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. MOTION AT THE SHOULDER JOINT 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, rotation of the is motion is more than has been The rotation of the scapula on the elevation of the is only possible of the motion at the two and the phase and of movement is between Elevation of the arm is accompanied by elevation of the at the This movement early and is the first degrees, when for every 10 degrees of elevation of the there are degrees of elevation of the degrees, motion at this joint is at the joint with that at the The total range is degrees and occurs both in the first 30 degrees of abduction, and after degrees of elevation of the these two there is no motion of this The sum of the movements at the and joints is to the range of movement the the two bones together by of their For this it was to motion of such occur at the joint, in of the that the is at its to the scapula the of the For motion to occur at the joint in the of elevation of the of this would to be and on first this would seem to be of the of the of the we a of the only by the on its so as to this to as a We to the of this rotation by insertion of a into the bone of the living subject. We that rotation was a fundamental of shoulder motion, for by rotation of the the range of motion at the shoulder was to degrees. out our of when we attempted to a of by the We that, this was to the beyond 60 degrees. We therefore, interested in an article by in which he a of by of the We would that, of this joint was carried out with the arm to less than 30 degrees, there would be of motion to degrees. was carried out with the arm 30 degrees, the be carried at to degrees these we would some of motion between and degrees of In of the of this joint the of the functional range of abduction, after of the glenohumeral joint, by of the of the MOTION After by of roentgenographic studies and direct the precise of the to one and the which motion, we to up and to the force for the of the flexion or the these at by theoretical and the on a force which at the glenohumeral joint in position of the a of is of these is by the of the extremity, acting at the of of the the the by the and the the of the acting the of rotation in a to that of the This force in the of for there is no muscle capable of in the which it components are the and of the head of the at the the of as the and action is the of rotation of the humeral In order to we two one acting at right to the of the and the to the of the scapula. The theoretical and for these in the range from 30 degrees to degrees of elevation, as 30 degrees is so great as to the highly The the force for elevation, a its at degrees of elevation, the of the force the of the extremity. It to at degrees, when the is in the position the no force is to that In the living mechanism, muscle power is the full range of The force is the which we have up into two components on the one force in the of the of on the the of and The shown in of the motion, is one of the and It with the of the the essential force necessary for elevation of the extremity. This force its at 60 degrees, its is the of the a greater than that of the degrees the force at degrees. The and the is a force of which its at degrees of elevation, it no less than the of the extremity. We should to the of the muscle force as an essential in the of elevation, as this be to in scapular as at the glenohumeral joint, essential the The first of these is the force necessary to the of the entire shoulder therefore, in a and The two the one acting from the of the in a and the from the angle in an or forward The force acting on the angle is by the acting on the is in and in The component is by the of the The or component is by the of the which in the of the girdle. The force acting from the is a the of both the and the The of the is a the entire range of motion, and is consequently by a The necessary to the and lower components of the force are but in and their at degrees of elevation, to at degrees. to the in position of the scapula elevation, the of the and components by the of the was to in its angle of action. This an mechanism in the action of the In the the muscle is entirely the first degrees of elevation, the angle of action of the muscle so that its force is equally between the and the to degrees, the muscle is more effective as a with its power at degrees, and beyond degrees its and its This mechanism of action of the is by elevation of the shoulder as a whole. In the is the more the muscle for this As rotation of the scapula in the angle of action of the muscle it to the of the more The of the the in to acting on the glenohumeral joint, is very The their at about degrees, when are only the of the extremity. In order to the and the of the in the comparative studies and by analysis of the of the shoulder mechanism, we have employed an entirely of The in the muscle motion in the living subject. The action of the muscle are in their The in potential so obtained are and The is not only for the of an muscle motion, but for the study of the phase of action of muscle in free The use of has to the action of a of have that a direct relationship between the in a muscle and the action current as by the of the This relationship is not a for in normal muscle the action potential more than the in the precise relationship the of the and is as a It is to that by somewhat In with the action potential far more than in the The of the is and the relationship We have not been