Observations of the Function of the Shoulder Joint : Clinical Orthopaedics and Related Research®

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Perspectives in Shoulder Research: The Classic

Observations of the Function of the Shoulder Joint

Inman, Verne; Saunders, J.; Abbott, Leroy

Editor(s): Bigliani, Louis

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Clinical Orthopaedics and Related Research 330():p 3-12, September 1996.
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Attendance in the concert hall or opera house brings the expectation of hearing equally talented individuals playing or singing together to produce the effect desired by the composer. Such ensemble performances are not often encountered in medical literature. The classic article for this symposium is an example of such an exception. John B de CM Saunders came to the University of California Medical School in San Francisco from Edinburgh's Royal Hospital for Sick Children. Born in Grahamstown, South Africa, he received a comprehensive classical education before going to the University of Edinburgh to study medicine. After graduation, he had postgraduate training in orthopaedic surgery. His initial appointment in San Francisco was in the Department of Anatomy. His interest in the history of medicine was shown by his important studies of Vesalius and Leonardo da Vinci. A man of great ability and energy, de CM Saunders was at 1 time the head of 2 departments, Anatomy and the History of Medicine, and dean of the medical school. Later he held the position of provost of the San Francisco campus.

Leroy C. Abbott was born in Madelia, MN. After graduating from the University of California School of Medicine in 1914, he spent a year as a house officer in orthopaedic surgery at the Massachusetts General Hospital in Boston before returning to his alma mater. Immediately after the United States entered into World War I, he went overseas as a member of the Goldthwaite Unit, the group of young American orthopaedic surgeons recruited by Joel E. Goldthwaite. During his period in the service, Abbott worked with Harold Stiles and Robert Jones. After the war, Abbott spent time on the faculty of the University of Michigan in Ann Arbor and Washington University in St. Louis before returning to San Francisco to become Chairman of the Division of Orthopaedic Surgery. Under his aegis, the division grew to become a great department. Abbott was a pioneer in the development of leg lengthening procedures. His work in the production of teaching films on surgical approaches made a valuable contribution to the teaching of residents everywhere.

Upon Abbott's retirement, he was succeeded by his pupil and colleague, Verne T. Inman who was born in San Jose, CA, and educated at the University of California, San Francisco. Inman's area of interest was functional anatomy. He was 1 of the first to use electromyography to analyze muscle function. After World War II, he became interested in lower limb prosthetics. This led to the founding of the Biomechanics Laboratory at the University of California in San Francisco and Berkeley which he directed for 16 years.

The following article, of necessity, has been greatly abbreviated. Anyone with a serious interest in the subject should seek out and read the original in its entirety. It reflects the collective wisdom of 3 gifted observers and serious students of kinesiology.

Leonard F. Peltier, MD, PhD

The manifest complexity of the mechanism of the shoulder joint is evident to anyone who has watched the progression of what Codman has so aptly described as “scapulohumeral rhythm”. This rhythm is participated in by the whole complex of joints constituting the shoulder girdle. Therefore, the term shoulder joint is somewhat misleading, unless we clearly bear in mind that this expression includes no less than four different joints: the sternoclavicular, the acromioclavicular, the scapulothoracic, and the glenohumeral; and that motion at the shoulder is the sum of movement contributed by synchronous participation of all these joint units. In the development of rational procedures for the correction and reconstruction of disabilities affecting the shoulder mechanism, it is necessary to break down this complex into its various components in order that we may uncover some of the fundamental principles underlying their action.

Many such analyses have been carried out in the past, but none, so far as we are aware, have attempted to solve or derive a comprehensive picture of the whole. Much of the early work is very contradictory, and nearly all is incomplete, due to lack of an adequate experimental method. For this reason many misconceptions exist, owing to the too ready facility with which investigators have evolved conceptions based on a priori reasoning from the inert cadaver. In studying functional mechanisms, there is only one touch-stone to which we can appeal, and that is the living body. It is this appeal which enlivens the observations derived from the cadaver to which nonetheless we are compelled to turn on occasion for certain basic information.

