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UNIT 3 – BIOMECHANICS OF THE UPPER LIMB AND SPINE
Dr M J Dolan
Department of Orthopaedic & Trauma Surgery
University of Dundee
Dr T Drew
Department of Orthopaedic & Trauma Surgery
University of Dundee
SECOND EDITION
Edited by
Dr T Drew
Department of Orthopaedic & Trauma Surgery
University of Dundee
Illustrations by
Mr I Christie
Published by
Distance Learning Section
Department of Orthopaedic & Trauma Surgery
University of Dundee
Second Edition published 2005: ISBN 1-903562-48-1
ISBN 978-1-903562-48-2
First Edition published 1994: ISBN 1-899476-51-2
Copyright © 2005 University of Dundee. All rights reserved. No part of this publication may be
reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic,
mechanical, photocopying, and recording or otherwise, without prior permission from the publisher.
The University of Dundee is a Scottish Registered Charity, No SC01509.
UNIT 3 - BIOMECHANICS OF THE UPPER LIMB AND SPINE
CONTENTS
1. THE SHOULDER
1.1 Glenohumeral Joint
1.2 Acromioclavicular Joint
1.3 Sternoclavicular Joint
1.4 Scapulothoracic Articulation
1.5 Range of Motion
1.6 Dislocations of the Shoulder
2. THE ELBOW
2.1 Flexion and Extension
2.2 Pronation and Supination
2.3 Range of Motion During Daily Activities
2.4 Stability
2.5 Joint Forces at the Elbow
3. THE WRIST
3.1 Articulations
3.2 Motion
4. THE HAND
4.1 Articulations
4.2 Motion and Range of Motion of the Fingers
4.3 Motion and Range of Motion of the Thumb
4.4 Interaction of the Hand and Wrist Motion
5. THE SPINE
5.1 Vertebrae
5.2 Intervertebral Discs
5.3 The Cervical Spine
5.4 The Thoracic Spine
5.5 The Lumbar Spine
5.6 The Sacrum and Coccyx
5.7 Range of Motion
5.8 Loadings on the Spine
SUMMARY
SAQ ANSWERS
END OF UNIT EXERCISE
UNIT 3 - BIOMECHANICS OF THE UPPER LIMB AND SPINE
OBJECTIVES
On completing your study of this unit you should be able to:
1. Describe the structure of the major upper limb joints.
2. Name the four shoulder joint articulations.
3. Explain the function of the rotator cuff.
4. Describe the motion and range of motion of the shoulder joint.
5. Explain how an anterior dislocation of the glenohumeral articulation may occur.
6. Name the three elbow joint articulations.
7. Describe the motion and range of motion of the elbow joint.
8. Describe the function of the annular ligament.
9. Discuss the joint forces at the elbow.
10. Explain the function of the pisiform bone.
11. Describe the motion and range of motion of the wrist.
12. List the principal joints of the hand.
13. Describe the motion of the carpometacarpal joint of the thumb.
14. Describe the motion and the range of motion of the fingers and thumb.
15. Discuss the importance of the motion of the thumb.
16. Discuss the interaction of the motion of the hand and wrist.
17. Describe the anatomy and function of the vertebrae and the interarticular discs.
18. Discuss the differences and similarities between the anatomy and function of
different regions of the spine.
19. Describe and discuss the motion and range of motion of the spine.
20. Explain how the loadings on the spine vary with the positioning of the trunk.
UNIT 3 - BIOMECHANICS OF THE UPPER LIMB & SPINE
INTRODUCTION
In this unit we will be looking at the major joints of the upper limb and the spine. In
particular, we will be examining the structure and function of the joints and their
stability, range of motion and loadings.
The upper limb consists of five parts - the shoulder girdle, the arm, the forearm, the
wrist and the hand (Figure 1). The shoulder girdle is formed by the clavicle and scapula,
the arm (upper arm) by the humerus, the forearm by the ulna and radius, the wrist by the
eight carpal bones and the hand by the metacarpals and phalanges. Three major joints
give the upper limb its wide range of motion. They are the shoulder, elbow and wrist
joints. The major function of the upper limb is to position the hand in space and it is
therefore designed to achieve a wide range of movement rather than for weight bearing.
shoulder
girdle
arm
spine
(vertebral column)
forearm
wrist
hand
FIGURE 1. THE UPPER LIMB AND SPINE.
The spine, or vertebral column, is made up of twenty-four vertebrae and the sacrum and
the coccyx. Its segmented structure gives it a remarkable amount of flexibility as well as
the strength required to support the upper body.
1. THE SHOULDER
The shoulder joint is the most intricate joint complex in the human body. It is formed
by the humerus, the shoulder girdle and the thorax and contains four distinct
articulations. Three of these articulations, the glenohumeral, the acromioclavicular
and the sternoclavicular, are synovial while the fourth, the scapulothoracic, is a boneon-muscle-on-bone articulation (Figure 2).
Unit 3 - Biomechanics of the Upper Limb & Spine 1
clavicle
acromioclavicular
articulation
acromion process
sternum
scapula
glenohumeral
articulation
sternoclavicular
articulation
humerus
scapulothoracic
articulation
FIGURE 2. THE SHOULDER JOINT.
1.1 Glenohumeral Joint
The glenohumeral joint is a ball-and-socket shaped synovial joint formed by the
humeral head and the glenoid fossa of the scapula (Figure 3). The glenoid fossa is
particularly shallow allowing for a wide range of motion. However, this means that the
articulation is less stable than it would be with a more developed socket. Dislocations of
the glenohumeral articulation are therefore not infrequent. To assist stability it has a
thick cartilaginous rim called the glenoid labrum (Figures 3 and 5). The articulation is
surrounded by a capsule and, more importantly, by the rotator cuff.
synovial cavity
humeral head
articular
cartilage
glenoid labrum
humerus
FIGURE 3. THE GLENOHUMERAL JOINT.
The rotator cuff is formed by a group of four muscles and their tendons: subscapularis,
infraspinatus, supraspinatus and teres minor (Figure 4A). They form a cuff of tissue,
like the fingers of a hand cupping a ball, around the glenohumeral articulation. They
provide dynamic restraints to anterior, posterior and inferior displacement. This is
illustrated in Figure 4(B). The rotator cuff pushes on the humeral head, preventing any
anterior-posterior movement, thereby stabilising the joint.
Unit 3 - Biomechanics of the Upper Limb & Spine 2
GLENOHUMERAL JOINT
HUMERAL HEAD
GLENOID FOSSA
GLENOID LABRUM
supraspinatus
posterior
(B)
(A)
subscapularis
humeral
head
infraspinatus
anterior
teres minor
FIGURE 4. (A) LATERAL VIEW OF THE LEFT SCAPULA SHOWING THE MUSCLES THAT FORM THE ROTATOR CUFF (B)
SUPERIOR VIEW OF THE LEFT GLENOHUMERAL JOINT SHOWING HOW THE ROTATOR CUFF MUSCLES STABILISE THE
JOINT.
