Glenohumeral Forces Research Articles

The Influence of Age at Pediatric-Onset Spinal Cord Injury and Years of Wheelchair Use on Shoulder Complex Joint Dynamics During Manual Wheelchair Propulsion.

To assess the association of age at pediatric-onset spinal cord injury (SCI) and years of manual wheelchair use with shoulder dynamics. Upper extremity kinematics and hand-rim kinetics were obtained during manual wheelchair propulsion. An inverse dynamics model computed three-dimensional acromioclavicular, sternoclavicular, and glenohumeral joint dynamics. Linear mixed effects models evaluated the association of age at injury onset and years of wheelchair use with shoulder dynamics. Motion laboratory within a children's hospital. Seventeen manual wheelchair users (N=17; 6 female, 11 male; mean age: 17.2 years, mean age at SCI onset: 11.5 years) with pediatric-onset SCI (levels: C4-T11) and International Standards for Neurological Classification of SCI grades: A (11), B (3), C (2), and N/A (2). Not applicable. Acromioclavicular, sternoclavicular, and glenohumeral angles and ranges of motion, and glenohumeral forces and moments. We observed a decrease in maximum acromioclavicular upward rotation (ß [95% confidence interval {CI}]=3.02 [0.15,5.89], P=.039) and an increase in acromioclavicular downward/upward rotation range of motion (ß [95% CI]=0.44 [0.08,0.80], P=.016) with increasing age at SCI onset. We found interactions between age at onset and years of use for maximum glenohumeral abduction (ß [95% CI]=0.16 [0.03,0.29], P=.017), acromioclavicular downward/upward rotation range of motion (ß [95% CI]=-0.05 [-0.09,-0.01], P=.008), minimum acromioclavicular upward rotation (ß [95% CI]=-0.34 [-0.64,-0.04], P=.026). A decrease in glenohumeral internal rotation moment (ß [95% CI]=-0.09 [-0.17,-0.009], P=.029) with increasing years of use was found. Age at injury and the years of wheelchair use are associated with shoulder complex biomechanics during wheelchair propulsion. These results are noteworthy, as both age at SCI onset and years of wheelchair use are considered important factors in the incidence of shoulder pain. These results suggest that investigations of biomechanical changes over the lifespan are critical.

Latissimus dorsi tendon transfer for massive unrepairable rotator cuff tears: A case report

Introduction: Massive and irreparable posterior-superior rotator cuff tears present a difficult treatment problem. Chronic rotator cuff tears can cause tendon retraction and fatty atrophy of the muscles. Full-thickness rotator cuff tears involves more than one tendon cause superior migration of the humeral head in relation to the glenoid with altered glenohumeral forces, and ultimately leading to rotator cuff arthropathy. Transfer of the latissimus dorsi muscle is considered a surgical option in the treatment of younger patients without glenohumeral arthritis and with functional deficits caused by an irreparable posterior-superior rotator cuff tear.Presentation of Case: This report presents a case of Right Supraspinatus and Infraspinatus Chronic tear with Subscapularis tear. The case was chronic with non-subsidence of symptoms in spite of conservative treatment. Our treatment in this case consisted of Right Subscapularis and Infraspinatus Repair with Lattisimus Dorsi Transfer to Greater Tubercle. Post operatively ultrasling was used to keep the shoulder in neutral with slight abduction. Rehabilitation protocol was begun after 3 weeks of immobilization. Discussion: Latissimus Dorsi Tendon Transfer has been considered to be a means of restoring function and providing pain relief in patients with irreparable tears involving the infraspinatus and supraspinatus tendons in the absence of substantial glenohumeral arthritis.The aim of this case presentation is to report an example for this case, highlight the difficulties in diagnosis and the treatment options.Conclusion: The Latissimus Dorsi Tendon Transfer for irreparable postero-superior cuff deficiency in selected patients reduce pain and improve shoulder function in the medium term.

