
2.4.1. History of robots ... 16
2.4.2. Humanoid robots ... 17
2.4.3. Application of robotic human arms ... 18
3. Requirements for the mechanical human arm ... 20
3.1. General requirements ... 20
3.2. Degrees of freedom ... 24
3.3. Range of motion... 25
3.4. Size, weight and force... 27
3.5. Materials ... 28
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4. Model of mechanical human arm... 29
4.1. First CAD-model... 29
4.1.1. Parts list and assembly... 30
4.1.2. Angled parts ... 31
4.1.3. Description of parts... 32
4.2. Redesign of CAD-model ... 41
4.3. Analysis of stresses and strains... 44
4.4. Prototype of physical model ... 45
4.4.1. Chosen material ... 45
4.4.2. Fastener system ... 47
4.4.3. Assembly of the first model ... 49
4.4.4. Assembly of the redesigned model... 52
4.5. Materials and serial production ... 54
5. Conclusions ... 55
6. Recommendations for further work... 58
Bibliography... 59
References ... 63
Appendix... 66
Appendix 1 - Parts of CAD-model and physical model ... 67
General parts... 67
Shoulder joint ... 68
Elbow joint ... 71
Wrist joint... 73
Appendix 2 - Test of first CAD-model... 75
Angles of shoulder joint ... 75
Angles of elbow joint... 79
Angles of wrist joint... 82
Appendix 3 - Images of Finite Element Analysis with ALGOR ... 86
Appendix 4 - Drawings of CAD-model... 91
Appendix 5 - Parts list ... 92
Appendix 6 - History of developing the CAD-model ... 93
Appendix 7 - Reports of meetings... 95
1
st
meeting... 95
2
nd
meeting ... 96
3
rd
meeting ... 97
4
th
meeting... 98
5
th
meeting... 99
6
th
meeting...100
7
th
meeting...101
8
th
meeting...102
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9
th
meeting...103
10
th
meeting...104
Appendix 8 - Project Plan for Individual Project ...105
1. Outline of my study and the research questions I am trying to answer...105
2. Series of tasks of study ...105
3. Gantt Chart...106
4. Main risks in the programme, how I mitigate them and my reordering of
sequence...106
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1. Introduction
The idea of replacing deformed or mutilated parts of the body has its origin since birth
of mankind. Since the mediaeval times, it has been possible to manufacture flexible
prostheses of extremities such as arms or legs. The early prostheses were made out
of wooden parts, mostly one single wooden part. The first known prostheses were by
the Egyptians in 2100 B. C. Since mediaeval times they were also produced out of
iron parts. The wooden parts were still common, but there were also prostheses
made out of iron and wood.
The development of modern prostheses as they are known today has its beginning in
the First and Second World Wars. Surgeons such as Ferdinand Sauerbruch or
Konrad Biesalski invented prostheses that allowed the wearer also more complex
range of motions. These prostheses allowed the wearer simple movements within the
integrated joints. Nowadays prostheses are assembled out of modern materials.
They often include microprocessors paired with tactile sensors, surfaces that imitate
the look and feeling of real skin and they provide almost all the functionality of real
human limbs.
Additionally, there are many applications of mechanical human arms in robotics.
Aims and objectives
The topic of this Individual Project is "design, model, prototype, test, analyse and
evaluate a mechanical human arm (shoulder to wrist)". The objectives are:
· to create a mechanical human arm which is as realistic in behaviour, capacity
and functionality as a real human arm. It will be created as a CAD-model and
as a physical model,
· to produce models of the most complex CAD-parts with Rapid-Prototyping,
· to analyse the stresses and strains of the model.
Managing the Individual Project
Information about how the Individual Project was managed can be found in the
Appendix (Chapters "Appendix 7 - Reports of meetings" and "Appendix 8 - Project
Plan for Individual Project").
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2. Literature Review
2.1. The human arm
2.1.1. General overview
The term "arm" is an anatomical term. The arm is also called the upper limb. The
human arm consists of three main parts, the upper arm between shoulder and elbow,
the forearm between elbow and wrist, and the hand. The term arm is often used to
describe the whole upper limb in colloquial speech (Wikipedia, 2006a), but the hand
is not part of it.
In evolutionary terms, it is a further development of the forefoot of animals to a
gripping tool for humans, based on the same vertebrate ancestor (O'Neil, 2007).
