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Tuesday, June 6, 2023

Observations on the Function of the Shoulder Joint

Brent Brookbush

Brent Brookbush


Research Review: Observations on the Function of the Shoulder Joint

By Jinny McGivern DPT, PT, Certified Yoga Instructor

Edited by Brent Brookbush DPT, PT, COMT, MS, PES, CES, CSCS, ACSM H/FS

Original Citation: Inman, V. T., Saunders, D.M., and Abbott, L.C. (1944). Observations on the Function of the Shoulder Joint. Journal of Bone and Joint Surgery, 26-A, 1-30. ABSTRACT

Why is this relevant?: According to Google scholar, Inman, Saunders and Abbott's article Observations on the Function of the Shoulder Joint (1944) has been cited in 1,375 publications between its original publication date and today (1/10/16). It provides an early comprehensive view of shoulder structure and upper extremity (UE) function, covering topics from anatomy to kinematics to EMG analysis. The concepts discussed in this paper provide a foundation for how human movement scientists view, research, assess and manage the shoulder girdle complex today.

Study Summary

Study Design Descriptive study
Level of Evidence VI - Evidence from a single descriptive or qualitative study
Subject Demographics Not provided
Outcome Measures Comparative Anatomy (compared to other species of primate and lower mammals)

The Scapula

  • Size/shape of the scapula
  • Size of the supraspinous fossa compared to the overall size of the scapula
  • Size of the infraspinous fossa compared to the overall size of the scapula
  • Size/shape of the acromion process

The Humerus

  • Location of the deltoid tuberosity
  • Quantity of torsion

The Scapulohumeral group of muscles (supraspinatus, infraspinatus, teres minor, subscapularis, deltoid, teres major)

  • Changes in mass of an individual muscle as compared to the group.

The Axioscapular group of muscles (trapezius, rhomboids, serratus anterior, levator scapula)

  • Changes in mass of an individual muscle as compared to the group.

The Axiohumeral group of muscles (pectoralis major, pectoralis minor, latissimus dorsi)

  • Changes in mass of an individual muscle as compared to the group.

The muscles of the upper arm (biceps brachii, triceps brachii)

  • Changes in mass of an individual muscle as compared to the group.

Observations of motion at the shoulder joint complex (made via X-ray and bone inserted pin motion analysis)

  • Early stages of elevation of the UE (0-30 degrees for abduction & 0-60 degrees for flexion)
  • Beyond 30 degrees of abduction or 60 degrees of flexion
  • Total range of glenohumeral motion
  • Total range of scapula motion
  • Motion at the SC joint
  • Motion at the AC joint
  • Motion of the clavicle

Mechanical Requirements for Shoulder Joint Motion (calculated theoretically, then tested on a force model)

  • Forces necessary for UE elevation
  • Maximal force requirements during elevation at the glenohumeral joint
  • Force requirements during elevation at the scapulothoracic joint

Myographic studies (Muscles grouped functionally as opposed to structurally. Amplitude of electrical activity was recorded via EMG for UE flexion and abduction)

Abductors and Flexors of the Humerus

Depressors of the humerus

Scapula Rotators

Results Comparative Anatomy (compared to other species of primate and lower mammals)

The Scapula

  • Size/shape of the scapula: In humans, the superior to inferior length of the scapula is greater than other primates.
  • Size of the supraspinous fossa compared with the size of the scapula: Similar between primates and humans
  • Size of the infrapinous fossa compared with the size of the scapula: Greatly increased in humans and larger primates; this accounts for much of the increased superior to inferior length of the scapula in humans
  • Size/shape of the acromion process: Increased mass & size in humans; increased coverage of the humeral head in humans and larger primates.

The Humerus

  • Location of the deltoid tuberosity: situated more distally in humans; thought to allow the deltoid greater mechanical advantage.
  • Quantity of torsion: increased in humans due to the dual needs of the head of the humerus to follow the scapula as its position shifts to the posterior aspect of the thoracic cage and for the axis of motion of the elbow to remain in the coronal plane so movement of the elbow occurs in the sagittal plane.
  • Altered position of the bicipital groove to be closer to the lesser tubercle (consequence of humeral torsion).