SAQ 1
(a) List the four articulations of the shoulder joint.
(b) Name the shallow depression in which the humeral head rests.
1.2 Acromioclavicular Joint
The acromioclavicular joint is a small synovial joint formed by the proximal acromion
of the scapula and the distal clavicle. It is stabilised by superior and inferior acromioclavicular ligaments which prevent the joint being pulled apart. Further stability is
provided by the two parts of the coracoclavicular ligament between the clavicle and
coracoid process of the scapula which limit the upward movement of the clavicle. The
range of motion of the joint is restricted by the thorax and the muscle attachments, being
limited to a few degrees during arm abduction.
clavicle
coracoclavicular
ligament
acromioclavicular joint
coracoacromial
ligament
coracoid
process
glenoid fossa
ACROMIOCLAVICULAR
JOINT
acromion
process
glenoid labrum
capsule
FIGURE 5. THE ACROMIOCLAVICULAR JOINT.
1.3 Sternoclavicular Joint
The sternoclavicular joint is small synovial joint between the manubrium of the
sternum and the proximal clavicle (Figure 6A). It is the only bony joint connecting the
shoulder girdle to the trunk. During arm elevation the clavicle also elevates at the
sternoclavicular joint. For the first 90 degrees of arm elevation the clavicle elevates by
around 4 degrees for every 10 degrees of arm elevation. Beyond 90 degrees of arm
Unit 3 - Biomechanics of the Upper Limb & Spine 3
STERNOCLAVICULAR
JOINT
elevation the elevation of the clavicle is almost negligible. During elevation and
depression the clavicle rotates about an axis determined by the attachment of the
costoclavicular ligament (Figure 6B).
anterior sternoclavicular
ligament
interclavicular
ligament
clavicle
(A)
first rib
manubrium
costoclavicular
ligament
articular disc
motion of
clavicle
articular
disc
manubrium
(B)
centre of
rotation
sternum
body
FIGURE 6. (A) THE LIGAMENTS OF THE STERNOCLAVICULAR JOINT (B) MOTION OF THE CLAVICLE DURING ELEVATION
AND DEPRESSION.
Activity: Place your left hand flat on your chest so that your index finger is aligned
along your right clavicle. Start with your right arm by your side and slowly raise your
arm, keeping your index finger fixed. Note the motion of the clavicle.
1.4 Scapulothoracic Articulation
The scapulothoracic articulation is the bone-muscle-bone articulation between the
scapula and the posterior thoracic wall. It is not a joint in the truest sense since there are
no direct bony or ligamentous connections between the scapula and thorax. However, it
contributes significantly to the wide range of motion of the scapula, which greatly
enhances the mobility of the entire shoulder complex.
The broad anterior surface of the scapula is separated from the posterior thorax by two
broad flat muscles: the serratus anterior and subscapularis muscles. The serratus
anterior originates on the upper eight or nine ribs and inserts on the anterior surface of
the scapula along its vertebral border. It helps to hold the scapula against the thorax,
thus preventing “winging”, and is a strong abductor that is useful in pulling or pushing
movements. The subscapularis originates from the subscapular fossa and inserts on the
lesser tubercle of the humerus. It is one of the rotator cuff muscles and acts to medially
rotate the humerus.
SAQ 2
(a) Name the ligament about whose attachment the clavicle rotates
during elevation and depression.
(b) Why is the scapulothoracic articulation not a joint in the truest
sense?
Unit 3 - Biomechanics of the Upper Limb & Spine 4
SCAPULOTHORACIC
ARTICULATION
SERRATUS ANTERIOR
SUBSCAPULARIS
1.5 Range of Motion
All four articulations of the shoulder joint act together to provide the wide range of
motion commonly found in the shoulder.
Shoulder elevation is the term used to describe the movement of the humerus away
from the side of the thorax in any plane. Similarly shoulder depression is the term used
to describe the movement of the humerus towards the side of the thorax (Figure 7A).
(A)
SHOULDER ELEVATION
SHOULDER
DEPRESSION
(B)
elevation
depression
angle of
elevation
FIGURE 7. SHOULDER ELEVATION AND DEPRESSION.
The amount of elevation is quantified as the angle of elevation. This is the angle
between the axis passing through the shoulder joint centre parallel to the longitudinal
axis of the trunk and the longitudinal axis of the humerus (Figure 7B). With a subject in
the anatomical position the longitudinal axis of the thorax will correspond to the
vertical. For example, for a subject in the anatomical position, with their arm against the
side of their trunk, the shoulder elevation angle will be zero degrees. With their arm
held out so that it is horizontal, the shoulder elevation angle will be 90 degrees. When
the arm is moved towards the trunk from an elevated position it is described as shoulder
depression.
In the sagittal plane, shoulder elevation is termed forward flexion when the arm
moves forward and backward extension or backward elevation when the arm moves
backwards (Figure 8).
(A)
FORWARD FLEXION
BACKWARD EXTENSION
(B)
FIGURE 8. (A) FORWARD FLEXION (B) BACKWARD EXTENSION.
In the coronal (frontal) plane, shoulder elevation is called abduction when the arm
moves away from the trunk and adduction as it moves towards the trunk. True
adduction in the coronal plane is limited by the trunk. However, adduction can be
Unit 3 - Biomechanics of the Upper Limb & Spine 5
ABDUCTION
ADDUCTION
achieved with slight forward flexion to adduct the arm in front of the trunk or with
slight backward extension to adduct the arm behind the trunk (Figure 9).
(A)
abduction
(B)
adduction
FIGURE 9. (A) ABDUCTION (B) ADDUCTION.
The normal range of forward flexion and abduction is about 180 degrees. The normal
range of backward extension is about 60 degrees. With the arm moving in front of the
trunk the range of adduction possible is around 75 degrees. Note that all these ranges
decrease with age, especially in the physically inactive.
Note: Some authors use the terms elevation and depression interchangeably with
abduction and adduction.
Another functionally important motion is the rotation about the longitudinal axis of the
humerus. This rotation is described as either internal or external rotation. The
definitions of these are most easily described using typical examples. Consider a subject
standing in the position shown in Figure 10A with their elbow flexed at 90°. Internal
rotation moves the forearm closer to the trunk and external rotation moves the forearm
away from the trunk. Now consider a subject with their arm elevated as shown in Figure
10B. Internal rotation moves the hand downwards and external rotation moves the hand
upwards. The range of internal and external rotation varies with the amount of shoulder
elevation, but in general each may be realised to about 90 degrees, giving a total range
of 180 degrees.
(A)
internal rotation
external rotation
(B)
external rotation
internal rotation
FIGURE 10. INTERNAL - EXTERNAL ROTATION (A) WITH UPPER ARM AT SIDE OF BODY (B) WITH UPPER ARM ABDUCTED
90 DEGREES.
Unit 3 - Biomechanics of the Upper Limb & Spine 6
INTERNAL/EXTERNAL
ROTATION
Motion in the transverse (horizontal) plane is termed horizontal flexion and horizontal
extension. Horizontal flexion is the forward motion of the arm (Figure 11A) and
horizontal extension is the backward motion of the arm (Figure 11B). Starting from a
position of 90 degrees of abduction (that is with the upper arm parallel with the ground)
the normal range of horizontal flexion and extension is about 135 degrees and 45
degrees respectively.