as to the of these in the abnormal As the of muscle upon the between the we are thus to the either of the entire muscle or of of its This is an important it that different of the muscle in their of upon the precise motion carried the of their action potential and phase of we have been to based upon from four to a of for each muscle in the shoulder of these that we group the into functional than into The and of the The The muscle its between and degrees of elevation, the between these In the total potential is of less than in the for It an initial at degrees, to degrees, and to the as that for when the arm is the The in in the various of the and these in turn with the of motion carried In no of the is In forward the head is a at degrees, and a at degrees. The of the head is the only of the muscle to in it is at degrees, to at degrees. The lower and are in both flexion and studies clearly that the head of the with the in forward and for the of the action potential of the in this The for the a which is of the force by In forward flexion it its before that of the the at degrees, that of is at degrees. The of the in both flexion and are the It is that the is the of The that this is not the The muscle together with the the entire range of we the action of all the we that and a which very with that from analysis of the force for elevation of the In abduction, the is at degrees and flexion at degrees. It be that the flexion is of somewhat greater than that of abduction, and further is to the We would for the in in flexion as due to the of the head of the whose force is in this movement, not only for elevation, but to the forward position of the The functional would for the of of the and its to the right when to the not to at degrees of elevation, that, the theoretical muscle is as is to be to the the However, in this position their is The of the The and a functional group as from the force as the or component of the force therefore, to both and In abduction, of the in to its at degrees. In the is somewhat is irregular, and two of the first at 60 degrees, and a of greater at 120 degrees. Thereafter the to with the The in a to that of the The is but at 120 degrees of In the total is to its between and 120 degrees, to with the of The of of the with of the and in that the picture is In this muscle the is of potential than the and its at degrees, is from there to and to The of the flexion is and its is at and thereafter with that for we the of these muscles, we that on the whole a remarkably to that of abduction, that the of the occurs a at to 120 degrees, the flexion in than that of an important and in the for which is between 60 and degrees. This occurs at the which force analysis is the period when the action of the lower force should be We that this is by the of two essential which are necessary for The first is due to the action and the to the of in their as this is is shown by the of these when the action is by the of a and so as to this movement in one or the We for example that we are to the of these by passively the power the of to of The more we with such the more we the power of in these we have in no the power of the by an of the power of the muscle for the of we have the lower of the and in its for The occupies a special position in the The muscle motion, but a in that it only into action when it is necessary to a In it its at about degrees. The in elevation of the extremity, therefore, is not a but is important in a its with the of The The scapular are a functional We have pointed out from comparative studies that both the and of division into and lower This is in the functional of these of the muscles. The and of the a whose are the The support of the elevation of the in is the component of the force necessary for scapular The of the group is by the that these an action current potential the arm is at rest. elevation of the extremity, both in flexion and abduction, the of their action current potential in its when the arm is the The degrees, for analysis has shown that the of the muscle at this point one with rotation of the scapula. The of the and the lower four of the the lower component of the scapular force and are to elevation of the in a The of their action is at and their at degrees of The of the and the and of the which are of one The with and in abduction, the and of the in The therefore, that the lower is the more component of the lower force in abduction, for in flexion it somewhat to the scapula to forward as a and the component of the lower force is by the lower of the The of the muscle is in abduction, when its potential to a at degrees, and at degrees. In forward its action potential in the early of movement, and up to degrees. The that the to the scapula in its of motion abduction, and somewhat in forward flexion to the scapula to the thorax. The in the as the are in In their is to between 60 and degrees, after which it to its at degrees. The that the and the are but in degrees is reached, a position different the arm has been carried to that by flexion or by abduction, the and the flexion and The to an understanding of the functional mechanism of the shoulder to with fundamental principles which are of to the in procedures for both and affecting this The obtained from comparative the in the of the the of the the theoretical force requirements, and the basic of the in the as by is and in the the have of and development of the with a of the and as with these the the of the scapula and acting on the by the necessary