Besides this, it is responsible for balancing the centre of mass of the upright walk with
its commuting movements.
There are in fact four joints which are part of the human arm (Moeslund and Granum,
2001): the shoulder joint; the elbow joint; the wrist joint; and the radioulnar joint. They
are all different kinds of joints which have different degrees of freedom.
Besides the bones, the arm consists mainly of sinews, nerves, muscles, blood
vessels and veins. (Hillman et al, 2000).
Movement is possible in a maximum of three dimensions, depending on the anatomy
of the joint. The range of movement of human joints is based on a model with a
neutral position. The neutral position of a human arm is as follows:
· Shoulder: The upper arm hangs directly downwards with an angle of 0° to
the trunk. The inner side of the elbow is turned to the trunk.
· Elbow:
Upper arm and forearm form an angle of 180°.
· Wrist:
The extension of the hand and the forearm form an angle of
180°. The inner surface of the hand is turned to the trunk.
The whole arm has seven degrees of freedom
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(Kyoto University, 2004) and can
change its ambient position via five parts of the body: the shoulder girdle, the upper
arm, the forearm, the hand and the fingers. For this report, the movements of the
shoulder girdle, the hand (except the wrist joint) and the fingers are not considered.
The human arm from shoulder to wrist contains 3 bones, the humerus of the upper
arm, and the radius and the ulna of the forearm (The Columbia Encyclopedia, Sixth
Edition, 2006). As said on the website of Norman (1999), the adjoining hand contains
27 bones; the carpus contains 8 of them which make up the wrist.
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Finger joints and shoulder girdle are not included in this figure.
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When the omoplate and the 8 bones of the wrist are added, there are 12 bones
which form the arm.
There are three planes that describe the kinematics of the human body (Lenarcic and
Umek, 1994). The horizontal x-axis goes through both shoulder joints, and the y-axis
is the vertical axis. The z-axis forms together with the x-axis a horizontal plane. The
three-dimensional coordinate system in Figure 2 is used for all descriptions within this
report. There are alternative technical terms for the planes:
· x-y-plane:
frontal
plane (showed in red)
· x-z-plane: transverse
plane (showed in blue, also called horizontal plane)
· y-z-plane: sagittal plane (showed in green)
Figure 2: three-dimensional coordinate system
(source: self-made)
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2.1.2. Shoulder joint
The shoulder joint as shown in Figure 3 is a ball and socket joint. It is also called the
glenohumeral joint (Smith et al, 2004). The upper arm has a ball attached to one end,
which has no perfect round shape. There is one single bone that forms the upper
arm, the humerus, and it ends in a hinged joint, the elbow. Beside the thighbone it is
one of the strongest bones of the human skeleton.
Figure 3: shoulder joint with omoplate and humerus
(source: http://www.orthopaedie-gewerbepark.de/pictures/schulter.jpg)
The shoulder joint has the largest range of motion of the joints of the upper limb
(Carlson, 2003) and of all socket and ball joints of the human body. The shoulder
blade is one single bone which contains the socket, and the bone of the upper arm is
ball shaped at the shoulder end. The ball is almost hemispherical in shape. This is
the basis for movements in each direction. The ball and socket touch each other in
any combination just at 1/3 of the area of the ball (Soames, 2003). The shoulder
allows a movement around 3 axes.
The movements of the upper arm in the y-z-plane are called flexion and extension.
Flexion is the movement towards the front of the body whereas extension describes
moving the arm away from behind the body. The range of flexion is 165°, the range
of extension 65°, in total 230°. Although there is not the full 360°, it is possible to
circle the arms 360°, because the arms do not circulate in a round circle lying on one
plane around the axis, it is more like an oval ellipse.
Raising the upper arm sideways in the x-y-plane is called abduction with a range of
motion of 90°. To drop it again is called adduction, which has a range of motion of
30°
2
. Both are 120° in total.
The movements in the x-z-plane are called lateral and medial rotation. Medial rotation
means moving towards the chest, in this direction at an angle of 70°. Lateral rotation
is the opposite and allows a range of motion of 60°.
2
This figure includes the range of motion of the shoulder girdle.