It is with such dynamic considerations uppermost, acting as the unifying theme, that we have attacked the problem anew, so as to obtain an unbiased and more vital point of view. Logically and chronologically, we have, therefore, examined the shoulder mechanism from several aspects,-namely the comparative anatomical, the roentgenographic analysis of motion, the theoretical force requirements, and the action current potential derived from the living muscle in motion, and from the data so obtained have attempted a resynthesis of the whole. Our studies have been rendered possible, thanks to the generosity of The National Foundation for Infantile Paralysis to whose support we acknowledge our deep indebtendness.


An essential preliminary to the analysis of the mechanism of the shoulder is an understanding of the sequence of motion which occurs at its component joints. The shoulder joint complex is composed of four independent articulations,-the sternoclavicular, acromioclavicular, scapulothoracic, and glenohumeral joints. While each of these is an independent entity, capable of independent motion, all contribute their share to the total in the normal functional mechanism of the extremity. Furthermore, the participation of each of these joints in the entire movement is simultaneous, and not successive.

We have employed for the elucidation of these movements both roentgenography and the direct insertion of pins into the bones of the living subject.

Elevation of the extremity, both in flexion and in abduction, at the glenohumeral articulation is simultaneously accompanied by scapulothoracic movement, an arrangement which critically enhances the power of the attendant muscles. In the first 30 to 60 degrees of elevation, the scapula seeks, in relationship to the humerus, a precise position of stability which it may obtain in one of several ways. Either the scapula remains fixed, motion occurring at the glenohumeral joint until the stable position is reached, or the scapula moves laterally or medially on the chest wall, or in rare instances oscillates until stabilization is attained. Hence the early phase of motion is highly irregular, and is characteristic for each individual. It would seem to depend upon the habitual position which the scapula occupies in the subject when at rest. This phase of motion is related to the setting action of the muscles, and we have, therefore, termed it “the setting phase”.

Once 30 degrees of abduction, or 60 degrees of forward flexion has been reached, the relationship of scapular to humeral motion remains remarkably constant. Thereafter a ratio of two of humeral to one of scapular motion obtains; and thus between 30 and 170 degrees of elevation, for every 15 degrees of motion, 10 degrees occurs at the glenohumeral joint, and 5 degrees by rotation of the scapula on the thorax.

It should be clearly recognized that the orthodox teaching on these motions is entirely incorrect. The standard textbooks state that glenohumeral motion occurs up to the right angle and that thereafter further elevation is brought about by rotation of the scapula. Roentgenography and examination of the living prove beyond a doubt that scapular and humeral motion are simultaneously continuous. As this ratio pertains, it is evident that the total range of scapular motion is not more than 60 degrees, nor that of the glenohumeral joint greater than 120 degrees. Under special and abnormal conditions, the motions of either one of these two joints can occur independently. For example, when the scapula is fixed, it is possible to raise the arm actively to the right angle, and passively to 120 degrees. However, observation and measurement demonstrate that the loss of the effective bone leverage, owing to lack of scapular participation, consequently diminishes power by a third. It should be pointed out in passing that, for free and full elevation of the extremity, lateral rotation of the humerus is essential.

Clavicular motion is more complicated than has been hitherto suspected. The continuous rotation of the scapula on the thoracic wall during elevation of the extremity is only possible because of the motion permitted at the two clavicular joints, and the phase and amount of movement is unequally distributed between them.

Elevation of the arm is accompanied by elevation of the clavicle at the sternoclavicular joint. This movement begins early and is almost complete during the first 90 degrees, when for every 10 degrees of elevation of the arm, there are 4 degrees of elevation of the clavicle. Above 90 degrees, clavicular motion at this joint is almost negligible.

Motion at the acromioclavicular joint contrasts markedly with that found at the sternoclavicular joint. The total range is approximately 20 degrees and occurs both early, in the first 30 degrees of abduction, and late, after 135 degrees of elevation of the arm. Between these two points there is almost no motion of this joint.

The sum of the movements at the sternoclavicular and acromioclavicular joints is naturally equal to the range of movement permitted the scapula, the two bones being welded together by means of their ligamentous attachments. For this reason, it was difficult to understand how motion of such extent could occur at the acromioclavicular joint, in view of the fact that the clavicle is rigidly attached at its lateral extremity to the scapula through the medium of the coracoclavicular ligament. For motion to occur at the acromioclavicular joint in the plane of elevation of the arm, elongation of this ligament would appear to be necessary, and on first sight this would seem to be impossible. Because of the marked curvature of the outer third of the clavicle, we could envisage a relative elongation of the coracoclavicular ligament, only by the clavicle rotating on its long axis, so as to allow this curvature to act as a crankshaft.

