Glenohumeral muscle force and moment mechanics in a position of shoulder instability

GLENOHUMERAL MUSCLE FORCE AND MOMENT MECHANICS IN A POSITION OF SHOULDER INSTABILITY A. 0. BROWNE, B. F. MORREY and K. N. AN*

R. W. B.\ss~rr,

Biomechanics Laboratory. Department of Orthopedics. Mayo Clinic/ Mayo Foundation, Rochester, MN 55905, U.S.A. Abstract-The three-dimensional orientation of the shoulder girdle musculature was studied in five cadaver shoulders in the position of function at 90” of abduction and 90” of external rotation using a method of computer assisted gross muscle cross-section analysis. The muscle volume. muscle fiber length, and physiological cross-sectional area were obtained by dissecting two specimens. The line of action. the magnitude and orientation of the moment were calculated for each muscle crossing the shoulder joint. The quantitative description of the moment potential of muscle forces influencing shoulder function was thus obtained. The most effective flexors of the shoulder which also appear to resist anterior dislocation in the position studied are the pectoral, the short head of the biceps, cordcobrachialis. anterior deltoid, and the subscapularis. Most of the rotator CUR muscles and the posterior deltoid acted as adductors. while the anterior deltoid, long and short head of the biceps, and supraspinatus were abductors. In this position. external rotation was effected by the long head of the biceps, coracobrachialis, and the posterior deltoid. while the majority of the remaining muscles acted as internal rotators.

cles across the abducted

ISTRODUCTION

shoulder

function with computer-assisted Over the past 40 years, a variety of methods have been

logical cross-sectional

used to calculate

shoulder joint

the forces across various

Because of the complexity

of the pomctry

joints. and the

multiple muscles involved, few studies have addressed the shoulder joint

using mechanical

In their pioneering simplified

two-dimensional Further

located

Similarly,

DeLuca

the instantaneous

shoulder on radiographs sectional cadaveric

by in-

cufi muscles and

line of action from the origin

of the supraspinatus

and Forrest

center of rotation and calculated

and deltoid

specimens

and

AgGt,

of the

the moments

muscles. The cross-

maximal

on

potential

forces

this study was limited

to two

muscles and was confined to a two-dimensional lysis. In 1978. Poppen and Walker prehensive

to

(1973)

areas of these muscles were measured

were estimated.

two-dimensional

published

force

shoulder and isometric abduction

analysis

anaa comof

the

in the plane of the

scapula. The authors took into account muscles which were active at each phase of abduction the electromyographic this analysis was limited comprchcnsivc

to

Again,

to two dimensions.

Before a

shoulder can be conducted, of the moment

according

data of Jones (1970).

three-dimensional

ing the magnitude

magnitude

and

moments of the shoulder

from these data.

orientation

musculature

force analysis of the

the difficult

and three-dimensional

ClATKRtAtS

to allow

was obtained

cluding only the deltoid and rotator the insertion.

potential

of

the

are prescntcd.

principles.

force diagram

simplification

assuming a straight

arcas of all muscles across the

were also measured

work, lnman et crl. (1944) used a

solution of the forces across the joint by planar vector analysis.

The

in the position of

analysis. The physio-

task of dcfinorientation

of each of the muscles crossing the

AND M~TitOOS

One shoulder from a female cadaver, aycd 72 years, and six shoulders from three male cadavers, aged 52, 67, and 74 years, with no history of shoulder pathology, were obtained

within

36 h of death and frozen.

Right and left forequarters the extrinsic

shoulder

were prepared to preserve

musculature.

specimens were transected inferiorly

and split in the midline

so doing,

the pectoralis

musculature

The

cadaveric

at the intercostal

major

sagittal

margin

plane.

and latissimus

By

dorsi

were preserved intact.