(A)
HORIZONTAL FLEXION
HORIZONTAL
EXTENSION
(B)
horizontal extension
horizontal
flexion
FIGURE 11. (A) HORIZONTAL FLEXION (B) HORIZONTAL EXTENSION.
SAQ 3 - Complete the following table of average ranges of shoulder
joint motion.
Forward flexion-backward extension
Abduction-adduction
Forward flexion
180°
Backward extension
60°
Range
240°
Abduction
Adduction
Range
Internal-external rotation
Internal rotation
External rotation
Range
Horizontal flexion-extension
Horizontal flexion
Horizontal extension
Range
Activity: Try for yourself the various motions of the shoulder joint described and
estimate your own range of motion. How do these compare to the average ranges in
the table above?
1.6 Dislocations of the Shoulder
All three of the synovial articulations of the shoulder joint are prone to dislocation. The
most common dislocation is the anterior dislocation of the glenohumeral
articulation. This is when the head of the humerus slips forward off the shallow
glenoid fossa. Generally, this may occur if the arm suffers a heavy blow when the
shoulder is abducted and extended horizontally. In this position the arm pivots about the
acromion and the ligaments and muscles act to prevent the humeral head slipping
Unit 3 - Biomechanics of the Upper Limb & Spine 7
ANTERIOR
DISLOCATION
GLENOHUMERAL
ARTICULATION
forward. If the blow is too heavy or the ligamental muscles too weak then a dislocation
occurs. In this way the arm and shoulder are functioning as a first class lever in the same
manner as a crowbar (Figure 12).
force
fulcrum at
the acromion
resistance force
FIGURE 12. ANTERIOR DISLOCATION OF THE GLENOHUMERAL ARTICULATION MAY OCCUR IF A FORCE IS APPLIED TO
THE ARM IN THE POSITION SHOWN.
With the arm fully extended the effort force is working a very large mechanical
advantage over the resistance force enabling even a comparatively small external force
to cause a dislocation. This can of course be likened to the action of a crowbar.
For example, if the distance from the acromion to the point of application of the external
force (dF) is 50 cm and the distance from the acromion to the point of application of the
resultant of all the resistant force (dR) is 5.0 cm, then the mechanical advantage (MA)
can be calculated as follows:
MA =
dF
50
=
= 10
d R 5.0
Thus, in this case the mechanical advantage is 10, and the magnitude of the resistance
force must be ten times that of the applied force.
SAQ 4 - If in the above examination of the mechanics of anterior
dislocation of the glenohumeral articulation, the magnitude of the
applied force was 100 N, what must the magnitude of the resistance
force be if it is to prevent a dislocation?
2. THE ELBOW
The elbow joint is formed by the distal surface of the humerus and the proximal
articular surfaces of the forearm bones, the radius and ulna. It consists of three synovial
articulations: the humeroradial articulation, the humeroulnar articulation and the
proximal radioulnar articulation (Figure 13).
Unit 3 - Biomechanics of the Upper Limb & Spine 8
ELBOW JOINT
(A)
humerus
trochlea
capitellum
humeroradial
articulation
proximal
radioulnar
articulation
humeroulnar
articulation
radius
ulna
humerus
(B)
humeroulnar
articulation
ulna
olecranon
trochlea fossa
FIGURE 13. THE ARTICULATIONS OF THE ELBOW JOINT (A) FRONTAL ASPECT (B) MEDIAL ASPECT.
The humeroradial articulation is formed by the articulation between the capitellum of
the distal humerus and the head of the radius.
HUMERORADIAL
ARTICULATION
The humeroulnar articulation is the articulation between the trochlea of the distal
humerus and the reciprocally shaped trochlear fossa of the proximal ulna.
HUMEROULNAR
ARTICULATION
The proximal radioulnar articulation is formed by the head of the radius and the
radial notch of the proximal ulna.
PROXIMAL RADIOULNAR
ARTICULATION
Here the functional motion of the elbow joint is of primary interest. The detailed motion
of each articulation is not of great importance and will therefore not be discussed.
Rather the overall motion resulting from the individual articulations will be described in
thee next two subsections.
2.1 Flexion and Extension
The two articulations that include the humerus, the humeroradial and humeroulnar
articulations, allow the elbow joint to flex and extend in a hinge-like manner (Figure
14). The axis of rotation passes through the middle of the trochlea and is roughly
parallel to the line joining the lateral and medial epicondyles of the humerus.
flexion
axis of
rotation
extension
FIGURE 14. FLEXION AND EXTENSION OF THE ELBOW.
Unit 3 - Biomechanics of the Upper Limb & Spine 9
The range of flexion-extension motion is around 140 degrees. When the elbow is fully
flexed the angle between the humerus and forearm is around 40 degrees and when it is
fully extended it is around 180 degrees giving a total range of motion of about 140
degrees made up of around 140 degrees of flexion and 0 degrees of extension (Figure
15).
full flexion
140°
180°
40°
full extension
FIGURE 15. RANGE OF FLEXION - EXTENSION MOTION OF THE ELBOW.
2.2 Pronation and Supination
The proximal radioulnar articulation allows rotation of the forearm about a longitudinal
axis. This rotation is termed pronation and supination (Figure 16). In pronation, the
palm of the hand faces posteriorly if the elbow is extended with the upper arm alongside
the trunk and downwards if the elbow is flexed 90 degrees.
PRONATION
SUPINATION
supination
pronation
FIGURE 16. SUPINATION AND PRONATION.
Pronation and supination are achieved by the rotation of the head of the radius in the
radial notch of the ulna in a pivot-like manner (Figure 17A). This occurs inside the
ligamentous sling which binds the radius to the ulna, the annular ligament (Figure
17B). The longitudinal axis passes through the radial head and the distal ulna articular
surface. The rotation about this axis results in the migration of the distal end of the
radius around the distal end of the ulna.
Unit 3 - Biomechanics of the Upper Limb & Spine 10
ANNULAR LIGAMENT
humerus
annular ligament
radius
ulna
radial collateral
ligament
(A)
(B)
FIGURE 17. RADIOULNAR ARTICULATION. (A) THE PIVOT-LIKE STRUCTURE (B) THE POSITION OF THE ANNULAR
LIGAMENT.
Activity: Place your forearm on a table with your palm facing upwards. Pronate your
forearm whilst keeping it on a table. Note how the radius moves around the almost
stationary ulna. If at the same time you feel the head of the radius (just distal to the
lateral epicondyle of the humerus) you will be able to feel the radius as it rotates and
rolls against the ulna.
The total range of pronation-supination is around 150 degrees. This is made up of
around 70 degrees of pronation and 80 degrees supination.