We were able to measure the excursion of this rotation experimentally by insertion of a pin into the bone of the living subject. We found that clavicular rotation was a fundamental feature of shoulder motion, for by manually preventing rotation of the pin, the range of motion at the shoulder was restricted to 110 degrees.

These findings bore out our experience of three years before, when we attempted to treat a case of acromioclavicular dislocation by nailing the joint. We found that, thereafter, this patient was unable to elevate the extremity beyond 60 degrees. We were, therefore, particularly interested in an article by Caldwell in which he discusses a series of cases treated by arthrodesis of the acromioclavicular joint. We would prognosticate that, if fusion of this joint was carried out with the arm elevated to less than 30 degrees, there would be limitation of shoulder-joint motion to slightly below 90 degrees. Whereas, if fusion was carried out with the arm abducted above 30 degrees, the extremity could be carried at least to 135 degrees without difficulty. Even under these circumstances, however, we would expect some limitation of motion between 135 and 180 degrees of elevation.

In addition, knowledge of the significance of this joint suggests the possibility of increasing the functional range of abduction, after arthrodesis of the glenohumeral joint, by excision of the outer end of the clavicle.


After measuring by means of roentgenographic studies and direct methods the precise relationships of the bony parts to one another, and the relative positions which they occupy during motion, we were able to set up equations and to calculate the force requirements for the maintenance of the extremity during flexion or abduction. By the technique employed, these calculations were arrived at by theoretical analysis, and the results were then checked on a force model which yielded empirical data.

To establish equilibrium at the glenohumeral joint in any position of the arm, a minimum of three forces is required. One of these is represented by the weight of the extremity, acting at the center of gravity of the limb; the second, the abducting musculature represented predominantly by the deltoid; and the third, the resultant of the former forces acting through the center of rotation in a direction opposite to that of the deltoid. This last force is, in turn, obviously the resultant of other components, for there is no muscle capable of exercising pull in the direction which it occupies. These components are undoubtedly: first, the pressure and friction of the head of the humerus at the glenoid; and, second, the downward pull of muscles-such as the subscapularis, infraspinatus, and teres minor-whose action is below the center of rotation of the humeral head. In order to represent them, we therefore selected two forces, one acting at right angles to the plane of the glenoid, and the other, parallel to the axillary border of the scapula.

The theoretical and empirical values for these forces were determined in the range from 30 degrees to 180 degrees of elevation, as below 30 degrees individual variation is so great as to make the findings highly questionable.

The curve representing the force requirements for elevation, almost a pure sine wave, reaches its summit at 90 degrees of elevation, where the magnitude of the force required represents 8.2 times the weight of the extremity. It then falls progressively to zero at 180 degrees, because, when the extremity is in the exact vertical position above the head, no force is theoretically required to maintain that position. In the living mechanism, muscle power is expended throughout the full range of motion. The second force is the resultant which we have broken up into two components representing, on the one hand, active force in the form of the downward pull of infraspinous muscles and, on the other, the passive resistance of pressure and friction. The active component, shown in myographic recordings of the muscles during motion, is one of the greatest significance and importance. It establishes, with the pull of the deltoid, the essential force couple necessary for elevation of the extremity. This force reaches its maximum at 60 degrees, where its magnitude is 9.6 times the weight of the extremity*, a pull greater than that of the elevating force. Beyond 90 degrees the force falls rapidly, reaching zero at 135 degrees.

The pressure and friction against the glenoid is a force of considerable size, which reaches its summit at 90 degrees of elevation, where it represents no less than 10.2 times the weight of the extremity.