The Scapulohumeral group of muscles

  • The Supraspinatus demonstrates a reduced relative mass in humans (although actual mass remains similar).
  • The Deltoids demonstrate a significant increase in relative and absolute mass in humans and in primates with greater use of a free upper extremity (UE). The mechanical advantage of the Deltoids is also increased due to the enlargement of the acromion and the distal migration of the deltoid tubercle.
  • The Teres Minor appears to originally have been a part of the Deltoids. It became a distinct structure as the superior to inferior dimensions of the scapula increased (primitive mammals have not been observed to have a teres minor). Humans demonstrate a larger relative teres minor as compared to other mammals, with the Gibbon and Chimpanzees having the closest size to humans. The line of pull of the teres minor has been altered so that it exerts a downward force on the humerus.
  • The Subscapularis demonstrates a slight decrease in relative size, as well as an alteration in its attachment point on the scapula in humans. Subscapularis is the largest muscle of this group in other mammals. Although somewhat decreased in relative size, humans possess an increased number of inferior fasciculi (another consequence of increased superior to inferior size of the scapula). This allows subscapularis to also have an inferior line of pull on the humerus, similar to Teres Minor.
  • The Infraspinatus also demonstrates an expansion of its inferior point of attachment as the length of the scapula increases from primates to humans. This allows Infraspinatus to join the Teres Minor and Subscapularis in exerting an inferior line of pull on the humerus.
  • The Teres Major demonstrates a decreased relative mass in humans as compared to other primates, especially those where climbing is a regular activity.

The Axioscapular group of muscles

  • The Trapezius muscle does not demonstrate major changes in size between primates and humans. It does demonstrate an increase in the quantity of muscle fibers along its superior and inferior borders and less development of its middle portion.
  • The Serratus Anterior and Levator Scapulae appear to be part of a single muscle sheet in lower mammals. In humans, an intermediate portion of this muscle sheet does not exist therefore the muscles are separate and distinct. The serratus anterior demonstrates changes similar to that of the Trapezius, where there is an increase in fiber concentration at its superior and inferior portions. This result in a less developed middle portion of this muscle.
  • The rhomboids do not demonstrate many differences in size and morphology between humans and lower mammals.

The Axiohumeral group of muscles

  • The Pectoralis Major and Minor appear to be joined as one structure in lower mammals. In humans, these muscles separate into superficial and deep layers. The superficial layer of Pectoralis Major expands more superiorly in humans than other primates (creating the clavicular head of the muscle). The proximal attachment for the deeper Pectoralis Minor is shifted to the scapula in humans, while in some species remains on the humerus. The subdeltoid bursa exists under the acromion in only chimps and humans.
  • Similar to the Teres Major, the Latissimus Dorsi demonstrates a slightly decreased relative mass in humans as compared to other primates, especially those where climbing is a regular activity.

The muscles of the upper arm

  • In lower mammals, the biceps brachii demonstrates a single proximal attachment point and performs upper extremity (UE) elevation in conjunction with a much larger Supraspinatus. Dual attachment points appear in primates, and remain in humans. Because of the altered location of the bicipital groove in humans, the biceps can assist in elevating the UE with the humerus in maximal external rotation.
  • The triceps brachii in humans demonstrates similar structure to that of other mammals & primates. The long head has a reduced length in humans.

Observations on motion of the shoulder joint complex (made via X-ray and bone inserted pin motion analysis)

  • Early stages of elevation of the UE (0-30 degrees for abduction & 0-60 degrees for flexion): the scapula and humerus adjust to attain a stable position with respect to each other. Motion may occur at the scapula or the humerus depending on the position of the UE at rest. There is a tremendous variability in strategy from person to person. The authors hypothesize that this occurs to allow efficient muscle contraction of the elevating musculature.
  • Beyond 30 degrees of abduction or 60 degrees of flexion: the humerus and scapula demonstrate a ratio of 2:1 motion (i.e. for every 30 degrees of motion, 20 degrees occur at the glenohumeral joint and 10 degrees occur at the scapulothoracic joint). There does not appear to be much variability in this strategy from individual to individual. Motion was observed to occur at both the glenohumeral and scapulothoracic joints throughout the range.
  • Total range of glenohumeral motion: 90 degrees actively/120 degrees passively.
  • Total range of scapula motion: 60 degrees (possible through motion at the sternoclavicular (SC) and Acromioclavicular (AC) joints)
  • Motion at the SC joint: Occurs primarily from 0-90 degrees of elevation. Minimal motion was observed at the SC joint above 90 degrees.
  • Motion at the AC joint: Occurs during 0-30 degrees of elevation and then again above 135 degrees. Minimal motion was observed at the AC joint between 30 and 135 degrees.
  • Motion of the clavicle: Rotation of the clavicle is essential for the UE to be able to attain full range of elevation. It was found that fixing the clavicle to prevent rotation allowed the subject to achieve a maximum of 110 degrees of elevation.