SAQ 5 - Complete the following table of the maximum ranges of
motion of the elbow.
Flexion-extension
Extension
Flexion
Pronation-supination
Pronation
Supination
Range
Range
2.3 Range of Motion During Daily Activities
The range of elbow joint motion required for various selected daily activities are
summarised in Table 1. Note that for these activities the required ranges are much less
than that which the elbow can achieve. Around 100 degrees of flexion motion is
required from 30 to 130 degrees and around 100 degrees of pronation and supination
motion is required from around 50 degrees pronation and 50 degrees supination.
Activity
Flexion (degrees)
Rotation (degrees)
minimum maximum arc
pronation
supination arc
Pouring from a jug
36
58
22
43
22
65
Putting fork to mouth
85
128
43
10
52
62
Opening a door
24
57
33
35
23
58
Putting glass to mouth
45
130
85
10
13
23
TABLE 1. THE RANGE OF ELBOW JOINT MOTION REQUIRED FOR VARIOUS DAILY ACTIVITIES.
2.4 Stability
The elbow is essentially a mechanically stable joint with the bony structure and its
associated ligaments and muscles all contributing to this stability.
The olecranon process, which in profile resembles the end of a spanner, is particularly
well suited to resist forces in the anteroposterior and posteroanterior directions, as it
holds the trochlea like a nut (Figure 13B). However, it does not provide much resistance
to forces acting in a lateral and medial direction.
Unit 3 - Biomechanics of the Upper Limb & Spine 11
The side to side stability is instead provided by the two collateral ligaments. The ulnar
or medial collateral ligament is the most important, preventing abduction of the elbow.
In contrast, the radial or lateral collateral ligament provides only limited resistance to
adduction forces. The lateral collateral ligament is assisted by the anconeus muscle
which is located on the lateral aspect of the elbow. The anconeus muscle has its origin
on the lateral epicondyle of the humerus and its insertion on the olecranon and superior
portion of the ulna shaft. This apparent weakness does not pose a significant problem
because valgus stability is much more important functionally than varus stability. This
is easily appreciated if you consider the forces acting during a throwing or hammering
action or a fall on an outstretched arm.
The stability of the elbow joint means that dislocations are much less common than
dislocations of the shoulder. However, a fall on a outstretched arm in almost full
extension can result in an anterior dislocation, whereby the distal end of the humerus
slides forward over the coronoid process.
SAQ 6 - By sketching a diagram of the arm decide which ligament will
prevent an elbow dislocating in a fall to the side with the arm straight
so that it breaks the fall.
2.5 Joint Forces at the Elbow
During common daily activities the elbow joint force can be as high as 2000 N which is
almost 2.5 to 3 times body weight. For example, the action of pulling an object, such as
a table, can generate a joint force of about 1900 N, and during dressing and eating
activities these can be around 300 N.
Worked Example
In this worked example the flexor muscle force and the elbow joint force will be
calculated for the situation shown in Figure 18. The subject had a total body mass of 85
kg and the length of his forearm from the elbow axis to the ulna styloid was measured
and found to be 27.5 cm. The distance from the elbow axis to the position of the
dumbbell was also measured and was found to be 35 cm. The mass of the dumbbell was
5.0 kg. The problem has been simplified by considering only the forces acting in the
sagittal plane and by grouping the actions of all the elbow flexors in to one force acting
vertically upwards with a lever arm about the elbow joint of 25 mm (remember that g =
9.8 m s-2).
muscle
force
2.5 cm
27.5 cm
35 cm
FIGURE 18.
Unit 3 - Biomechanics of the Upper Limb & Spine 12
The first step in solving any biomechanical problem is to draw a free body diagram (as
shown in Figure 19). Note that the elbow joint force, FELBOW, is drawn as acting
vertically downwards. The direction was deduced by considering the other forces. None
of the other forces have a horizontal component, therefore the elbow joint force can not
have a horizontal component - if it did the forearm would be accelerating! The direction
upwards or downwards was deduced by considering the moments about the insertion of
the muscle force. Both weights would cause a clockwise moment. The elbow joint force
must counteract this with an anticlockwise moment. It must therefore be acting
downwards.
FM
FELBOW
IM
ICOM
mFHg
ID
m Dg
FIGURE 19. FREE BODY DIAGRAM OF FOREARM.
SAQ 7 - Could the problem be solved without first deducing the
direction of the elbow joint force?
The mass of the forearm and hand, mFH, and the distance from the elbow joint to the
centre of mass, lCOM, are unknown but can both be calculated using the anthropometric
data contained in Table 1 of Unit 1 - Biomechanical Analysis. Thus, for the mass of the
forearm and hand:
mFH = 0.022 × 85 = 1.87 kg
For the distance:
lCOM = 0.682 × 27.5 = 18.755 cm = 0.18755 m
To calculate the flexor muscle force we need to eliminate the other unknown, the elbow
joint force. This is achieved by summing the moments about elbow joint. Since the arm
is in static equilibrium the sum of all the moments should be zero. Thus, taking
anticlockwise as positive:
∑M
ELBOW
= FMlM – mFHglCOM – mDglD = 0
Rearranging for FM:
FM =
m FH gl COM + m D gl D 1.87 × 9.8 × 0.18755 + 5.0 × 9.8 × 0.35
=
lM
0.025
FM =
3.4370413 + 17.15
= 823.482 = 820 N
0.025
Thus the flexor muscle force is 820 N. To calculate the elbow joint force, two methods
can be employed. Either the vertical forces can be summed or the moments about any
point other than the elbow joint can be summed.
Unit 3 - Biomechanics of the Upper Limb & Spine 13
Summing the vertical forces (upwards positive):
∑F
VERTICAL
= -FELBOW + FM – mFHg – mDg = 0
FELBOW = FM – mFHg – mdg = 823.482 – 1.87 × 9.8 – 5 × 9.8 = 756.156 = 760 N
Alternatively, summing the moments about the point of application of FM:
∑M
FM
= FElM – mFHg(lCOM – lM) – mDg(lD – lM) = 0
Rearranging for FE:
FELBOW =
m FH g(l COM − l M ) + m D g(l D − l M )
lM
FELBOW =
1.87 × 9.8 × (0.18755 − 0.025) + 5 × 9.8 × (0.35 − 0.025)
0.025
FELBOW =
2.9788913 + 15.925
= 756.156 = 760 N
0.025
Thus the joint force is equal to 760 N and the flexor muscle force is equal to 820 N.
SAQ 8
(a) Express the flexor muscle force and the elbow joint force found in
the worked example as ratios of the subject’s total body weight.
(b) Comment on the effect on the joint force if the flexor muscle joint
force was not acting vertically.