We should like to emphasize the importance of the muscle force couple as an essential principle in the mechanics of elevation, as this same principle will be seen to operate in scapular rotation. Here, as at the glenohumeral joint, three essential forces participate the mechanism. The first of these is the force necessary to counteract the weight of the entire shoulder girdle which, therefore, acts in a vertical and upward direction. The other two forces establish the rotary couple, one acting from the region of the acromion process in a medial direction, and the other from the inferior angle in an outward or forward direction. The force acting on the inferior angle is represented predominantly by the serratus anterior. That acting on the acromion is in part passive, and in part active. The passive component is resisted by the antagonistic pressure of the clavicle. The active or kinetic component is supplied by the upper portion of the trapezius, which must, however, also, act in supporting the weight of the pectoral girdle. The force acting from the acromion is a resultant, fulfilling the requirements of both the supportive and the rotary roles.

The weight of the extremity is a constant during the entire range of motion, and is consequently represented by a straight line. The forces necessary to establish the upper and lower components of the rotary force couple are equal but opposite in direction, and reach their summit at 90 degrees of elevation, theoretically falling to zero at 180 degrees. Owing to the changes in position of the scapula during elevation, the resultant of the supportive and rotary components supplied by the upper portion of the trapezius was found to fluctuate in its angle of action. This fluctuation reveals an interesting mechanism in the action of the trapezius. In the resting position, the muscle is entirely supportive. With the first 35 degrees of elevation, the angle of action of the muscle changes, so that its force is equally divided between the supportive and the rotary rôles. From 35 to 140 degrees, the muscle is increasingly more effective as a rotator, with its maximum power at 90 degrees, and beyond 140 degrees its rotary efficiency decreases and its supportive rises. This complicated mechanism of action of the upper trapezius is achieved by simple elevation of the shoulder girdle as a whole. In consequence, where the supportive rôle is the more important, the muscle acts predominantly for this purpose. As rotation of the scapula develops, progressive changes in the angle of action of the muscle enable it to serve the requirements of the rotary couple more effectively.

The magnitude of the forces rotating the scapula, in contrast to those acting on the glenohumeral joint, is very much smaller. The forces reach their maximum at about 90 degrees, when they are approximately only twice the weight of the extremity.


In order to correlate the findings and test the validity of the conclusions reached in the comparative anatomical studies and by analysis of the mechanical-force requirements of the shoulder mechanism, we have employed an entirely new method of approach. The method consists in sampling the muscle activity developed during motion in the living subject. The electrical action potentials of the muscle are drawn off through electrodes, implanted in their substance. The differences in potential so obtained are amplified and mechanically recorded. The procedure is valuable, not only for analyzing the activity of an individual muscle during motion, but also for the study of the phase of action of muscle groups participating in any free coordinated movement. The use of six separate amplifiers has enabled us to examine the action of a like number of muscles simultaneously.

Experiments have determined that a direct relationship between the tension developed in a muscle and the action current potential, as recorded by the magnitude of the amplitude, exists. This relationship is not however a linear one, for in normal muscle the action potential rises more rapidly than the increase in tension, the precise relationship being the function of the square, and is expressed as a quadratic equation. It is interesting to observe that muscles affected by poliomyelitis behave somewhat differently. In postpoliomyelitic paralysis with increasing tension, the action potential rises far more rapidly than in the normal. The shape of the curve is changed and the relationship now becomes logarithmic. We have not been able as yet to establish the significance of these changes in the abnormal muscle.

As the sampling of muscle depends upon the distance between the inserted electrodes, we are thus able to record the activity either of the entire muscle or of any of its parts. This is an important asset, because it reveals that different portions of the same muscle differ in their degree of activity, depending upon the precise motion carried out.

From the records of their action potential and phase of activity, we have been able to construct, based upon averages from four to twenty individual observations, a series of curves for each muscle participating in the shoulder motion. Analysis of these reveals that we must group the muscles into functional rather than into topographical groups.

The Abductors and Flexors of the Humerus

The Deltoid

The deltoid muscle exhibits its greatest activity between 90 and 180 degrees of elevation, the curve plateauing between these points. In flexion, the total potential is of less amplitude than in the corresponding curve for abduction. It reaches an initial peak at 110 degrees, plateaus to 130 degrees, and finally rises abruptly to attain the same level as that for abduction when the arm is fully elevated above the head.