Arthroscopic Subscapularis Repair Using a Clever Hook and Lasso Loop Technique in the Beach Chair Position: A Simple and Reproducible Guide

Addressing subscapularis tendon pathology has garnered increased attention during shoulder arthroscopy in attempt to adequately restore glenohumeral force couples. The appropriate rebalancing of force couples of the rotator cuff musculature by repairing subscapularis tendon tears in patients with large rotator cuff tears has been shown to improve functional outcomes while decreasing retear rates. However, subscapularis tendon tears may be particularly challenging to diagnose and present a significant degree of technical difficulty with the description of multiple arthroscopic and open surgical techniques. In this comprehensive guide, we put forth a simple, concise, and reproducible arthroscopic technique using a Clever Hook and Lasso Loop stitch technique for repairing both high-grade partial and full-thickness tears of the subscapularis tendon.

Instantaneous helical axis estimation of glenohumeral kinematics: The impact of rotator cuff pathology

The rotator cuff is theorized to contribute to force couples required to produce glenohumeral kinematics. Impairment in these force couples would theoretically result in impaired ball-and-socket kinematics. Although less frequently used than traditional kinematic descriptors (e.g., Euler angles, joint translations), helical axes are capable of identifying alterations in ball-and-socket kinematics by quantifying the variability (i.e., dispersion) in axis orientation and position during motion. Consequently, assessing glenohumeral helical dispersion may provide indirect evidence of rotator cuff function. The purpose of this exploratory study was to determine the extent to which rotator cuff pathology is associated with alterations in ball-and-socket kinematics. Fifty-one participants were classified into one of five groups based on an assessment of the supraspinatus using diagnostic imaging: asymptomatic healthy, asymptomatic tendinosis, asymptomatic partial-thickness tear, asymptomatic full-thickness tear, symptomatic full-thickness tear. Glenohumeral kinematics were quantified during coronal plane abduction using a biplane x-ray system and described using instantaneous helical axes. The degree to which glenohumeral motion coincided with ball-and-socket kinematics was described using the angular and positional dispersion about the optimal helical axis and pivot, respectively. No statistically significant difference was observed between groups in angular dispersion. However, symptomatic individuals with a full-thickness supraspinatus tear had significantly more positional dispersion than asymptomatic individuals with a healthy supraspinatus or tendinosis. These findings suggest that symptomatic individuals with a full-thickness supraspinatus tear exhibit impaired ball-and-socket kinematics, which is believed to be associated with a disruption of the glenohumeral force couples.

EMG-Assisted Algorithm to Account for Shoulder Muscles Co-Contraction in Overhead Manual Handling

Glenohumeral stability is essential for a healthy function of the shoulder. It is ensured partly by the scapulohumeral muscular balance. Accordingly, modelling muscle interactions is a key factor in the understanding of occupational pathologies, and the development of ergonomic interventions. While static optimization is commonly used to estimate muscle activations, it tends to underestimate the role of shoulder’s antagonist muscles. The purpose of this study was to implement experimental electromyographic (EMG) data to predict muscle activations that could account for the stabilizing role of the shoulder muscles. Kinematics and EMG were recorded from 36 participants while lifting a box from hip to eye level. Muscle activations and glenohumeral joint reactions were estimated using an EMG-assisted algorithm and compared to those obtained using static optimization with a generic and calibrated model. Muscle activations predicted with the EMG-assisted method were generally larger. Additionally, more interactions between the different rotator cuff muscles, as well as between primer actuators and stabilizers, were predicted with the EMG-assisted method. Finally, glenohumeral forces calculated from a calibrated model remained within the boundaries of the glenoid stability cone. These findings suggest that EMG-assisted methods could account for scapulohumeral muscle co-contraction, and thus their contribution to the glenohumeral stability.

Alterations in Glenohumeral Force Distribution Following Rotator Cuff Injury and Repair

Rotator cuff repair is associated with an increased incidence of osteoarthritic changes of the glenohumeral joint. Such degeneration may be secondary to improper tensioning of muscular stabilizers during surgical intervention; however, existing studies have not specifically examined changes in joint mechanics following rotator cuff repair. Therefore, this study’s purpose was to assess for changes in glenohumeral contact forces following the creation and repair of rotator cuff injury.