3. THE WRIST
The wrist joint is a complex structure that allows the hand to move relative to the
forearm and transmit loads between the forearm and hand. The wrist joint complex is
formed by the distal radius, the structures within the ulnocarpal space, the carpal bones
and proximal ends of the metacarpals (Figure 20).
capitate
trapezoid
five
metacarpals
trapezium
hamate
pisiform
triquetrum
eight
carpal
bones
lunate
ulnocarpal
space
ulna
scaphoid
radius
FIGURE 20. ANTERIOR VIEW OF THE BONES OF THE WRIST.
Unit 3 - Biomechanics of the Upper Limb & Spine 14
Seven of the eight carpal bones are arranged into two rows.
The proximal row is made up of three bones:
PROXIMAL ROW
¾ triquetrum
¾ lunate
¾ scaphoid
The distal row is made up of four bones:
DISTAL ROW
¾ hamate
¾ capitate
¾ trapezoid
¾ trapezium
The eighth carpal bone, the pisiform, is positioned anteriorly to the triquetrum.
The pisiform bone is the only one of the carpal bones that is easily palpated. It projects
anteriorly on the little finger side of the hand as a small rounded elevation. It is the
insertion point of the flexor carpi ulnaris muscle which flexes and adducts the wrist.
The tendon of the flexor carpi ulnaris can also be easily palpated when the wrist is
flexed. The arrangement of the pisiform and the flexor carpi ulnaris is comparable to the
arrangement of the patella and the knee extensor muscles in that the pisiform increases
the lever arm of the flexor carpi ulnaris.
pisiform
tendon of the
flexor carpi ulnaris
FIGURE 21. THE PISIFORM INCREASES THE LEVER ARM OF THE FLEXOR CARPI ULNARIS MUSCLE.
Despite its wide range of motion the wrist is a comparatively stable joint. Its stability is
derived from the intricate ligamentous structures and the precise opposition of the
multifaceted articular surfaces rather than from any inherent bony stability, such as that
found in the hip joint.
SAQ 9
(a) List the bones that form the proximal and distal rows of the wrist.
(b) How is the lever arm of the flexor carpi ulnaris muscle increased?
3.1 Articulations
The motion of the wrist joint is dependent on the numerous articulations formed
between the carpal bones and between the carpal bones and adjacent bones of the hand
Unit 3 - Biomechanics of the Upper Limb & Spine 15
PISIFORM
and forearm. These include the radiocarpal joint, the midcarpal joints, the carpometacarpal joints, and the intercarpal joints. Two of the most interesting articulations of
the wrist are the radiocarpal joint and the articulation between the triquetrum and distal
ulna.
The lunate and scaphoid articulate with the distal end of the radius, forming the
radiocarpal joint. It is a condyloid joint, whereby an oval-shaped condyle fits into an
elliptical depression. It allows flexion and extension, abduction and adduction and
circumduction (Figure 22).
RADIOCARPAL JOINT
CONDYLOID JOINT
FIGURE 22. A CONDYLOID JOINT.
The triquetrum articulates with the distal ulna via a triangular shaped inter-articular disc
which occupies the ulnocarpal space. This is attached at its apex to the styloid process
of the ulna and at its base to the ulnar notch of the radius (Figure 23).
radius
ulna
styloid
process
articular cartilage
FIGURE 23. END ON VIEW OF THE DISTAL FOREARM SHOWING THE ARTICULAR CARTILAGE IN THE ULNOCARPAL
SPACE.
3.2 Motion
The overall motion of the wrist is a result of the complex interaction of the bones and
ligaments that form the joint. These interactions are as yet still not understood
completely so here we will be concentrating on examining the gross functional motion
of the wrist.
The wrist allows flexion and extension, and abduction and adduction (Figure 24). When
in the anatomical position:
¾ flexion results in the hand tilting forwards
¾ extension results in the hand tilting backwards
Unit 3 - Biomechanics of the Upper Limb & Spine 16
ULNOCARPAL SPACE
¾ abduction results in the hand tilting outwards
¾ adduction results in the hand tilting inwards
flexion
extension
abduction
adduction
FIGURE 24. THE MOTION OF THE WRIST.
The wrist allows approximately 80 to 90 degrees of flexion and 70 to 80 degrees of
extension. The range of flexion generally exceeds the range of extension by an average
of 10 degrees. Approximately 60% of flexion occurs at the midcarpal joint and the rest,
40%, in the radiocarpal joint (Figure 25). The opposite is true for extension with around
two thirds, 67%, of extension occurring in the radiocarpal joint and a third, 33%, at the
midcarpal joint. However, these do vary widely between individuals.
26°
40%
40°
60%
flexion
extension
18°
37°
33.5%
66.5%
FIGURE 25. THE CONTRIBUTIONS OF THE PROXIMAL AND DISTAL ARTICULATIONS TO THE OVERALL FLEXION EXTENSION MOTION OF THE WRIST.
The wrist joint allows approximately 15 to 20 degrees of abduction and around 35
degrees of adduction giving a total range of motion of around 50 degrees.
To perform most of the activities of daily living (such as eating, reading, using a
telephone) a wrist joint with 10 degrees of flexion to 35 degrees of extension is
generally satisfactory. The maximum range of extension is most critical and the ability
Unit 3 - Biomechanics of the Upper Limb & Spine 17
to perform tasks is generally reduced as extension capability is lost. For immobilised
wrist joints a fixed extension of around 15 degrees allows most activities of daily living
to be performed quite satisfactory.
SAQ 10
(a) What are the principal motions of the wrist joint?
(b) At which articulation does most wrist extension occur?
(c) What is the most functional position for an immobilised wrist
joint?
4. THE HAND
The hand is the most distal structure of the upper limb. It is formed by the metacarpals
and the phalanges. In total there are five metacarpals (one at the base of each digit) and
fourteen phalanges (three for each finger and two for the thumb).
3 phalanges
2 phalanges
5 metacarpals
carpal bones
FIGURE 26. THE BONES OF THE HAND AND WRIST.
4.1 Articulations
The joints of the hand include the:
¾ carpometacarpal joints
¾ intermetacarpal joints
¾ metacarpophalangeal joints
¾ proximal interphalangeal joints
¾ distal interphalangeal joints
The carpometacarpal (CMC) joints are formed by the carpal bones of the wrist and the
metacarpals of the hand. The first carpometacarpal joint, the articulation between the
trapezium and the first metacarpal, at the base of the thumb, is of great significance. It is
the most freely moving carpometacarpal joint. It allows the thumb to oppose the fingers
giving the human hand much greater dexterity than the forepaw of any other animal and
allows us to manipulate our environment so effectively. It is a saddle joint, with the
articulating surfaces resembling reciprocally shaped saddles, which allows the first
metacarpal to flex and extend, and abduct and adduct. The remaining carpometacarpal
joints are essentially modified saddle joints. All the carpometacarpal joints are
surrounded by joint capsules which are reinforced by several ligaments.
Unit 3 - Biomechanics of the Upper Limb & Spine 18
CARPOMETACARPAL
JOINT
second
metacarpal
first
metacarpal
trapezium
trapezium
FIGURE 27. THE CARPOMETACARPAL JOINT OF THE THUMB FUNCTIONS AS A SADDLE JOINT.