Two specimens were dissected in order to determine the muscle

fiber length

sectional

area (PCSA)

tablished

technique

An

cr ul.,

and the physiologic

(Steno,

1981). For

cross-

of the muscles using an es1667; Brand er al.. 1981;

each muscle,

the origin

and

insertion ol’each muscle were identified. The tendon of insertion was sectioned distal to the muscle fiber and was allowed to rotate freely about the bone or tendon of origin. formed

With

careful

manipulation,

the shape of a parullslcpipcdon.

the

muscle

The muscle

tiber length was established from one measurement individual

iibers lying obliquely

of

pnrallcl between tcn-

don plates of origin and insertion using a micrometer.

shoulder joint must be defined. In this study, we have

The muscles were then disscctcd free and their volume

assessed the three-dimensional

mcasurcd

orientation

of the mus-

PCSA Receirrd infirkrljorm 2 Xfurch l9R9. *To whom correspondence should be addressed. Tel. (507) 2&t-2589.

by a water

displaccmcnt

was then calculated

technique.

by dividing

The

volume

by

mean fiber length. The remaining acquire

live shoulder specimens were used to

the gross muscle cross-section

data

to be

R. W. BASSETT tr ui.

406

presented by this study. The orientation of each specimen required careful positioning with the assistanceof radiography so as to place the brachium at approximately 90” abduction, 90” of external rotation, and 0’ Rexion and extension. All shoulder positions were based on the thoracohumeral angle. The forearm was placed in 90’ of flexion and neutral pronation and supination. The elbow was incised and the biceps and triceps tendons isolated. The tendons were sutured to the distal end of the humerus through drill holes, thus maintaining their orientation and length. The elbow was then disarticulated. The prepared specimens were placed in the freezer in such a way as to avoid passive muscle sag. Because of the significant clinical problem of shoulder instability, two commonly performed surgical procedures involving the alteration of muscle orientation were done on three specimens. On two, a Bristow-type procedure was performed, transplanting the tip of the coracoid (just distal to the insertion of the pectoralis minor) with the conjoined tendon of the short head of the biceps and the coracobrachialis onto the anterior rim of the glenoid (Halley and Olix, 1975; Lombard0 et al.. 1976; May, 1970) and fixed with a wooden peg. Glcnoid exposure was achieved by subscapularis split at the junction of the mid and lower third of the muscle and later m-approximated around the transposed conjoined tendon with nylon sutures. A Magnuson -Stack (1943) proccdurc was pcrformcd on one shoulder specimen through an axillary skin incision. The subscapularis was sharply dissected from the lesser tubcrosity and transplanted 2 cm laterally and I cm distally along the proximal humeral shaft. The tendon was anchored with nylon sutures through drill holes. After freezing, each specimen was placed in a 50 x 50 x 35 cm plexiglass container and embedded in a rapidly setting firm resin elastomer before the speci-

(4 Fig. 1. (a) Radio-opaque

\

men had thawed. The abducted humeral shaft was parallel to the long axis of the container. Once the elastomer had hardened, the specimen was placed back in the freezer until it was studied. Biplanar radiographs were taken of the specimensat right angles to one another on a special radio-opaque grid consisting of 2.5 x 2.5 cm squares (Fig. la). The location of the humeral head within the glenoid fossa could be confirmed and accurately determined, thus avoiding possible subluxation resulting in a change in some moment arms. These radiographs allowed precise three-dimensional location of bony landmarks from which a coordinate system could be defined (Morrey and Chao. 1976). The angular position of the shoulder waS calculated with respect to rotations about the X (axial rotation), Y(adduction, abduction), and Z (llexion and extension) axes at the position studied (Fig. I b). Once the orientation had been determined. the embedded specimens were cut with a band saw to expose serial cross-sections of the humeral shaft and shoulder musculature, proceeding proximally at I cm intervals from the humcral condyles (Fig. Ic). The section intervals were marked prior to cutting to prevent accumulative errors in section width, and the cuts were pcrpcndicular to the long (X) axis of the cube. After each cut, the surface was clcancd and muscle boundaries emphasized prior to being photographed from a fixed distance with a 35 mm camera. The dcvclopcd 2 x 2 inch slides (Fig. 2) of each cross-section were projected on a semi-opaque screen, positioned between two arms of a sonic digitizing system (Model 6P-3, Science AccessoriesCorporation, Southport. Connecticut). The circumference of each muscle was defined by digitizing the X and Y coordinates of points on the periphery and stored in a computer. From these data, the centroid and volume of each muscle were calculated. The exact position of

(b) grid. (b) Humcrd

(a

axis system. (c) Plane of cross-sections.

(h)

(a) Fig. 2. (a) A typid

cross-section

in the sagittd idcntilicdun

plane through

the ccnkr

of the muscles.