Scapulothoracic rhythm affects glenohumeral joint force

HypothesisMusculoskeletal computer models provide valuable insights into shoulder biomechanics. The shoulder is a complex joint composed of glenohumeral, scapulothoracic, acromioclavicular, and sternoclavicular articulations, whose function is largely dependent on the many muscles spanning these joints. However, the range of patient-to-patient variability in shoulder function is largely unknown. We therefore assessed the sensitivity of glenohumeral forces to population-based model input parameters that were likely to influence shoulder function.MethodsWe constructed musculoskeletal models of the shoulder in the AnyBody Modeling System (AnyBody Technology, Aalborg, Denmark). We used inverse dynamics and static optimization to solve for glenohumeral joint forces during a simulated shoulder elevation. We generated 1000 AnyBody models by uniformly distributing the following input parameters: subject height, scapulohumeral rhythm, humeral head radius, and acromiohumeral interval.ResultsIncreasing body height increased glenohumeral joint forces. Increasing the ratio of scapulothoracic to glenohumeral elevation also increased forces. Increasing humeral head radius and acromiohumeral interval decreased forces. The relative sensitivity of glenohumeral joint forces to input parameters was dependent on the angle of shoulder elevation. We developed an efficient method of generating and simulating musculoskeletal models representing a large population of shoulder arthroplasty patients. We found that scapulohumeral rhythm had a significant influence on glenohumeral joint force.ConclusionsThis finding underscores the importance of more accurately measuring and simulating scapulothoracic motion rather than using fixed ratios or average scapulothoracic motion. This modeling approach can be used to generate virtual populations for conducting efficient simulations and generating statistical conclusions.

Biomechanical comparison of 2 augmented glenoid designs: an integrated kinematic finite element analysis

Augmented glenoid implants are available to help restore the biomechanics of the glenohumeral joint with excessive retroversion. It is imperative to understand their behavior to make a knowledgeable preoperative decision. Therefore, our goal was to identify an optimal augmented glenoid design based on finite element analysis (FEA) under maximum physiological loading. FEA models of 2 augmented glenoid designs-wedge and step-were created per the manufacturers' specifications and virtually implanted in a scapula model to correct 20° of retroversion. Simulation of shoulder abduction was performed using the FEA shoulder model. The glenohumeral force ratio, relative micromotion, and stress levels on the cement mantle, glenoid vault, and backside of the implants were compared between the 2 designs. The force ratio was 0.56 for the wedge design and 0.87 for the step design. Micromotion (combination of distraction, translation, and compression) was greater for the step design than the wedge design. Distraction measured 0.05 mm for the wedge design and 0.14 mm for the step component. Both implants showed a similar pattern for translation; however, compression was almost 3 times greater for the step component. Both implants showed high stress levels on the cement mantle. At the glenoid vault and on the implants, the stress levels were 1.65 MPa and 6.62 MPa, respectively, for the wedge design and 3.78 MPa and 13.25 MPa, respectively, for the step design. Implant design slightly affects joint stability; however, it plays a major role regarding long-term survival. Overall, the augmented wedge design provides better implant fixation and stress profiles with less micromotion.

Sensitivity of Shoulder Musculoskeletal Model Predictions to Muscle-Tendon Properties.

