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
The Humerus
The Scapulohumeral group of muscles (supraspinatus, infraspinatus, teres minor, subscapularis, deltoid, teres major)
The Axioscapular group of muscles (trapezius, rhomboids, serratus anterior, levator scapula)
The Axiohumeral group of muscles (pectoralis major, pectoralis minor, latissimus dorsi)
The muscles of the upper arm (biceps brachii, triceps brachii)
Observations of motion at the shoulder joint complex (made via X-ray and bone inserted pin motion analysis)
Mechanical Requirements for Shoulder Joint Motion (calculated theoretically, then tested on a force model)
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
The Humerus
The Scapulohumeral group of muscles
The Axioscapular group of muscles
The Axiohumeral group of muscles
The muscles of the upper arm
Observations on motion of the shoulder joint complex (made via X-ray and bone inserted pin motion analysis)
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:
Force requirements during elevation at the scapulothoracic joint:
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
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
Teres Minor - demonstrates a curve similar to infraspinatus;
Subscapularis
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.
Scapula Rotators - organized into functional groups Upper Trapezius, Levator Scapula, Upper Portion of the Serratus Anterior
Lower Trapezius and inferior portion of Serratus Anterior - the shape of both curves is very similar
The Middle Trapezius
|
| 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
Questions, comments, and criticisms are welcomed and encouraged -