407

of the humcrd

head. (h) With

Glenohumeral

the humeral

head relative

further confirmed

to the glenoid

muscle brcc and moment mechanics

fossa was

on these serial gross muscle cross-

section studies. The muscle belly lengths were calculated

by measur-

ing the distance between the origin and insertion of the muscle-tendon

junction

based on the cross-sectional

slices. In most

instances,

distinguishable

(Fig.

boundary and

the muscles were clearly

2); however,

since no distinct

could be made between

teres minor

the infraspinatus

on the photographic

projections,

these were treated as a single muscle. This assumption was felt to be reasonable, as these muscles were nearly always simultaneously majian.

active on EMG

1971). As it was impossible

separate heads of the deltoid distally, also outlined acromion,

as a single entity

where the anterior

testing (Basto identify

the

this muscle was

to the level of the

and posterior parts were

more clearly definable.

CALCtiLATION

To facilitate

OF MOMENTS

the calculation

of the muscle moment

arm. a coordinate

system was established.

of the coordinate

system was placed in the center of

the humernl

The center

head, as it had been shown IO coincide

with the center of rotation (1976). The coordinate of both humcral

by Poppen

was d&cd

and Walker

as follows: the tips

condylcs wcrc used to dcfinc the Y

axis: a lint from the ccntcr of the humeral point pcrpcndicular

the % axis was then calculated the unit vectors of X and mined abduction the position

as the cross-product

Y. The

and adduction

studied.

internal-external Finally,

head to a

on the Y axis dcfincd the X axis;

The

X

rotation

of the shoulder

axis

axis

the 2 axis corresponded

of

Y axis thus deterrcprcsented

of

the

in the

shoulder.

to flexion and exten-

sion of this joint (Fig. I b). The moment of each muscle at the shoulder joint

was calculated

by first defining

the position of the centroid ofcach muscle with respect to the reference coordinate

system using the technique

described by Jensen and Davy (1975). The unit vector of the muscle force was then detined tangent to the line joining

based on the

the centroid of each muscle.

The moment arms were calculated

based on the force

vector and position vectors at the section through the center of the humeral

head.

RFStiLTS

Shouldrr

orirntotion

The angular

relationships

were defined as the posi-

tion of the arm with respect to the torso. The precise Eulerian

angle calculations

based on the coordinate

system for the five specimens revealed a mean abduction angle of 86” (range 78-89”); O-8’);

and external

Further, averaged

rotation

the glenohumeral 64’,

acic rotation.

Rexion of 4” (range

of 92” (range 89-101”).

contribution

while 22’ occurred

to abduction

from scapulothor-

410

R. W. BASSETT et al.

Muscle rolumc, fiber lmyth, and physioloyicul crosssrctionul urea The muscle volumes for the two specimens using the immersion

technique

and the calculated

umes for the remaining

muscle vol-

five specimens using the gross

muscle cross-section method are presented in Table Reasonable mersion

agreement

method

calculation.

I.

was observed between the im-

and

the cross-section

Extremely

close agreement

method

tion or delineation calculations.

For example,

of the cross-section fore, the anterior By combining

and 5, and 6 and 7). thus establishing

rotation

ility of the cross-section specimen,

variation

the data

percentages

technique. occurring

were expressed

of the entire

vided a relationship

To eliminate

the

from specimen to as normalized

muscle volume.

This pro-

of the size of the muscle which was

also observed to be reasonably

The muscle fiber length measured directly from the immersion

specimens

calculated

from

and the muscle

the specimens

belly lengths

sectional areas of each muscle calculated

cross-

by dividing

moments

related

The component

mo-

of the moment arm of each muscle with respect to a specitic

axis for a typical specimen are shown in Figs 3 and 4. studied,

Rotation

tion (-

in the normalized

represents forward

extremity

cross-

the average potential

to rotate the shoulder joint

muscle cross-section

to varying dcgrbws of muscle utilization.

by

1968)

(Table 5).