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All ranges of motion of the shoulder are larger if the support of the shoulder girdle is
also counted as defined by Berg (1999). That is the reason why the arm can be
laterally raised up to 180° whereas the movement within the shoulder joint counts just
90° of this movement. Without the supporting movements of the shoulder girdle, the
upper arm loses much of its range of motion.
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2.1.3. Elbow joint
At the other end of the upper arm is the elbow joint. The elbow joint is often regarded
as a joint which allows movements around two axes, but this assumption is incorrect.
Only in combination with the radioulnar joint more complex movements are possible
(Gray, 2005). The elbow joint itself is a hinge joint with possible movement around
one axis. At the other side of the elbow joint, there is the forearm. The forearm
consists of two bones, the radius and the ulna which are both connected to the elbow
joint. Therefore, the elbow joint has two distinct articulations as Figure 4 shows. They
are called humeroulnar and humeroradial joints (Soames, 2003).
Figure 4: elbow joint with upper arm, radius and ulna bones
(source: http://www.orthopaedie-gewerbepark.de/pictures/ellbogen.jpg)
The elbow of a female person allows a total range of movements of 160°, divided into
150° for flexion and 10° for extension. Corney (1971) shows that 85% of females are
able to reach an angle of extension, but males normally can not. Both movements
take place in the y-z-plane. When the elbow is elongated in its neutral position, the
angle between upper arm and forearm is 180°.
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2.1.4. Radioulnar joint
The two bones in the forearm (Figure 5), the radius and ulna, form another joint in
the arm, the radioulnar joint. The radius rotates around the ulna, and both bones end
in the wrist. The ulna is the longer and thicker of the two bones (Wikipedia, 2006b); it
lies at the side of the small fingers.
Figure 5: radius and ulna
(source:
http://www.uwyo.edu/RealLearning/injuries/pictures/HR188%20Bent%20Ulna/dscn3855.JPG)
The bones of the forearm are not in contact with each other at all. They are both
connected to the wrist and elbow joints. Because of this, a radial movement around
the y-axis is possible. But this movement is not part of the elbow joint itself. It can be
regarded as a fourth joint which exists because of the unique arrangement of the
bones between elbow and wrist. It provides the arm with another axis of movement
and is important for flexible positioning of the hand and its work carried out (Elbow
Pain Info, 2007).
There are two joints which allow the movement of radius and ulna, the superior
radioulnar joint, located towards the elbow joint, and the inferior radioulnar joint
located towards the wrist joint. Radius and ulna each have two joints. Although they
are directly positioned in the elbow joint in a similar way to the wrist joint, they are not
the same as the elbow joint or the wrist joint.
The radioulnar joint has a range of movement of 95° in each direction, or a total
movement of 190°. The movements are called pronation and supination. In
supination, the bones cross each other whereas they are parallel to each other in
pronation.
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2.1.5. Wrist joint
The wrist joint consists of a double row of four small short bones each (Marshall,
2007) as shown in Figure 6. The bones which are next to the forearm form the
radiocarpal joint. The second row of bones next to the bones of the fingers forms the
intercarpal joints. The intercarpal joints are not considered any further in the
investigations of this report, because they are not needed for the functionality of the
wrist joint leading to the forearm.
Figure 6: wrist joint with radius, ulna and digital bones
(source: http://www.ruhr-uni-bochum.de/radiologie-josefhospital/download/3d_handgelenk.jpg)
The radiocarpal joint of the wrist joint is an ellipsoid synovial joint (Soames, 2003). It
allows two degrees of freedom, but it has limits in comparison to the socket and ball
joint of the shoulder joint because of the oval shape of the ball. Turning movements
around the elongated axis of the forearm are not possible. But because of the ability
to turn the forearm about 95° in the radioulnar joint, the limit of the wrist joint is
nullified.
The wrist allows movements in two planes. The movements of the hand up and down
are called extension (up) and flexion (down) and take place in the y-z-plane. The
second pair of movements, moving the hand to the left and to the right, is called
abduction (away from the body) and adduction (towards the body). In the model of
neutral positions, these two movements take place in the x-y-plane.
The range of motion for flexion and extension decreases with age in the wrist joint
more than in other joints. For a 20-year-old person the flexion is 75° and the
extension 73°, in total 148°. Abduction is 20° whereas adduction is 36°, in total 56°.