components of a force together with the about the motion necessary for The the and by functional and from the lower and lower have the components of a force acting upon the scapula. have been in of the angle of the together with of muscle such as the has a the group of the glenohumeral joint, and at the time in the arm for the action of the lower and as a for the scapula. such as the of the insertion of the and the of the are in to the by this of the and should be upon as the essential The studies of joint motion prove that elevation of the in either the or is on free motion in all the joints of the shoulder The that to the right angle entirely at the glenohumeral joint, and that, full elevation is by motion of the scapula on the chest wall, is incorrect. the it should be that motion occurs in all the joints of the each its share to the of the The of the rhythm of and motion joints and the of power in the that We may in that early of elevation of the the joint its of movement, and in the the the glenohumeral and scapulothoracic the from the to the of the is two to so that for every 15 degrees of elevation, the glenohumeral 10 degrees, and the scapulothoracic 5 degrees. It is in to that of rhythm or loss of motion in one phase of motion may that this is due to loss of at the joint or joints which contribute the share of movement that Furthermore, of one of the joints in the complex a loss in of movement in direct to the of movement contributed by that In of the joint, as by some surgeons for its a loss of in greater or less in the of this movement, should be at the joint not only abduction, but greatly its the an of the glenohumeral joint, he should be that the range is but 60 degrees of that further experimental that this range be by of the of the with of the The studies of the basic in the living clearly the of the of for example, no such as a as are only of action. This and the down by when he with to the that it of the action of the muscles, but only of We a of motion carried to the Furthermore, we are to that the principles as by are entirely incorrect. that an muscle has but one function. of various of an muscle that can but in with the total of a This great is fundamental for rational procedures of muscle of be by of the muscle can only be brought into by out the precise motion which brings into It is our that the rational for the employed in the of and motion is the only to in phase and the muscle for the of the as a whole. A further is in the of certain which a in the of a of the extremity. be in mind when are made to such as in reconstruction procedures. This action has been for example, when the to the to motion of the and came into only when a position was
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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. MOTION AT THE SHOULDER JOINT 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, rotation of the is motion is more than has been The rotation of the scapula on the elevation of the is only possible of the motion at the two and the phase and of movement is between Elevation of the arm is accompanied by elevation of the at the This movement early and is the first degrees, when for every 10 degrees of elevation of the there are degrees of elevation of the degrees, motion at this joint is at the joint with that at the The total range is degrees and occurs both in the first 30 degrees of abduction, and after degrees of elevation of the these two there is no motion of this The sum of the movements at the and joints is to the range of movement the the two bones together by of their For this it was to motion of such occur at the joint, in of the that the is at its to the scapula the of the For motion to occur at the joint in the of elevation of the of this would to be and on first this would seem to be of the of the of the we a of the only by the on its so as to this to as a We to the of this rotation by insertion of a into the bone of the living subject. We that rotation was a fundamental of shoulder motion, for by rotation of the the range of motion at the shoulder was to degrees. out our of when we attempted to a of by the We that, this was to the beyond 60 degrees. We therefore, interested in an article by in which he a of by of the We would that, of this joint was carried out with the arm to less than 30 degrees, there would be of motion to degrees. was carried out with the arm 30 degrees, the be carried at to degrees these we would some of motion between and degrees of In of the of this joint the of the functional range of abduction, after of the glenohumeral joint, by of the of the MOTION After by of roentgenographic studies and direct the precise of the to one and the which motion, we to up and to the force for the of the flexion or the these at by theoretical and the on a force which at the glenohumeral joint in position of the a of is of these is by the of the extremity, acting at the of of the the the by the and the the of the acting the of rotation in a to that of the This force in the of for there is no muscle capable of in the which it components are the and of the head of the at the the of as the and action is the of rotation of the humeral In order to we two one acting at right to the of the and the to the of the scapula. The theoretical and for these in the range from 30 degrees to degrees of elevation, as 30 degrees is so great as to the highly The the force for elevation, a its at degrees of elevation, the of the force the of the extremity. It to at