Mechanical Requirements for Shoulder Joint Motion (calculated theoretically, then tested on a force model)

3 forces are necessary in order for UE elevation:

1: Force to counteract the weight of the shoulder girdle - vertical direction (primarily generated by the trapezius)

2: Rotary force couple part A - Force from the acromion process pulling superiomedially (generated via both passively from the outward force of the clavicle acting as a strut and actively via contraction of the trapezius)

3: Rotary force couple part B - Force from the lateral aspect of the scapula pulling superolaterally (generated actively by serratus anterior).

*The trapezius participates in generating both the supportive force and the rotary force at different points in the arc of movement.

Maximal force requirements during elevation at the glenohumeral joint:

  • Gradually increased up to and peaked at 90 degrees of motion. It then decreased until the UE reached 180 degrees of elevation.
  • Myographic recordings demonstrate that maximal muscle force between the Delotids and the rotator cuff occur at 60 degrees of elevation and produces a force equivalent to 9.6 times the weight of the limb.
  • The friction force between the head of the humerus and glenoid was observed to peak at 90 degrees of elevation and is equivalent to 10.2 times the weight of the extremity.

Force requirements during elevation at the scapulothoracic joint:

  • The forces acting on the scapula are observed to be less than those acting at the glenohumeral joint. They are observed to peak at 90 degrees of elevation and represent approximately 2 times the weight of the upper extremity.

Theoretically, no muscle force is required at 180 degrees of elevation because of the vertical orientation of the limb (although this was not found to be true in a practical setting).

Myographic studies (Muscles grouped functionally as opposed to structurally and measured using EMG)

Abductors and Flexors of the Humerus


  • Abduction: greatest electrical activity occurs between 90 and 180 degrees
  • Flexion: greatest activity between 110 and 180 degrees;
  • Amplitudes: reduced activity during flexion as compared with abduction below 130 degrees; Amplitude of activity similar for flexion and abduction above 130 degrees.

Pectoralis Major:

  • Abduction: negligible activity
  • Flexion: clavicular head is the most active portion of the muscle with peaks at 75 degrees and 115 degrees; manubrial head active to a much lesser degree with peaks of activity at 110 degrees and a cessation of activity above 145 degrees; the lower sternal and abdominal heads are not active during flexion or abduction.


  • Abduction: Active throughout the range of motion with peak at 100 degrees
  • Flexion: Active throughout the range of motion with peak at 80 degrees
  • Amplitudes: similar between flexion and abduction

Sum of all contributions of the flexing/abducting musculature: produce a force comparable to the necessary force predicted in the theoretical analysis. The flexion curve demonstrates a slightly higher amplitude when compared to the abduction curve. The major difference from the theoretical model is that muscle activity remains ongoing at 180 degrees of elevation, although it is reduced.

Depressors of the Humerus - all were found to act continuously throughout the range of flexion and abduction


  • Abduction: linear increase in activity throughout the range
  • Flexion: linear increase in activity with two peaks at 60 degrees and 120 degrees, before leveling off to match the abduction curve.
  • Amplitudes: Greater for flexion than abduction until above 160 degrees of elevation

Teres Minor - demonstrates a curve similar to infraspinatus;

  • Abduction: curve has a linear shape with peak at 120 degrees
  • Flexion: curve peaks between 90 and 120 degrees
  • Amplitudes: Higher for flexion than abduction


  • Abduction: peak activity recorded from 90 to 130 degrees with activity noted to decrease above 130 degrees
  • Flexion: peak activity occurred between 110 and 130 degrees with a sharp decrease in activity beyond that.
  • Amplitudes: greater amplitudes during abduction than flexion

The summed contributions of the depressors of the humeral head were noted to have higher amplitudes during flexion as compared to abduction. It was also noted that there were 2 points of peak activity: at 60-80 degrees and again from 110-120 degrees. The authors hypothesize that the first peak of activity was related to these muscles functioning in the capacity of depressors of the humeral head and that the second peak was related to these muscles functioning as rotators of the glenohumeral joint. The authors noted that when a weight was used to increase resistance to rotation the power of abduction was decreased.

Teres Major

  • Observed to be electrically silent during motion
  • Active during periods where a static position was maintained
  • Peak activity for stabilizing a position was when the arm was held at 90 degrees of elevation.