These values are also relatively consistent. particularly attributed

of

ment of each muscle for the shoulder to be calculated

rotation).

influence of the dominant

and Fukunaga.

muscle force and physiological

In the position

may be

(Ikai

area, permitting

represents the external

form. Some discrepancy

the

by the physiological

area. This term was then multiplied

a constant of 3.5 kgcm-*

method and by the muscle belly length for the gross in Table 3.

area,

of each muscle for shoulder

of each muscle, the magnitude

the volume by the muscle fiber length in the immersion method are dcpictcd

of the mo-

cross-sectional

arm was multiplied

cross-sectional

in the cross-section

studies are shown in Table 2. The physiological

calculated.

the data for magnitude

moment

the moment

which

head: therecontribution

were established (Table 5). To determine

potential

sectional

consistent.

deltoid

ment arm and the physiological the potential

large individual

through the humeral or posterior

could not be accurately

between shoulders of the same cadavers (specimens 4 the reproducib-

to be included in the

in specimens 3 and 4. the

deltoid was still present as a single muscle at the level

of

was noted

of one of the muscles could not be

defined with sufficient certainty

rotation axial

about

the X axis

(-A’.

rotation

internal

about the Yaxis represents adduc-

Y, abduction).

while rotation

flexion

about the % axis

( -Z, extension).

the

and the possibility

DISCUSSION

of hypertrophy. In

the past, several problems

definition The magnitude

of the moment

arm for the muscle

magnitude

studies in the five specimens are recorded in Table 4.

across

The

orientation

altered

specimens formed

moments in which

are excluded

of the muscles for the three a surgical from

procedure

was per-

the calculations

mean value. In some instances, an accurate

for the orienta-

scapula body.

have precluded

of the three-dimensional of the moments

the shoulder.

orientation

and muscle distribution

A definition

of the plane

is not the same as the coronal In addition,

changes direction

Humerus (digitized length) Biceps (LH) Biceps (SH) Coracobrachailis Deltoid am. Deltoid mid.

n

plane of the

the orientation

of the muscles

from longitudinal

to transverse in

I

I

L R Fiber length I 2

L

-

31 31 I9 20

35 36 22 20

34 34 20 I9

34 34 I9 I9

:: I9 I7

I9

I9

20

23

I9

5 12 I6

5 10 I8

8 ::

4 9 I7

3 9 17

24

I6

I8

23

20

IO II

IO 8 II 33

IS I2 18 31

8 8 ::

IO 10 II 29

Deltoid post. Inka. + I. minor Latissimus dorsi Pee. major (sternal) Pet. major (clav.)

12.9 14.6 7.1 9.6 7.8 9.9 8.5 21.7 13.9 13.2

16.3 19.6 9.9 II.9 10.8 16.9 9.3 34.6 25.7 19.4

Subscapularis Supraspinatus Teres major Triceps (LH)

1.4 7.0 7.8 10.6

8.7 6.9 16.8 12.1

of

is of some concern, since the plane of the

Table 2. Muscle Ii&r lengths (cm) and muscle belly lengths (cm)

Side Method Spccimcn

the and

3

R

L R Muscle belly length 4 5 6

L 7

Glcnohumcral muscle force and moment mechwks the abducted arm so that the cross-sectional difficult

I

JI

area is

to determine with certainty. The correlation of

which muscles may be active for a given function, well as the relationship sectional area with

of the physiological

its potential

been the source of difficulty

force has. likewise,

with respect to defining

the forces which occur at the shoulder joint. reason, the previous DeLuca

and Forrest

as

cross-

works

by Inman

For this

et al. (19JJ),

(1973). and Poppen and Walker

(1976). while contributing

valuable information

with

respect to the problem. have not provided data which allow

an

understanding

of

the

three-dimensional

forces occurring at this joint. The

present work seeks to provide a quantitative

description

of the muscles of the abducted and extern-

ally rotated shoulder cross-section

in three dimensions

method. The

cross-section

using the method for

the study of muscle force and moment arm mechanics has certain limitations.

Due to its invasive nature. each

specimen can only be studied for a given configuration simulating

one particular

loading condition.

In ad-

dition. using this method, a knowledge of the location of the center of rotation

of the glcnohumcral

joint

is

required for measurement of moment arms. An altcrnative method used for such studies and analysis

is

based on tendon joint excursion (An rt ~1.. 1983). With this

technique.

conditions

one may simulate

and pathological

specimen. Nevertheless, provided

us with

examination

multiple

states utilizing

loading the same

the cross-section method has

valuable information

and allowed

of the change in moment arm magnitude

and direction

for certain

proccdurcs.