While the sensitivity of estimated muscle forces to muscle-tendon properties is well documented for the lower limbs, little is known about the shoulder and upper limbs. The purpose of this study was to assess the sensitivity of estimated shoulder muscle forces and scapulohumeral joint force to muscle-tendon properties. One healthy male participant performed arm flexions and simulated throwing maneuvers. Kinematics were recorded using intra-cortical pins. Muscle forces were estimated using static optimization with the generic delft shoulder and elbow in OpenSim, and scapulohumeral joint forces were calculated from the estimated forces. Then, variations from -25% to +25% of the nominal values of the tendon slack length, the optimal fiber length, the maximal isometric force, and the pennation angle were applied to the musculoskeletal model to compute affected muscle forces and scapulohumeral joint force. The variations in muscle-tendon properties led to changes up to 9.6N or 174% in the muscle nominal forces. The more sensitive muscles were those that produced the greatest force: the rotator cuff muscles and the prime movers specific to the task. Among the four muscle-tendon properties, the maximal isometric force and the optimal fiber length had the greatest influence on the muscle force variability. Glenohumeral force was slightly influenced by muscle-tendon properties (<8%). Generic models (i.e., those without personalization of muscle-tendon properties) can lead to misinterpretations of muscle force. Efforts should focus on the maximal isometric force and the optimal fiber length of the rotator cuff muscles and prime movers.

Alterations in Glenohumeral Forces Following Rotator Cuff Injury and Repair

Objectives:Rotator cuff repair is associated with an unusually high incidence of osteoarthritic changes and cartilage damage in the glenohumeral joint. Such degeneration may be secondary to improper tensioning of muscular stabilizers during surgical intervention; however, existing studies have not specifically examined changes in joint congruity following rotator cuff repair. Therefore, the purpose of this study was to assess for changes in glenohumeral contact forces following the repair of rotator cuff injury.Methods:Transduction mapping was performed on the glenohumeral joint of ten fresh-frozen cadaveric shoulder specimens. A calibrated pressure-mapping sensor was introduced through the rotator interval and secured along the concavity of the glenoid labrum. Following a baseline force measurements, analysis of force intensity and total glenohumeral contact area was performed in each specimen for 6 simulated injury and treatment conditions: A) A 1 cm supraspinatus lesion; B) 2-suture repair of the 1 cm lesion; C) removal of the 2-suture repair; D) a 2 cm supraspinatus lesion; E) 3-suture repair of the 2 cm lesion and; F) removal of the 3-suture repair. All repairs were performed via bone tunnels in the standard method described. Data were recorded over 60s intervals at a rate of 4 frames per second and included raw force, area, and force per unit area. Values for lesion, repair, and post-repair conditions were expressed as a proportion of initial baseline measurements. Means and standard deviations were then calculated for each condition and compared via Student’s t-tests.Results:For baseline measurements, the mean intact glenohumeral force was 38.55 ± 24.79 N and the mean contact area was 313 ± 84.09 mm2. In comparison to baseline values, 3-suture repair yielded a significant increase in both total glenohumeral force (mean proportion: 2.16 ± 3.26; p=0.046) as well as proportion of force per unit area (1.73 ± 1.86 N/mm2; p=0.024). Both the 2 cm lesion and the 2-suture repair removal yielded significant decreases in contact area when compared to baseline, with the former exhibiting a proportion of 0.76 ± 0.19 (p=0.040) and the latter yielding a proportion of 0.60 ± 0.29 (p=0.004). No other conditions exhibited significant changes from baseline measurements.Conclusion:Rotator cuff injury leads to alterations in glenohumeral forces, with significant increases in articular contact-pressures following repair of larger supraspinatus lesions. These findings offer a possible explanation for the high rate of degenerative changes demonstrated following rotator cuff repair. Further study is warranted to determine how current treatment methods might be improved to result in glenohumeral contact pressures resembling those experienced prior to injury.