Scapula Rotators - organized into functional groups

Upper Trapezius, Levator Scapula, Upper Portion of the Serratus Anterior

  • Demonstrate ongoing electrical activity at rest indicating a role in postural support
  • Abduction and Flexion: demonstrate a linear increase in amplitude throughout the arc of elevation peaking at 180 degrees
  • Perform dual roles of elevating the shoulder girdle as well as participating in the superior aspect of the force couple that rotates the scapula.

Lower Trapezius and inferior portion of Serratus Anterior - the shape of both curves is very similar

  • Abduction: activity increases in a linear fashion and peaks at 180 degrees.
  • Flexion: activity increases in a linear fashion and peaks at 180 degrees.
  • Amplitudes: the lower trapezius demonstrates slightly higher amplitude during abduction; the inferior serratus anterior demonstrates a slightly higher amplitude during flexion (the authors hypothesize that the lower trapezius relaxes to allow the shoulder girdle to move anteriorly during flexion).

The Middle Trapezius

  • Abduction: peak activity at 90 degrees with a slight decrease at 180 degrees.
  • Flexion: increases in a linear fashion an peaks at 180 degrees
  • Amplitudes:significantly greater for abduction as compared to flexion (the authors hypothesize that decreased activity during flexion allows more efficient anterior motion of shoulder girdle).


  • Demonstrate similar activity to that of the middle Trapezius
  • Abduction: Linear increase with a peak at 180 degrees; smaller peak at 130 degrees.
  • Flexion: minimal increase in activity from 0-150, with sharp increase from 150-180 degrees.
  • Amplitudes: higher levels of activity during abduction throughout the range, however activity increases considerably to levels similar abduction above 150 degrees of flexion.
Conclusions This article provides evidence of the rotator cuff's role in creating optimal conditions for elevation of the UE, via their action of depressing the humeral head in the glenoid fossa. This article also provides support for the role of the axioscapular musculature in elevation of the UE via upward rotation of the scapula.
Conclusions of the Researchers The authors hypothesize that many of the morphological changes associated with the bony structure are related to altered muscular function based on the functional demands imposed by the freedom of the UE (i.e. not requiring the UE for a weight-bearing function). These observations are essential to consider when planning surgeries to aid those affected by paralysis of the muscles surrounding the shoulder.

"Gray409" by Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (See "Book" section below)Bartleby.com: Gray's Anatomy, Plate 409. Licensed under Public Domain via Wikimedia Commons - https://commons.wikimedia.org/wiki/File:Gray409.png#/media/File:Gray409.png

Review & Commentary:

This article presents a tremendous amount and breadth of information relating to the structure and function of the shoulder joint complex. Some of the methods utilized are not particularly common today, such as the comparison of human structures to the equivalents in other mammals, mainly primates. Other methods, such as EMG analysis of muscle function during movement were considered avant-garde at the time and are now relatively commonplace.

There were many strengths to the design of this research. The authors stated that they set out to provide a comprehensive understanding of the shoulder due to the lack of such an analysis in the literature at the time. They report that research on the shoulder was often conducted on cadavers and that the results were highly contradictory. A major strength of this study was that it was conducted on live subjects. The comparison of human structure to that of other mammals, allowed the authors to observe the anatomical differences present between organisms where the upper extremity (UE) is primarily used freely in open-chain tasks, rather than in weight-bearing activities (changes in the size and proportions of the scapula; changes in absolute versus relative size of various muscles). The authors also observed the differences between primates who utilized their UE very intensively in climbing as compared to humans who spend less time performing this task (the relative differences in size of teres major and latissimus dorsi) . With respect to observing function, the authors utilized both theoretical force models and EMG analysis collected during flexion and abduction of the shoulder. This allowed them to compare the performance of their models to "live" scenarios. In an effort to understand the relationships between different muscles during movement, the researchers collected EMG data from multiple muscles simultaneously during shoulder flexion and abduction. The authors highlight that their observations of muscle function during movement were novel at the time their research was conducted and that most researchers studied muscle function during an isolated isometric contraction.

There were also weaknesses in the methodology, and more specifically the reporting of the procedures used in this research. There were no descriptions of any of the subjects or tissue samples used. There was no information included in the paper on how bony changes or changes in muscle mass were measured, therefore it is impossible to know if these means were validated by other authors. There was no discussion of procedures for EMG analysis or of normalization of raw EMG recordings. Methods of statistical analysis and specific values of results were not included. It is unlikely that this information was deliberately not reported; it is more likely that it was not customary to include this information in publications at the time this article was presented.