The

moment

arms for the live specimens were relatively consistent in their total lengths, but due to mild diffcrcnces in position,

there was slightly

individual The

greater variation

components along the X.

moment

in the

Y and Z axes.

arms for the latissimus

dorsi,

tcres

major, and pectoralis major appeared to be somewhat excessive in some of the specimens. mobility

of these muscles and their

The

distant

origin in relation to their humeral insertions ish them from the rest of the shoulder Their

sites of distingu-

musculature.

tendency to sag and the need to extend the

centroid

line

humeral

head for calculations

of the humerus

sections undoubtedly in the calculation The

extreme

method

proximally

past the

of proximal

contributed

muscle

to some inaccuracy

of these moment arms. used

for

the

muscle

Faber length

measurement and thus PCSA calculation as described by Brand

et ul. (1981) was based on the remarkable

insight of the muscle geometry. They confirmed the lengths of the libers arc constant throughout individual

muscle. We occasionally

fibers at the proximal were longer

that each

observed a few

or distal end of the muscle that

than the others.

As also observed

by

Brand et ul.. the variation of fiber length was common for those muscles when the fiber wrapped around or crossed the joint.

The

fbcrs

closer to the center of rotation further

away.

that crossed the joint were shorter than those

R. W. BASSEIT et 01.

412

Table 4. Moment arm of shoulder muscles about the center of the humeral head (cm)

Side Specimennumber Biceps(LH) Biceps (SH) Coracobrachialis Deltoid Post. deltoid Infraspin. + I. minor Latissimus doni Pectoralis major Subscapularis Supraspinatus Teres major Triceps (LH)

n

n

L

R

L

R

L

3

4

5

6

7

Mean

2.4 3.8 3.1

2.6 3.6 3.7 3.8 t 2.8 17.5 9.5 3.1 1.5 5.9 4.5

2.3 5.4’ 5.2. 5.1 5.0 2.6 6.0 6.6 2.8 2.0 5.4 4.7

2.5 4.2’ 3.8’ 3.9 5.4 3.6 ;::

2.2 2.9 3.5 2.8 5.6 3.5 18.1 3.7 4.6: t 6.5 6.0

2.4 3.4 3.6 4.2 5.3 3.1 Il.7 6.2 2.8 2.1 5.9 4.9

: 3.2 8.7 5.9 2.4 1.9 5.6 4.7

2.7 3.1 6.0 4.7

*Moment arm was altered by a surgical intervention to simulate a Bristow-type procedure and was excluded from calculation of the mean. tCalculation was omitted as delineation proved difficult. IMoment arm was altered by a surgical intervention to simulate a Magnuson-Stack-type procedure and was excluded from calculation of the mean.

Table 5. Potential moment generated by shoulder muscles (Ncm-‘)

r--l

l---l

L 3

R 4

L 5

R 6

L 7

Biceps (LH) Biceps (SH)

13.5 32.1

15.7 20.8

I3.Y 35.0.

21.5 34.4’

19.4 23.4

16.8 25.4

3.5 5.9

Coracobrach. Deltoid Deltoid post. Infra. + I. min. Lat. dorsi Pet. major Subscapularis Supraspinatus Teres major Triceps (LH)

14.8 :

16.8 181.3 t 113.4 730.1 427.6 153.1 30.2 130.3 43.8

25.19 222.8 59.5 74.4 214.4 254.3 90.5 26.7 73.9 48.4

26.1’ 269.9 97.0 214.2 349.0 253.9 245.6 76.9 232. I 75.8

27.7I 233. 126.0 211.4 782.4 201.9 335.8$

19.1 226.8 94.2 155.6 495.6 285.6 146.0 43.9 183.4 69.5

3z 33:4 61.2 248.2 85.4 72.3 23.0 80.3 26.3

Side Specimen number

164.6 402.2 290.3 94.Y 41.6 207.6 69.6

27:.0 109.8

Mean

SD.

*Potential moment altered by a surgical intervention to simulate a Bristow-type procedure and was excluded from calculation of the mean and standard deviation. tCalculation was omitted as delineation proved ditlicult. :Potential moment was altered by a surgical intervention to simulate a Magnuson-Stack-type procedure and was excluded rrom calculation of the mean and standard deviation.