degrees, when the is in the position the no force is to that In the living mechanism, muscle power is the full range of The force is the which we have up into two components on the one force in the of the of on the the of and The shown in of the motion, is one of the and It with the of the the essential force necessary for elevation of the extremity. This force its at 60 degrees, its is the of the a greater than that of the degrees the force at degrees. The and the is a force of which its at degrees of elevation, it no less than the of the extremity. We should to the of the muscle force as an essential in the of elevation, as this be to in scapular as at the glenohumeral joint, essential the The first of these is the force necessary to the of the entire shoulder therefore, in a and The two the one acting from the of the in a and the from the angle in an or forward The force acting on the angle is by the acting on the is in and in The component is by the of the The or component is by the of the which in the of the girdle. The force acting from the is a the of both the and the The of the is a the entire range of motion, and is consequently by a The necessary to the and lower components of the force are but in and their at degrees of elevation, to at degrees. to the in position of the scapula elevation, the of the and components by the of the was to in its angle of action. This an mechanism in the action of the In the the muscle is entirely the first degrees of elevation, the angle of action of the muscle so that its force is equally between the and the to degrees, the muscle is more effective as a with its power at degrees, and beyond degrees its and its This mechanism of action of the is by elevation of the shoulder as a whole. In the is the more the muscle for this As rotation of the scapula in the angle of action of the muscle it to the of the more The of the the in to acting on the glenohumeral joint, is very The their at about degrees, when are only the of the extremity. In order to the and the of the in the comparative studies and by analysis of the of the shoulder mechanism, we have employed an entirely of The in the muscle motion in the living subject. The action of the muscle are in their The in potential so obtained are and The is not only for the of an muscle motion, but for the study of the phase of action of muscle in free The use of has to the action of a of have that a direct relationship between the in a muscle and the action current as by the of the This relationship is not a for in normal muscle the action potential more than the in the precise relationship the of the and is as a It is to that by somewhat In with the action potential far more than in the The of the is and the relationship We have not been as to the of these in the abnormal As the of muscle upon the between the we are thus to the either of the entire muscle or of of its This is an important it that different of the muscle in their of upon the precise motion carried the of their action potential and phase of we have been to based upon from four to a of for each muscle in the shoulder of these that we group the into functional than into The and of the The The muscle its between and degrees of elevation, the between these In the total potential is of less than in the for It an initial at degrees, to degrees, and to the as that for when the arm is the The in in the various of the and these in turn with the of motion carried In no of the is In forward the head is a at degrees, and a at degrees. The of the head is the only of the muscle to in it is at degrees, to at degrees. The lower and are in both flexion and studies clearly that the head of the with the in forward and for the of the action potential of the in this The for the a which is of the force by In forward flexion it its before that of the the at degrees, that of is at degrees. The of the in both flexion and are the It is that the is the of The that this is not the The muscle together with the the entire range of we the action of all the we that and a which very with that from analysis of the force for elevation of the In abduction, the is at degrees and flexion at degrees. It be that the flexion is of somewhat greater than that of abduction, and further is to the We would for the in in flexion as due to the of the head of the whose force is in this movement, not only for elevation, but to the forward position of the The functional would for the of of the and its to the right when to the not to at degrees of elevation, that, the theoretical muscle is as is to be to the the However, in this position their is The of the The and a functional group as from the force as the or component of the force therefore, to both and In abduction, of the in to its at degrees. In the is somewhat is irregular, and two of the first at 60 degrees, and a of greater at 120 degrees. Thereafter the to with the The in a to that of the The is but at 120 degrees of In the total is to its between and 120 degrees, to with the of The of of the with of the and in that the picture is In this muscle the is of potential than the and its at degrees, is from there to and to The of the flexion is and its is at and thereafter with that for we the of these muscles, we that on the whole a remarkably to that of abduction, that the of the occurs a at to 120 degrees, the flexion in than that of an important and in the for which is between 60 and degrees. This occurs at the which force analysis is the period when the action of the lower force