A major difference between the findings presented in this study and the way the muscles of the shoulder joint complex are classified today is in the discussion of the Levator Scapulae . Inman et al. (1944), describe the Levator Scapulae as an elevator and upward rotator of the scapula, working in conjunction with the Serratus Anterior . Currently, this muscle is described as a downward rotator of the scapula (Levangie and Norkin, 2011, pg 177), thus opposing the action of Serratus Anterior . The authors note that in some mammals the Levator Scapula and Serratus Anterior are part of a single sheet of muscle. It is possible that this may have lead Inman et. al to be predisposed to think of this muscle functionally as an upward rotator. The authors did not include a separate graph to report the EMG activity of Levator Scapula , therefore it is not possible to know if the Levator Scapulae would have demonstrated different levels of activity from the Serratus Anterior . In the model of postural dysfunction for the upper body (UBD) , The Brookbush Institute proposes that Levator Scapula is a muscle with a propensity to become short and overactive. Hypothetically, if Inman et. al (1944) had noted that the Levator Scapula was active during overhead motion, might this be an observation of a piece of UBD, or might Levator Scapula be functioning more in a role of elevator of the shoulder girdle?

Why is this study important?

This study provides a foundation of concepts essential to the human movement professional's understanding of the fluid interplay between the structure and function of the shoulder joint complex. Infraspinatus and Teres Minor were shown to be active throughout the range of overhead motion, thus confirming their importance as depressors of the humeral head. The role of the axioscapular muscles was depicted in the theoretical force models describing the necessary forces for overhead motion of the UE, and was then confirmed via analysis of EMG recordings of the muscles that make up this group during overhead motion. This study refuted the description of scapulohumeral rhythm that was widely used at the time of its publication. Scapulohumeral rhythm initially described all motion occurring from 0-90 degrees as glenohumeral and all motion above 90 degrees as scapulothoracic. Inman et. al proposed that motion occurs at both the scapulothroacic and glenohumeral joints throughout the range of motion, which is how we conceptualize this relationship today.

How does it affect practice?

This article outlines and supports many of the tenets and concepts that we use today in the management of shoulder pathologies, pain and functional limitations. In discussing the functional groupings of the muscles of the shoulder complex, the authors described groupings that we continue to use: flexors and abductors of the humerus (i.e. prime movers), depressors of the humerus (stabilizers and synergists), upward and downward rotators of the scapula (prime movers and stabilizers/synergists). Knowledge of how these groups of muscles work together allow the human movement professional to gain a sense of how one might start to structure a corrective exercise routine. For example, the authors report that the Trapezius , Levator Scapulae , and Serratus Anterior were observed to be active during static standing, thus indicating a possible role in postural stabilization. Conversely, the Deltoid s was one muscle that was only active during motion. Depending upon the goal of a specific session, it may be preferable to design activities that challenge the postural stabilizers over the prime movers or vice versa (or more optimally to train the postural stabilizers prior to training the prime movers).

How does it relate to Brookbush Institute Content?

Many aspects of this research support the Brookbush Institute's model of Upper Body Dysfunction (UBD) . The Brookbush Institute highlights the role of the rotator cuff muscles in depression of the humeral head during overhead motion. However, it takes this concept one step further in the description of UBD and presents the idea that imbalances can develop within the rotator cuff muscles. Infraspinatus and Teres Minor may have a propensity toward under-activity and Subscapularis may tend to become over-active. Inman et. al (1944), note that the activation curves of Infraspinatus and Teres Minor steadily increased throughout overhead motion. Subscapularis demonstrated an increase in activity until approximately 140 degrees of overhead motion, then a sharp drop off beyond this point. This may allude to how the Subscapularis contributes to dysfunction should it become overactive - it could create motion dysfunction at the glenohumeral joint during the upper limits of overhead motion. Performing inhibitory techniques for the Subscapularis and activation techniques for the Teres Minor and Infraspinatus help to re-instate optimal neuromuscular balance within this group. Below are a series of videos describing examples of these techniques.

Subscapularis SA Static Release

Crucifixion Stretch -Pectoralis Major, Pectoralis Minor and Subscapularis Static Stretch

Shoulder External Rotation Isolated Activation

External Rotation Progression

Supplemental References:

Levangie, P. K., & Norkin, C. C. (2011). Joint structure and function: a comprehensive analysis. FA Davis.

© 2016 Brent Brookbush

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