Pectoralis Major

The pectoralis major exhibits differences in activity in the various portions of the muscle, and these in turn vary with the type of motion carried out. In abduction no portion of the pectoralis major is active. In forward flexion, the clavicular head is most active, reaching a primary peak at 75 degrees, and a secondary peak at 115 degrees. The manubrial segment of the sternocostal head is the only other part of the muscle to show any activity in flexion. Its activity is, however, slight; it is maximum at 110 degrees, and, thereafter, falls abruptly to zero at 145 degrees. The lower sternal and abdominal portions are inactive in both flexion and abduction. These studies clearly reveal that the clavicular head of the pectoralis major works synchronously with the anterior deltoid in forward flexion, and accounts for the relatively low level of the action potential of the deltoid found in this movement.


The curve for the supraspinatus describes almost a pure sine wave, which is strongly reminiscent of the force curve elaborated by mechanical analysis. In forward flexion it reaches its peak slightly before that of the abduction curve, the maximum being at 80 degrees, while that of abduction is greatest at 100 degrees. The heights of the curves in both flexion and abduction are the same. It is commonly considered that the supraspinatus is the initiator of abduction. The action-potential curves, however, definitely show that this is not the case. The muscle acts together with the deltoid progressively throughout the entire range of motion.

When we summate the action potentials of all the abducting musculature, we find that individual irregularities disappear, and a smooth curve results which compares very closely with that established from analysis of the force requirements for elevation of the arm. In abduction, the summit is reached at 90 degrees and flexion slightly later at 110 degrees. It will be noted that the flexion curve is of somewhat greater amplitude than that of abduction, and further is displaced slightly to the right. We would account for the difference in amplitude in flexion as being due to the activity of the clavicular head of the pectoralis major, whose force is utilized in this movement, not only for elevation, but partly to maintain the forward position of the arm. The additional functional requirement would account for the slight increase of amplitude of the curve and its displacement to the right when compared to the force-requirement curve. Muscle activity does not fall to zero at 180 degrees of elevation, indicating that, unlike the theoretical calculations, muscle activity is still required, as is to be expected, to maintain the extremity above the head. However, in this position their amplitude is reduced.

The Depressors of the Humerus

The subscapularis, infraspinatus, and teres minor constitute a functional group which, as already revealed from the force studies, act as the second or inferior component of the force couple. These muscles were, therefore, found to act continuously throughout both abduction and flexion.

In abduction, activity of the infraspinatus rises in almost linear fashion to attain its summit at 180 degrees. In flexion, the curve is somewhat higher, is irregular, and exhibits two major peaks of activity, the first at 60 degrees, and a second of greater amplitude at 120 degrees. Thereafter the curve falls to coincide with the abduction curve.

The teres minor behaves in a similar fashion to that of the infraspinatus. The abduction curve is almost linear, but slightly peaked at 120 degrees of abduction. In flexion, the total amplitude is higher, rising to its maximum between 90 and 120 degrees, thence falling slightly to coincide with the curve of abduction.

The curve of activity of the subscapularis contrasts with those of the infraspinatus and teres minor in that the picture is reversed. In this muscle the abduction curve is of higher potential than the flexion, and reaches its summit at 90 degrees, is maintained from there to 130, and thence falls abruptly to 180. The amplitude of the flexion curve is lower, and its summit is arrived at later-between 110 and 130 degrees-and thereafter coincides with that for abduction.

If we summate the amplitudes of these three depressor muscles, we find that on the whole they describe a curve remarkably similar to that of abduction, excepting that the summit of the curve occurs a little later,-namely, at 110 to 120 degrees, the flexion curve being slightly higher in amplitude than that of abduction. There is, however, an important and remarkable secondary peak prominent in the curve for abduction which is maximal between 60 and 80 degrees. This secondary peak occurs at the level which force analysis indicates is the period when the depressor action of the lower force couple should be most active. We believe that this curve is formed by the superimposition of two essential forces which are necessary for abduction. The first peak is unquestionably due to the depressor action and the second to the activity of those muscles in their function as rotators. That this interpretation is correct is shown by the behavior of these muscles when the rotary action is minimized by the addition of a lever and weight so as to resist this movement in one or the other direction. We find for example that we are totally unable to suppress the activity of these muscles by passively supplying the rotary power through the medium of weights applied to varying lengths of lever arm. The more we increase rotary resistance with such apparatus, the more we weaken the power of abduction. Although in these experiments, we have in no way altered the power of the abducting musculature, nonetheless, by absorbing an increasing amount of the depressor power of the muscle for the purposes of rotation, we have weakened the lower element of the couple, and in consequence minimized its effectiveness for abduction.