The intermetacarpal joints are irregular articulations formed between the proximal ends
of adjacent metacarpals. They share the joint capsules of the carpometacarpal joints.
The metacarpophalangeal (MCP) joints are condyloid joints formed by the rounded
distal heads of the metacarpals and the concave proximal ends of the phalanges. These
joints form the knuckles of the hand. Each joint is enclosed in a capsule and stabilised
by strong collateral ligaments. The metacarpophalangeal joint of the thumb is
strengthened by an additional dorsal ligament.
The proximal interphalangeal (PIP) and distal interphalangeal (DIP) are all
essentially hinge joints allowing only flexion and extension motion. In contrast to the
other digits the thumb has only one interphalangeal (IP) joint.
4.2 Motion and Range of Motion of the Fingers
The range of motion of each of the carpometacarpal, metacarpophalangeal and interphalangeal joints of the fingers is dependent on their bone structure and the surrounding
ligaments.
The second and third metacarpals are basically immobile whilst the fourth and fifth
metacarpals permit a small amount of flexion and extension. This is in the region of 10
to 15 degrees at the fourth and 20 to 30 degrees at the fifth.
The metacarpophalangeal joints allow flexion-extension and abduction-adduction. The
maximum amount of flexion is around 90 degrees. The amount of extension varies
considerably between individuals depending on the laxity of their ligaments (Figure
28A).
0°
neutral
metacarpophalangeal joint
(A)
0°
neutral
90°
proximal interphalangeal joint
(B)
100°
distal interphalangeal joint
(C)
90°
0° neutral
FIGURE 28. THE APPROXIMATE RANGE OF MOTIONS OF THE FINGER JOINTS.
Unit 3 - Biomechanics of the Upper Limb & Spine 19
INTERPHALANGEAL
JOINT
The proximal and distal interphalangeal joints permit only flexion-extension. The
largest amount of flexion occurs at the proximal joints. This is around 100 to 110
degrees (Figure 28B). At the distal joints the maximum amount of flexion is about 90
degrees (Figure 28C). Extension beyond the neutral position, with the fingers straight, is
termed hyperextension. As with the MCP joints it is dependent largely on ligament
laxity.
SAQ 11 - How does the maximum amount of flexion at the metacarpophalangeal joint vary from finger to finger?
4.3 Motion and Range of Motion of the Thumb
The metacarpophalangeal and interphalangeal joints of the thumb resemble those of the
fingers in structure and function. The metacarpophalangeal joint, however, does not
generally allow as large an amount of flexion as those of the fingers. The range varies
from as little as 30 degrees to as much as 90 degrees. The amount of extension though is
generally greater with normally around 15 degrees being possible.
The carpometacarpal joint of the thumb is of particular importance functionally. The
motion of the thumb in the plane of the palm is of particular interest. This is described
as flexion as the thumb moves across the palm (Figure 29A) and extension as the thumb
moves away to the side from the palm. Around 15 degrees of flexion and 20 degrees of
extension is possible. Abduction occurs when the thumb moves away from the hand
with around 60 degrees being possible. A small amount of rotation is also possible. For
example, if the thumb is moved across the palm so that the tip touches the base of the
little finger, this is achieved by the flexion and rotation of the carpometacarpal joint and
flexion of the metacarpophalangeal and interphalangeal joints (Figure 29C). If the
thumb is then moved to touch the tip of the little finger, adduction is required (Figure
29D).
(A)
(B)
flexion
15°
abduction
60°
rotation
flexion
adduction
(C)
(D)
FIGURE 29. MOTION OF THUMB.
Unit 3 - Biomechanics of the Upper Limb & Spine 20
4.4 Interaction of the Hand and Wrist Motion
The principal muscles that control the movements of the digits are actually located in
the forearm. Their distal tendons cross the wrist and possibly several joints of the digits
before they are inserted. For example, the flexor digitorum profundus originates from
the anterior aspect of the ulna and has insertions on the distal phalanges, allowing it to
flex the distal interphalangeal joints (Figure 30).
flexor digitorum profundus
FIGURE 30. FLEXOR DIGITORUM PROFUNDUS MUSCLE.
As the wrist changes its position it also alters the functional lengths of the muscle
tendons that cross it. For example, when the wrist is straight the fingers can be easily
clenched into a tight fist, however, if the wrist is flexed first of all then it becomes
difficult to fully flex the fingers. Similarly the range of wrist flexion is dependent on
whether the fingers are straight or flexed. This is illustrated in Figure 31. With the
fingers extended the wrist can flex to almost 90º, but with the fingers clenched into a
fist the range of wrist flexion is significantly reduced.
FIGURE 31. THE INTERACTIONS OF THE WRIST AND HAND MOTION CAUSES THE RANGE OF WRIST FLEXION TO BE
DEPENDENT IN PART ON THE POSITIONS OF THE DIGITS.
Activity: With your wrist straight grasp a pen in your hand making a fist around it.
With your other hand gently push your wrist into flexion. Notice how your grip on the
pen is loosened.
Unit 3 - Biomechanics of the Upper Limb & Spine 21
5. THE SPINE
The spine (or vertebral column) forms the longitudinal axis of the skeleton. It is a
complex structure consisting of 24 unfused vertebrae, the sacrum and coccyx (Figure
32). Its principal functions are to protect the spinal cord, transfer loads from the head
and trunk to the pelvis, and allow movement of the trunk.
C1
cervical
7 vertebrae
C7
T1
dorsal (thoracic)
12 vertebrae
T12
L1
lumbar
5 vertebrae
L5
sacrum and coccyx
(5 and 4 fused vertebrae)
FIGURE 32. THE SPINE.
The spine is divided into five regions, as shown in Figure 32. They are the:
¾ cervical
¾ thoracic
¾ lumbar
¾ sacrum
¾ coccyx.
In each region of the spine the vertebrae are slightly different in structure, as is the
overall structure, function and range of motion in each region. The vertebrae of the
cervical, thoracic and lumbar regions are numbered from the top downwards and
labelled C, T and L respectively, as shown in Figure 32.
SAQ 12 - Name the five regions of the spine.
5.1 Vertebrae
All the unfused vertebrae, except the first two cervical vertebrae, resemble each other in
certain features. They all have a flat, rounded body placed anteriorly and centrally,
called the vertebral body, an arch of bone, called the neural arch, that forms the
spinal foramen through which the spinal cord passes, a spinous process projecting
inferiorly in the posterior mid-line and two transverse processes projecting laterally.
These processes provide anchorage sites for the ligaments and muscles which stabilise
and move the spine. A typical vertebra is shown in Figure 33.
VERTEBRAL BODY
NEURAL ARCH
SPINAL FORAMEN
SPINOUS PROCESS
TRANSVERSE PROCESS
Unit 3 - Biomechanics of the Upper Limb & Spine 22
(A)
spinous process
vertebral body
articular surfaces
of a facet joint
spinal foramen
spinous process
(B)
neural arch
FIGURE 33. A TYPICAL VERTEBRA (A) FROM THE SIDE (B) FROM ABOVE.