There were some differences between the muscle fiber lengths in the two immersion specimens and the muscle belly lengths taken from the cross-section studies, in particular the long head of biceps and coracobrachialis. However, this appears to be compensated by the larger volumes as seen in Table 1. resulting in comparable physiological cross-sectional areas as in Table 3. The apparent discrepancies between these two methods for the latissimus dorsi and posterior deltoid may be explained by muscle sag. resulting in a shorter muscle belly observed with the cross-section technique. Two other assumptions which may have affected the study were individual muscle structure and scapu-

lothoracic rhythm concurrent with glenohumeral abduction. The assumption that centroid lines are true representations of force vectors is invalid for unipennate and asymmetrical muscles. This is not a large factor in the shoulder, as the muscles are of a complex bipennate or multipennate nature. Finally, the scapulothoracic motion accompanying active shoulder abduction was possibly not exactly duplicated in these cadavers in which the position was from passive rotations. Though Poppen and Walker (1976, 1978) found the glenohumeral rhythm of cadavers to be within the normal in viuo range, it is possible that more motion occurred at the glenohumeral joint and less at the scapulothoracic interface than would be seen in

Glenohumeral muscle force and moment mechanics

rotation about - Y (abduction)

BKW! *.

4

Swrawh.aluus, rotation about + 2 (torward flexion)

rotation about -2 (extension)

4

rotation about (adduction)

\’

LsUsslmus

Oorri

PectoralIs

Maior

lY

Fig. 3. Illustration of moment arm magnitude components and direction for a typical specimen in resisting load applied at the glenohumeral joint in llcxion, exlension. abduction. and adduction. For example, in the

position studied. the pectoralis major is a strong forward flexor and adductor. rotation about -Y (abduction)

bCsm I

rotation

about +X

(external

rotation)

rotation about -X (Internal rotation)

Fig. 4. IllustraGon of moment arm magnitude components and direction for a typical specimen in resisting load applied at the glenohumeral joint in abduction, adduction. internal and external rotation. For example. in the position studied. the latissimus dorsi is a strong internal rotator and adductor.

R. W. BASEIT et al.

-11-l

living subjects. Potential the rotator

muscle atrophy

cuff (i.e. supraspinatus

or tears of

and long head of

the biceps) may alter the resultant moment of adjacent muscles and should be a consideration

with such a

study. However. no such tear was observed particular

shoulder

in these

specimens. Consideration

should

be given to the fact that due to the small number of cadaveric specimens, the data presented here may not

be representative of the population as a whole. The most effective flexors of the shoulder

are the

pectoral, the short head of the biceps. the coracobrachialis.

the anterior

deltoid.

and the subscapularis

(Figs 3 and 4). These are also the structures

which

appear to most effectively resist anterior dislocation the humerus.

In the position of 907 of external

of

rota-

tion, even the latissimus dorsi, teres major, and triceps have weak tlexion moment arms. From this location, the abductors of the shoulder are the anterior

deltoid.

the short head of the biceps, the long head of the biceps, and minimally

supraspinatus.

In this externally

rotated and abducted position, most of the rotator cuff muscles and the posterior

deltoid

actually

became

be as an extensor and external rotator. Unfortunately, no other muscles are comparably oriented to allow comparison in this fashion. All of the specimens demonstrated moderate variation in moment arm orientation. The major variation was the effect of alignment that influenced abduction and external rotation. In the position studied. the effect of the modified Bristow procedure was to change the short head of the biceps and coracobrachialis from weak abductors and external rotators to poor adductors and internal rotators, while the flexion moment remained unchanged. The moment arm of the subcapularis appeared to be increased by the Magnuson-Stack procedure in specimen 7 and can be explained by the more distal transfer of that muscle. With this procedure the subscapularis can change from being an adductor to an abductor, depending on the exact placement. A more distal transposition will retain, if not increase, adduction and increase both internal rotation and flexion moments.

adductors. There are few muscles acting as external rotators of

REFERENCES

the shoulder in the position studied, which is in near maximal

external

rotation

(Fig. 4). The long head of

the biceps, coracobrachialis. orionted

and posterior deltoid are

to increase external

rotation

from this po-

sition. The short hcnd of the biceps acts as a minor external

rotator,

most of the other

powerful

internal

rotation

The kinematics

moment

of the shoulder are c?mplica\ed

the fact that a muscle’s function upon

its line of action

rotation,

referable

at any given time. Figures

with slight changes in joint position,

and teres minor internal

to the center

of

3 and 4

the fact that any vector lying on or close to

an axis will have the potential abducted

by

changes depending

and this is dependent on the specific position

of the shoulder illustrate

muscles having arms.