Teres Major

The teres major occupies a special position in the scapulohumeral musculature. The muscle never exhibits any activity during motion, but plays a peculiar rôle in that it only comes into action when it is necessary to maintain a static position. In static positions, it reaches its maximum activity at about 90 degrees. The teres major, in elevation of the extremity, therefore, is not a kinetic muscle, but is important in maintaining a static posture, its activity directly increasing with the increase of weight.

The Scapular Rotators

The scapular rotators are a distinct functional group. We have already pointed out from comparative anatomical studies that both the trapezius and serratus anterior show evidence of division into separate upper, intermediate, and lower components. This anatomical separation is reflected in the functional activity of these portions of the muscles.

The upper trapezius, levator scapulae, and upper digitations of the serratus anterior constitute a unit whose activities are essentially the same. The unit performs three functions: passive support of the shoulder, active elevation of the shoulder, and, in addition is the upper component of the force couple necessary for scapular rotation. The postural function of the group is evidenced by the fact that these muscles exhibit an action current potential while the arm is at rest. With elevation of the extremity, both in flexion and abduction, the amplitude of their action current potential rises in linear form reaching its maximum when the arm is above the head. The curve undulates slightly above 90 degrees, for mechanical analysis has shown that the supportive function of the muscle at this point becomes one predominantly assocaited with rotation of the scapula.

The inferior portion of the trapezius and the lower four digitations of the serratus anterior constitute the lower component of the scapular rotary force couple, and are found to act throughout elevation of the extremity in a complementary manner. The general shape of their action curves is similar, starting at zero and reaching their maximum at 180 degrees of elevation. The inferior portion of the trapezius and the seventh and eighth digitations of the serratus anterior describe curves which are mirror images of one another. The inferior trapezius acts with slight undulations continuously and predominantly in abduction, the seventh and eighth digitations of the serratus predominantly in flexion. The curves demonstrate, therefore, that the lower trapezius is the more active component of the lower force couple in abduction, for in flexion it relaxes somewhat to allow the scapula to migrate forward as a whole, and the major component of the lower force couple is then supplied by the lower digitations of the serratus anterior.

The intermediate portion of the trapezius muscle is most active in abduction, when its potential rises to a maximum at 90 degrees, flattens off, and finally falls slightly at 180 degrees. In forward flexion, its action potential decreases in amplitude during the early ranges of movement, and then builds up slightly to 180 degrees. The findings indicate that the middle trapezius functionally serves to fix the scapula in its plane of motion during abduction, and relaxes somewhat in forward flexion to allow the scapula to rotate around the thorax.

The rhomboid muscles function in much the same manner as the middle trapezius, and, like it, are most active in abduction. In flexion, their amplitude is found to flatten off between 60 and 150 degrees, after which it rises sharply to its maximum at 180 degrees. The curves indicate that the rhomboid and the middle trapezius are but little active in flexion. Once, however, 140 degrees is reached, a position little different whether the arm has been carried to that level by flexion or by abduction, the muscles strongly contract and the flexion and abduction curves superimpose.


The eclectic approach to an understanding of the functional mechanism of the shoulder enables us to lay down, with considerable certainty, fundamental principles which are of supreme significance to the surgeon in planning reconstructive procedures for both postpoliomyelitic paralysis and other disorders affecting this region. The evidence obtained from comparative anatomical trends, the changes in the relative size of the musculature, the elaboration of the bony parts, the theoretical force requirements, and the basic behavior of the muscles in the living, as revealed by myographic analysis, is consistent, and points in the same general direction.