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Mechanical Human Arm
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2.1.6. Range of motion
Gender and age are the two main reasons for different ranges of motion between
individual humans (Bell and Hoshizaki, 1981). Additionally, heredity and individual
circumstance such as fitness also influence the ranges of motion as cited by Vlach,
2007.
Generally, female persons are more agile than male ones, but there is only a little
difference and in some joints there is no relevant difference. The older the person,
the less agility there is and the movement is more and more confined. In neonates
the range of motion is the largest and it decreases throughout life. Children lose the
largest relative percentage of their range of motion during their first five years of life.
Other reasons for differences in the range of motions have genetic causes. Chart 1
shows the terms of the different possible movements of the human arm from
shoulder joint to wrist joint. These movements limit the ranges of motion.
Movement Explanation
flexion
deflecting a joint, raising forward
extension
stretching a joint
abduction
strutting a limb away from the body, to raise to side
adduction
pulling up a limb towards the body or axis of the limb
lateral rotation
turning a limb away from the body
medial rotation
turning a limb towards the body
pronation
turning towards the centre, rotation to a palm-up position
supination
turning away from the centre, rotation to a palm-down position
Chart 1: movements of the joints of the arm and their explanations
(source: self-made)
Chart 2 shows an overview of the minimum and maximum values of the range of
motion. The maximum values are those of neonates and the minimum values are
those of 60 to 70-year-old persons. Measurements have been taken from both
female and male persons. These values are of average nature and exceptions are
possible.
Shoulder joint
Elbow Joint
Radioulnar
joint
Wrist joint
Flexion
150° - 180°
139° - 158°
50° - 96°
Extension
30° - 89°
6° - 10°
40° - 89°
Abduction
90°
3
20° - 40°
Adduction
0° - 30°
28° - 40°
lateral rotation
60° - 134°
medial rotation
65° - 90°
Pronation
68° - 90°
Supination
80° - 90°
Chart 2: ranges of motion of all joints of the human arm with minimum and maximum values
(source: self-made)
3
No further values found in literature. Nevertheless, this range of motion also decreases at higher age
as all others do.
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Mechanical Human Arm
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10
2.1.7. Size, weight, and force
Size, weight and force of the human arm are also dependent on gender, age,
heredity and individual circumstances.
The science of measuring lengths, circumferences, diameters and distances of the
human body and other measurements e. g. volume or mass is called anthropometry
(NASA, 2006). Because of the varying individual lengths and distances, there is a
system necessary that groups average lengths together. Due to this, the percentile is
an important term. The measurements made on a preferably large group of humans
are divided with the help of the percentile. There are three main figures related to the
percentile:
· the 5 percentile (P5)
· the 50 percentile (P50)
· the 95 percentile (P95)
When it is necessary to ensure that 90% of the population are in a range of
measurements, the difference between the 95 percentile and the 5 percentile is
taken. The 50 percentile shows the average value. In Chart 3, the basic values of the
human arm are shown for the 50 percentile.
Measured items
percentile value (mm) value (inch)
distance from shoulder joint to elbow joint
P50
282
11.1
distance from elbow joint to wrist joint
P50
251
9.9
diameter of upper arm
P50
99
3.9
circumference of upper arm
P50
311
12.25
diameter of forearm
P50
94
3.7
circumference of forearm
P50
295
11.62
diameter of shoulder joint
P50
102
4
distance from wrist joint to fingertips
P50
191
7.5
Chart 3: lengths, circumferences and diameter of the human arm (50 percentile)
(source: Dreyfuss Associates, 2002)
Both arms have a weight of 10% to 12% of the total weight of the human body as
defined by Croney (1971b). This means for a person that weighs 80 kg that one arm
weighs from 4 kg to 4.8 kg. The percentage allotment of the bones on the total
weight of the human body is about 12% to 14%. This results in a weight of 0.48 kg to
0.672 kg for the bones of one arm
4
. These figures are important for the choice of
materials for the mechanical human arm.
Force is not as easy to determine as weight or size. It is heavily dependent on the
individual fitness. Because of these huge biological differences, the force that a
human arm can produce is difficult to ascertain. It is not further covered in this
chapter, but later discussed in the requirements.
4
Calculation of boundaries: 4 kg * 0.12 = 0.48 kg; 4.8 kg * 0.14 = 0.672 kg
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