Contributions of Muscle Imbalance and Impaired Growth to Postural and Osseous Shoulder Deformity Following Brachial Plexus Birth Palsy: A Computational Simulation Analysis

Two potential mechanisms leading to postural and osseous shoulder deformity after brachial plexus birth palsy are muscle imbalance between functioning internal rotators and paralyzed external rotators and impaired longitudinal growth of paralyzed muscles. Our goal was to evaluate the combined and isolated effects of these 2 mechanisms on transverse plane shoulder forces using a computational model of C5-6 brachial plexus injury. We modeled a C5-6 injury using a computational musculoskeletal upper limb model. Muscles expected to be denervated by C5-6 injury were classified as affected, with the remaining shoulder muscles classified as unaffected. To model muscle imbalance, affected muscles were given no resting tone whereas unaffected muscles were given resting tone at 30% of maximal activation. To model impaired growth, affected muscles were reduced in length by 30% compared with normal whereas unaffected muscles remained normal in length. Four scenarios were simulated: normal, muscle imbalance only, impaired growth only, and both muscle imbalance and impaired growth. Passive shoulder rotation range of motion and glenohumeral joint reaction forces were evaluated to assess postural and osseous deformity. All impaired scenarios exhibited restricted range of motion and increased and posteriorly directed compressive glenohumeral joint forces. Individually, impaired muscle growth caused worse restriction in range of motion and higher and more posteriorly directed glenohumeral forces than did muscle imbalance. Combined muscle imbalance and impaired growth caused the most restricted joint range of motion and the highest joint reaction force of all scenarios. Both muscle imbalance and impaired longitudinal growth contributed to range of motion and force changes consistent with clinically observed deformity, although the most substantial effects resulted from impaired muscle growth. Simulations suggest that treatment strategies emphasizing treatment of impaired longitudinal growth are warranted for reducing deformity after brachial plexus birth palsy.

Augmented wedge-shaped glenoid component for the correction of glenoid retroversion: a finite element analysis

This study undertook a computational analysis of a wedged glenoid component for correction of retroverted glenoid arthritic deformity to determine whether a wedge-shaped glenoid component design with a built-in correction for version reduces excessive stresses in the implant, cement, and glenoid bone. Recommendations for correcting retroversion deformity are asymmetric reaming of the anterior glenoid, bone grafting of the posterior glenoid, or a glenoid component with posterior augmentation. Eccentric reaming has the disadvantages of removing normal bone, reducing structural support for the glenoid component, and increasing the risk of bone perforation by the fixation pegs. Bone grafting to correct retroverted deformity does not consistently generate successful results. Finite element models of 2 scapulae models representing a normal and an arthritic retroverted glenoid were implanted with a standard glenoid component (in retroversion or neutral alignment) or a wedged component. Glenohumeral forces representing invivo loading were applied and stresses and strains computed in the bone, cement, and glenoid component. The retroverted glenoid components generated the highest compressive stresses and decreased cyclic fatigue life predictions for trabecular bone. Correction of retroversion by the wedged glenoid component significantly decreased stresses and predicted greater bone fatigue life. The cement volume estimated to survive 10 million cycles was the lowest for the retroverted components and the highest for neutrally implanted glenoid components and for wedged components. A wedged glenoid implant is a viable option to correct severe arthritic retroversion, reducing the need for eccentric reaming and the risk for implant failure.

Shoulder External Rotation Fatigue and Scapular Muscle Activation and Kinematics in Overhead Athletes

Glenohumeral external rotation (GH ER) muscle fatigue might contribute to shoulder injuries in overhead athletes. Few researchers have examined the effect of such fatigue on scapular kinematics and muscle activation during a functional movement pattern. To examine the effects of GH ER muscle fatigue on upper trapezius, lower trapezius, serratus anterior, and infraspinatus muscle activation and to examine scapular kinematics during a diagonal movement task in overhead athletes. Human performance research laboratory. Descriptive laboratory study. Our study included 25 overhead athletes (15 men, 10 women; age = 20 ± 2 years, height = 180 ± 11 cm, mass = 80 ± 11 kg) without a history of shoulder pain on the dominant side. We tested the healthy, dominant shoulder through a diagonal movement task before and after a fatiguing exercise involving low-resistance, high-repetition, prone GH ER from 0° to 75° with the shoulder in 90° of abduction. Surface electromyography was used to measure muscle activity for the upper trapezius, lower trapezius, serratus anterior, and infraspinatus. An electromyographic motion analysis system was used to assess 3-dimensional scapular kinematics. Repeated-measures analyses of variance (phase × condition) were used to test for differences. We found a decrease in ascending-phase and descending-phase lower trapezius activity (F(1,25) = 5.098, P = .03) and an increase in descending-phase infraspinatus activity (F(1,25) = 5.534, P = .03) after the fatigue protocol. We also found an increase in scapular upward rotation (F(1,24) = 3.7, P = .04) postfatigue. The GH ER muscle fatigue protocol used in this study caused decreased lower trapezius and increased infraspinatus activation concurrent with increased scapular upward rotation range of motion during the functional task. This highlights the interdependence of scapular and glenohumeral force couples. Fatigue-induced alterations in the lower trapezius might predispose the infraspinatus to injury through chronically increased activation.