rotators,

to change its function position.

the supraspinatus,

Hence, in the infraspinatus,

can act as either minor depending

external

or

on the exact rotational

position of the humerus (Fig. 4). Table 4 shows the mean magnitude

of moment arm

for each muscle. The values logically

progress as one

considers the muscles going from deep to superficial. The only previous study to quote measurements

for

muscle moment arms was that of Poppen and Walker

(1976). Their study of the shoulder in abduction expressed the moment arm values in two-dimensional terms of the plane of abduction, ignoring the anterior-posterior component. At 90” of shoulder abduction with neutral rotation. the supraspinatus is oriented to act as an abductor with little, if any, function as a flexor or extensor. The moment arm of the muscle described here is 21 mm which compares favorably with the 22 mm value described by Poppen and Walker (1976). However, in the position of abduction and external rotation, we observed the major component of the moment arm of the supraspinatus to

An, K. N., Hui. F. C.. Morrcy. 8. F., Linsehcid. R. L. and Chao, E. Y. (1981) Muscles across the elbow joint: a biomcchanieal analysis. J. Binmechunics 14, 659-669. An. K. N.. Ueba. Y.. Chao. E. Y.. Cooncy, W. P. and Linscheid. R. L. (1983)Tcndon excursion and moment arm of index linger muscles. J. Biomcchunics 16.419-425. Basmajian. J. V. (1971) Musckes Alioe--Their Fun&m Reueded by Elecrromyography. Williams & Wilkins. Baltimore. Brand, P. W.. Beach. M. A. and Thompson, D. E. (1981) Relative tension and potential excursion of muscles in the forearm and hand. J. ffund Surg, 6. 209-219. DcLuca, C. J. and Forrest, W. J. (1973) Force analysis of individual muscles acting rimultancously on the shoulder joint during isometric abduction. J. Biomechunics 6, 385393. Halley, D. K. and Olix. M. L. (1975) A review of the Bristow operation for recurrent anterior shoulder dislocation in athletes. Clin. Orlhop. 106, 175-179. Ikai. M. and Fukunaga. T. (1968) Calculation of muscle strength per unit of cross-sectional arca of human muscle. Z. Anyew. Physiol. Einschl. Argeilphysiof. 26, 26. Inman, V. T., Sanders, M. and Abbott, L. C. (1944) Obscrvations on the function of the shoulder joint. J. Bone Jr Sury. 26A. I-30. Jensen. R. H. and Davy, D. T. (1975) An investigation of muscle lines of action about the hip: a centroid line approach vs the straight line approach. J. Bbmerhunics 8, 103-l IO. Jones, D. W. (1970) The role of shoulder muscles in control of humeral position (an electromyographic study). Master’s Thesis, Case Wcstcrn Rcscrve University. Lombardo. S. J.. Kerlan, R. K.. Jobc. F. W.. Carter. V. S., Blazina. M. E. and Shields, C. L. (1976) The modified Bristow proecdure for recurrent dislocation of the shoulder. J. Bone Jr Surg. SSA. 256-261. Magnuson. P. 8. and Stack, J. K. (1943) Recurrent dislocation of the shoulder. J. Am. med. Ass. 12X889-892. May. V. R. (1970) A modified Bristow operation for anterior recurrent dislocation of the shoulder. J. Bone Jr Sura. 52A. 1010-1016. Morrcy. 8. F. and Chao, E. Y. S. (1976) Passive motion of the elbow joint. J. Bone Jr Surg. 58A, 501408.

Glcnohumcral

Poppcn,

N. K. and Walker.

normal

motion

195-201. Poppcn. N. K. glenohumcral 165-170.

P. S. (1976)

of the shoulder.

muscle force and moment mrchamcs

Normal

J. Bone JI

and ab

Surg.

IA,

and Walker, P. S. (1978) Forces at the joint in abduction. Clin. Orthop. 135,

415

Steno, N. (1667). Efemvntorum myokogiae specimen s. muscufi descriprio geomerriccl. Inman. V. (Ed.) ( I9 IO) Opera Philosophico, Vol. 2. p. 108. Copenhagen. Quoted in Bastholm. E. (195C) The History of ~%fuscle Physiology. Ejnar Munksgaard, Copenhagen.