Each vertebra articulates with each adjacent vertebra at three points. The main
articulation is at the vertebral body via an intervertebral disc, the other two are facet
joints. These synovial joints are positioned on either side of the arch. The upper facets
articulate with the lower facets of the vertebra above, and the lower facets articulate
with the upper facets of the vertebra below.
FACET JOINTS
5.2 Intervertebral Discs
The intervertebral disc is of great mechanical and functional importance. It has a dual
role of bearing and distributing loads and of restraining excessive motion. The
cylindrical discs are made up of the inner nucleus pulposus and the outer annulus
fibrosus (Figure 34).
The nucleus pulposus lies directly in the centre of all discs except those in the lumber
segments where it is slightly posterior. It is formed by a strongly hydrophilic (waterloving) gel that is enmeshed in a random collagen matrix. The hydrophilic gel produces
a high water content and an elevated nucleus pressure. The internal pressure balances
the applied compressive stress. If the applied stress is increased water is driven out of
the disc until a new steady state is reached. Likewise, when the applied stress is
decreased the disc rehydrates. However, this mechanism is not capable of maintaining a
constant level of hydration over a long period of time. The reduction in hydration over
time results in a decreased disc height which is evident in the loss of standing height
over the day which can be as much as 1 cm.
The annulus fibrosus is a tough layer which surrounds the nucleus pulposus. It is
composed of collagen fibres. These form concentric layers (lamellae) with alternating
orientations of the collagen fibres (Figure 34). This arrangement resists high bending
and torsional loads.
annulus fibrosus
nucleus pulposus
lamellae of
annulus fibrosus
FIGURE 34. SECTION THROUGH AN INTERVERTEBRAL DISC.
Unit 3 - Biomechanics of the Upper Limb & Spine 23
NUCLEUS PULPOSUS
ANNULUS FIBROSUS
5.3 The Cervical Spine
The cervical spine consists of the seven vertebrae that form the neck. This is the most
mobile region of the spine.
CERVICAL SPINE
The two most superior vertebrae, the atlas (C1) and the axis (C2) are particularly
mobile (Figure 35). In structure they are quite different from the other cervical
vertebrae. The atlas has no body but is composed of a ring within which an oval fossa
articulates with the axis. The axis has an articular process, the dens, which protrudes
superiorly from the vertebral body. A small synovial joint is formed between the
anterior tip of the dens and the oval fossa of the atlas. The atlas rotates about the dens
but the motion is restricted by several ligaments that are attached to the top of the dens.
dens
atlas
axis
FIGURE 35. TWO CERVICAL VERTEBRAE - THE ATLAS AND THE AXIS.
5.4 The Thoracic Spine
The thoracic spine or dorsal spine consists of the twelve vertebrae located in the
posterior part of the thoracic region of the trunk (the chest). Each of these vertebrae is
attached to a pair of ribs. Each rib articulates with the body and the transverse process
of its corresponding thoracic vertebra. The head of each rib articulates with the body
and the tubercle of each rib articulates with the transverse process. In addition the
second through to ninth ribs articulate with the body of the vertebra above. These
articulations allow the ribs to move up and down as we breathe. The ribs give added
rigidity to the thoracic spine which effectively limits its mobility to only a very limited
degree of flexion and extension and little rotation.
spinous process
transverse
process
rib
facet joint
articulations
vertebral body
spinal foramen
FIGURE 36. A TYPICAL THORACIC (DORSAL) VERTEBRA SHOWING THE ARTICULATION WITH A RIB.
Unit 3 - Biomechanics of the Upper Limb & Spine 24
THORACIC SPINE
5.5 The Lumbar Spine
The lumbar spine consists of the five vertebrae of the lower back. The lumbar
vertebrae are subjected to significantly greater loads than the vertebrae in the rest of the
spine. This is apparent in their larger bodies.
LUMBAR SPINE
5.6 The Sacrum and Coccyx
The sacrum and coccyx form the distal portion of the spine, lying below the lumbar
vertebrae. In adults the sacrum is a single triangular-shaped bone that has resulted from
the fusion of five separate vertebrae and the coccyx is a single bone that has resulted
from the fusion of four or five vertebra.
The sacrum forms the link between the lumber spine and the pelvic girdle. The junction
between the sacrum and the lumber spine is very mobile. The sacrum is joined to the
two innominate bones of the pelvis by two fibrous joints that allow only a small amount
of relative motion (Figure 37).
sacrum
ilium
coccyx
ischium
FIGURE 37.
5.7 Range of Motion
The overall range of motion of the spine is large. The actual ranges, however, vary
considerably between individuals depending on their sex and age. The range of motion
is particularly dependent upon age, in old age the range is around half that in youth. All
the movements of the spine are a result of the combined movement of several vertebrae,
the amount of motion between adjacent segments is generally small and does not occur
independently.
5.7.1 Flexion-extension
Flexion of the spine occurs when bending forward and extension when bending
backwards (Figure 38). The range of flexion-extension motion between adjacent
segments varies between different regions of the spine. It is greatest in the cervical
spine, with a total range of flexion-extension of around 21 degrees between C4 and C5,
and smallest in the thoracic spine, with a total range of around 3 degrees between T9
and T10. The maximum amount of flexion and the maximum amount of extension
between adjacent segments also varies. This is particularly true in the lumbar spine
where the maximum range of flexion averages about 10 degrees and the maximum
range of extension averages only around 4 degrees.
Unit 3 - Biomechanics of the Upper Limb & Spine 25
SACRUM
COCCYX
flexion
extension
FIGURE 38. FLEXION AND EXTENSION OF THE SPINE.
During actual movements the motion of the spine is complex and often combined with
motion in other parts of the skeletal system. For example, during forward bending the
first 50 to 60 degrees of flexion occurs in the lumbar spine with any further flexion
being achieved by the tilting of the pelvis forward.
50°
50°
pelvic tilting
FIGURE 39. FORWARD FLEXION OF THE TRUNK.
5.7.2 Lateral bending
Lateral bending is the motion from side to side in the frontal plane. The amount of
lateral motion has a similar distribution to that of flexion and extension, with the
cervical spine being the most mobile and the thoracic spine the least (Figure 40A).
Notably there is no lateral bending between the first two cervical vertebrae, the atlas
(C1) and axis (C2), and no rotation between the atlas (C1) and the occipital (OC) bone
of the skull.
Unit 3 - Biomechanics of the Upper Limb & Spine 26
(A) lateral flexion
(B) rotation
FIGURE 40. RANGE OF MOTION BETWEEN ADJACENT VERTEBRAE. (A) LATERAL FLEXION (B) ROTATION.
5.7.3 Rotation
The amount of rotation about the longitudinal axis of the spine generally decreases
down the spine (Figure 40). Notably, the range of rotation is considerably larger
between the atlas (C1) and axis (C2) than between any other vertebrae. This is due to
their unique structure.