Briefly, the morphological changes have consisted of elaboration and massive development of the deltoid, with a corresponding suppression of the supraspinatus and biceps muscles as active abductors. Associated with these changes, the muscles lying below the spine of the scapula and acting on the humerus assume increasing importance; by establishing the necessary components of a force couple, they, together with the deltoid, bring about the rotary motion necessary for elevation. The upper trapezius, the upper serratus, and levator angulae scapulae, by functional and morphological separation from the lower trapezius and lower serratus, have produced the components of a similar force couple acting upon the scapula. These changes have been attained structurally in surprisingly simple fashion. Elongation of the inferior angle of the scapula, together with minor migrations of muscle masses, such as the teres minor, has accomplished a twofold object informing the depressor group of the glenohumeral joint, and at the same time in increasing the lever arm for the action of the lower trapezius and serratus muscles as a rotary unit for the scapula. Alterations, such as the distal migration of the insertion of the deltoid and the increasing mass of the acromion, are relatively minor in comparison to the effects achieved by this extension of the scapula, and should be looked upon as adjustments perfecting the essential mechanism.

The studies of joint motion prove conclusively that complete elevation of the arm, in either the coronal or frontal plane, is dependent on free motion in all the joints of the shoulder complex. The old teaching, still commonly accepted, that abduction to the right angle takes place entirely at the glenohumeral joint, and that, thereafter, full elevation is completed by motion of the scapula on the chest wall, is incorrect. On the contrary, it should be stressed that motion occurs in all the joints of the region simultaneously, each contributing its share to the completion of the movement. The maintenance of the rhythm of smooth and coordinated motion requires intact joints and the preservation of power in the muscles that move them.

We may say in summary, that during early phases of elevation of the arm, the sternoclavicular joint passes through its greatest ranges of movement, and in the terminal phase, the acromioclavicular. At the glenohumeral and scapulothoracic articulations, the ratio, from almost the beginning to the termination of the arc, is respectively two to one, so that for every 15 degrees of elevation, the glenohumeral contributes 10 degrees, and the scapulothoracic 5 degrees.

It is helpful in diagnosis to recognize that disturbance of rhythm or actual loss of motion in any one phase of motion may indicate that this disturbance is due to loss of function at the joint or joints which contribute the major share of movement during that phase. Furthermore, ankylosis of any one of the joints associated in the complex will cause a permanent loss in degree of movement in direct proportion to the amount of movement contributed by that joint. In fusion of the acromioclavicular joint, as practiced by some surgeons for its dislocation, a permanent loss of abduction in greater or less degree in the terminal phases of this movement, should be anticipated. Restriction at the sternoclavicular joint will not only limit abduction, but will greatly diminish its power. When the surgeon performs an arthrodesis of the glenohumeral joint, he should be aware that the maximum range is but 60 degrees of actual motion. Evidence, that awaits further experimental confirmation, suggests that this range might be increased by resection of the outer end of the clavicle with preservation of the coracoclavicular ligaments.

The myographic studies of the basic activity in the living clearly reveal the nature of the pattern of muscular activity. There is, for example, no such thing as a prime mover, as ordinarily understood. There are only patterns of action. This conception extends and amplifies the axiom laid down by Beevor when he said with regard to the brain that it knows nothing of the action of the individual muscles, but only of movement. We can, then, see a central pattern of motion carried to the periphery. Furthermore, we are forced to recognize that the principles as adumbrated by MacKenzie are entirely incorrect. MacKenzie contends that an individual muscle has but one function. Sampling of various portions of an individual muscle shows that they can act independently, but synchronously in association with the total pattern of a specific movement. This great principle is absolutely fundamental for establishing rational procedures of muscle re-education. These patterns of muscular activity cannot be reduplicated by voluntary contraction of the muscle alone. They can only be brought into play by carrying out the precise motion itself, which probably brings into play proprioceptive mechanisms. It is our opinion that here lies the rational explanation for the empirical methods employed in the Kenny treatment of poliomyelitis. Motion, and motion alone, is the only known stimulus able to engender, in phase and degree, the muscle activity requisite for the establishment of the pattern as a whole. A further principle is demonstrated in the existence of certain muscles which play a static rôle in the maintenance of a posture of the extremity. These must be borne in mind when attempts are made to utilize such muscles as kinetic elements in reconstruction procedures. This static action has been confirmed clinically, for example, when the teres major, transplanted to replace the infraspinatus group, failed to act during motion of the extremity and came into play forcibly only when a static position was maintained.

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