should be We that this is by the of two essential which are necessary for The first is due to the action and the to the of in their as this is is shown by the of these when the action is by the of a and so as to this movement in one or the We for example that we are to the of these by passively the power the of to of The more we with such the more we the power of in these we have in no the power of the by an of the power of the muscle for the of we have the lower of the and in its for The occupies a special position in the The muscle motion, but a in that it only into action when it is necessary to a In it its at about degrees. The in elevation of the extremity, therefore, is not a but is important in a its with the of The The scapular are a functional We have pointed out from comparative studies that both the and of division into and lower This is in the functional of these of the muscles. The and of the a whose are the The support of the elevation of the in is the component of the force necessary for scapular The of the group is by the that these an action current potential the arm is at rest. elevation of the extremity, both in flexion and abduction, the of their action current potential in its when the arm is the The degrees, for analysis has shown that the of the muscle at this point one with rotation of the scapula. The of the and the lower four of the the lower component of the scapular force and are to elevation of the in a The of their action is at and their at degrees of The of the and the and of the which are of one The with and in abduction, the and of the in The therefore, that the lower is the more component of the lower force in abduction, for in flexion it somewhat to the scapula to forward as a and the component of the lower force is by the lower of the The of the muscle is in abduction, when its potential to a at degrees, and at degrees. In forward its action potential in the early of movement, and up to degrees. The that the to the scapula in its of motion abduction, and somewhat in forward flexion to the scapula to the thorax. The in the as the are in In their is to between 60 and degrees, after which it to its at degrees. The that the and the are but in degrees is reached, a position different the arm has been carried to that by flexion or by abduction, the and the flexion and The to an understanding of the functional mechanism of the shoulder to with fundamental principles which are of to the in procedures for both and affecting this The obtained from comparative the in the of the the of the the theoretical force requirements, and the basic of the in the as by is and in the the have of and development of the with a of the and as with these the the of the scapula and acting on the by the necessary components of a force together with the about the motion necessary for The the and by functional and from the lower and lower have the components of a force acting upon the scapula. have been in of the angle of the together with of muscle such as the has a the group of the glenohumeral joint, and at the time in the arm for the action of the lower and as a for the scapula. such as the of the insertion of the and the of the are in to the by this of the and should be upon as the essential The studies of joint motion prove that elevation of the in either the or is on free motion in all the joints of the shoulder The that to the right angle entirely at the glenohumeral joint, and that, full elevation is by motion of the scapula on the chest wall, is incorrect. the it should be that motion occurs in all the joints of the each its share to the of the The of the rhythm of and motion joints and the of power in the that We may in that early of elevation of the the joint its of movement, and in the the the glenohumeral and scapulothoracic the from the to the of the is two to so that for every 15 degrees of elevation, the glenohumeral 10 degrees, and the scapulothoracic 5 degrees. It is in to that of rhythm or loss of motion in one phase of motion may that this is due to loss of at the joint or joints which contribute the share of movement that Furthermore, of one of the joints in the complex a loss in of movement in direct to the of movement contributed by that In of the joint, as by some surgeons for its a loss of in greater or less in the of this movement, should be at the joint not only abduction, but greatly its the an of the glenohumeral joint, he should be that the range is but 60 degrees of that further experimental that this range be by of the of the with of the The studies of the basic in the living clearly the of the of for example, no such as a as are only of action. This and the down by when he with to the that it of the action of the muscles, but only of We a of motion carried to the Furthermore, we are to that the principles as by are entirely incorrect. that an muscle has but one function. of various of an muscle that can but in with the total of a This great is fundamental for rational procedures of muscle of be by of the muscle can only be brought into by out the precise motion which brings into It is our that the rational for the employed in the of and motion is the only to in phase and the muscle for the of the as a whole. A further is in the of certain which a in the of a of the extremity. be in mind when are made to such as in reconstruction procedures. This action has been for example, when the to the to motion of the and came into only when a position was
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