SAQ 13 - Try to estimate the amount of range of rotation in your
lumbar and thoracic spine. First of all stand up in front of a wall with
your arms folded and twist gently from side to side whilst keeping
your arms fixed relative to your chest. Note, how the angle between
your folded arms and the wall changes. Does this give a true
representation of the range of rotation in your lumbar and thoracic
spine?
5.8 Loadings on the Spine
The loadings on the spine are mainly due to the weight of the upper body, muscle
activity and externally applied loads. Not surprisingly the lumbar spine carries the
highest loadings and this is reflected in the larger vertebra found in the lumbar region as
compared to those in the thoracic and cervical regions. Despite this lower back pain in
the lumbar region is very prevalent and usually results from a bad posture during
standing or sitting, or by bad bending or lifting techniques.
Studies of the loads on the lumbar spine during various postures have demonstrated that
bad postures increase the loads on the vertebrae. For example, Figure 41 shows a bar
chart of the comparative loads on the third lumbar intervertebral disc for different
postures. The load during standing is defined as 100% during upright standing. Note
that during sitting the loading is actually larger than during upright standing and how
slumping forwards can almost double the load.
Unit 3 - Biomechanics of the Upper Limb & Spine 27
200%
150%
100%
50%
prone
erect
standing
erect
sitting
bent
forward
standing
relaxed
sitting
FIGURE 41. LOADINGS ON THE LUMBAR SPINE WITH DIFFERENT POSTURES.
These results can be explained by considering the position of the upper body relative to
the lumbar spine. In Figure 42 the vertical arrow shows the direction of the weight of
the upper body in two cases: for upright standing and relaxed unsupported sitting. The
moment arm of the upper body mass about the lumbar spine is increased in the case of
relaxed sitting due to the backward tilt of the pelvis. The flexion moment produced by
the upper body weight and the moment arm must be counterbalanced by an extension
moment produced by the posterior back muscles. The larger the moment arm is the
greater the muscle forces need to be. The muscles produce a compressive load on the
spine which increases with increasing muscle force. Thus any change in posture that
causes the upper body to be in a position offset from the lumbar spine effectively
increases the load that it must carry. Even during erect sitting the moment arm is
slightly greater than that during upright standing, giving a slightly increased load on the
lumbar spine.
moment arm
moment arm
weight
weight
(A)
(B)
FIGURE 42. LUMBAR SPINE MOMENT DURING (A) STANDING AND (B) SITTING.
Unit 3 - Biomechanics of the Upper Limb & Spine 28
Similarly, during lifting the load on the lumbar spine is increased in accordance with the
increase in the moment arm produced by weight of the object being lifted. This is
illustrated in Figure 43. In the first case (Figure 43A), the moment arms of the weight of
both upper body and the object are greater than when lifting the same object with the
knees bent (Figure 43B). This bad lifting technique results in a considerably larger load
on the lumbar spine which could be avoided by bending the knees and keeping the
object closer to the body.
(A)
(B)
FIGURE 43. LUMBAR SPINE MOMENT ARMS WITH (A) BAD LIFTING POSTURE (B) GOOD LIFTING POSTURE.
SAQ 14 - As the moment arm of the upper body weight increases does
the loading on the spine increase or decrease?
Unit 3 - Biomechanics of the Upper Limb & Spine 29
SUMMARY
In this unit you have been introduced to the biomechanics of the upper limb and spine.
Mechanically the upper limb is designed for the delicate manoeuvring of the hand.
Nevertheless it must also withstand the high forces generated at joints caused by muscle
contractions and external loads.
Overall the spine displays a considerable range of flexibility but between adjacent
vertebrae the range of motion is generally small. The loadings on the spine can be
considerable especially in the lumbar region.
Unit 3 - Biomechanics of the Upper Limb & Spine 30
SAQ ANSWERS
SAQ 1
(a) The four articulations of the shoulder joint are the glenohumeral, acromioclavicular, sternoclavicular and the scapulothoracic.
(b) The shallow depression in which the humeral head rests is called the
glenoid fossa.
SAQ 2
(a) The ligament about whose attachment the clavicle rotates during elevation
and depression is the costoclavicular.
(b) The scapulothoracic articulation is not a joint in the truest sense - there are
no direct bony or ligamentous connections.
SAQ 3
Forward flexion-backward extension
Abduction-adduction
Internal-external rotation
Horizontal flexion-extension
Forward flexion
180°
Backward extension
60°
Range
240°
Abduction
180°
Adduction
75°
Range
255°
Internal rotation
90°
External rotation
90°
Range
180°
Horizontal flexion
135°
Horizontal extension
45°
Range
180°
SAQ 4
The resistance force must be 10 times the magnitude of the applied force, that
is 1000N.
SAQ 5
Flexion-extension
Pronation-supination
Extension
Flexion
Range
Pronation
Supination
Range
0°
140°
140°
70°
80°
150°
Unit 3 - Biomechanics of the Upper Limb & Spine 31
SAQ 6
lateral
collateral
ligament
SAQ 7
Yes, the elbow joint force could be assumed to have both horizontal and vertical
components. Summing the horizontal force components would then give a
result of zero for the horizontal elbow joint component and summing the vertical
forces would give a negative result if the vertical component was defined as
acting upwards.
SAQ 8
(a) Total body weight = 9.8 × 85 = 833 N.
The flexor muscle force is 820 N, therefore it is equal to 0.98 of total body
weight.
The elbow joint force is 760 N, therefore it is equal to 0.91 of total body
weight.
(b) If the flexor muscle force did not act vertically then the elbow joint force
would need to have an equal but opposite horizontal component to
maintain static equilibrium.
SAQ 9
(a) The bones that form the proximal row of the wrist are the triquetrum, lunate
and scaphoid and those that form the distal row are the hamate, capitate,
trapezoid and trapezium.
(b) The lever arm of the flexor carpi ulnaris muscle is increased by the pisiform
bone.
SAQ 10
(a) The principal motions of the wrist joint are flexion-extension and adbuctionadduction.
(b) Most wrist extension occurs at the radiocarpal joint.
(c) The most functional position for an immobilised wrist joint is around 15
degrees of extension.
Unit 3 - Biomechanics of the Upper Limb & Spine 32
SAQ 11
The maximum amount of flexion at the metacarpophalangeal joints generally
decreases from the fifth to the second joint. At the fifth joint (little finger) it is
around 95 to 100 degrees and at the second joint (index finger) it is about 70
degrees.
SAQ 12
The five regions of the spine are the cervical, thoracic, lumbar, sacrum and
coccyx.
SAQ 13
No. When standing and gently twisting from side to side motion also occurs in
the lower limbs. Do the same again but this time sitting down. This should give
you a better estimate of the range of rotation in your lumbar and thoracic spine.
SAQ 14
The loading on the spine increases as the moment arm of the upper body
weight increases.
Unit 3 - Biomechanics of the Upper Limb & Spine 33