Effect of supraspinatus deficiency on humerus translation and glenohumeral contact force during abduction

Supraspinatus deficiency is the most frequent and important problem associated to rotator cuff pathologies. It reduces shoulder stability and can lead to osteoarthritis. The goal of this study was to develop a numerical model of the shoulder to analyse the biomechanical consequences of this pathology. A 3D finite element model of the shoulder was developed from a normal cadaver specimen. It included the scapula, the humerus and the major abduction muscles. Instead of the usual ball-socket assumption, which prevents the natural translation of the humerus, shoulder stability was actively achieved by muscles. A feedback algorithm was developed to synchronise muscle forces during abduction. The numerical algorithm was validated against an algebraic model, and the calculated muscle moment arms were compared to the literature. Two cases were considered: a normal shoulder and the same one without supraspinatus. For the normal shoulder, the model predicted the initial upward migration of the humeral head. The maximal humerus translation occurred at 30 degrees of abduction and was 0.75 mm above its ideal centered position. Without supraspinatus, it was 1.6 times higher and the contact point in the glenoid fossa was more eccentric. For the normal shoulder, the maximal glenohumeral force was 81% of the body weight, at 82 degrees of abduction. Without supraspinatus, it increased by 8%, while the increase of muscle forces was 30%. Supraspinatus deficiency increased the upward migration of the humerus, the eccentric loading, and the joint and muscle forces, which may cause a limitation of active abduction and degenerative glenohumeral changes (osteoarthritis and the rotator cuff tear).

Biomechanical effects of glenoid retroversion in total shoulder arthroplasty

The purpose of this study was to evaluate the effects of glenoid retroversion after total shoulder arthroplasty on glenohumeral joint contact pressures, contact areas, and forces. We studied 8 cadaveric shoulders with a custom shoulder testing system that permits anatomic loading of the rotator cuff and the humerothoracic muscles. Each shoulder was tested intact and after total shoulder arthroplasty with the glenoid component in a neutral position and with the glenoid component retroverted 15 degrees . Glenohumeral contact pressures and areas were measured with Fuji pressure-sensitive film, and the glenohumeral forces were measured with a 6-df load cell. Glenohumeral contact areas decreased significantly, and contact pressures increased after total shoulder arthroplasty with both neutral glenoid components and retroverted glenoid components (P < .0002 to .04). Inferior forces decreased significantly after total shoulder arthroplasty with the retroverted glenoid components when compared with the intact, native shoulder (P < .02). Posterior forces also decreased significantly with the retroverted glenoid when compared with the neutral glenoid (P < .03). Placement of the glenoid in 15 degrees of retroversion significantly decreased glenohumeral contact area, increased contact pressure, and decreased inferior and posterior glenohumeral forces. Glenoid component retroversion may result in eccentric loading of the glenoid component and possibly lead to wear and loosening, signifying the importance of correct anatomic placement during total shoulder arthroplasty.

Development of cadaveric models of a thrower’s shoulder

Two novel cadaveric models of a thrower's shoulder were evaluated. Both models included the capsuloligamentous and osseous components of the glenohumeral joint. In model 1 the coracohumeral ligament was retained and the glenoid was positioned parallel to the ground whereas the humerus was positioned at 60 degrees of glenohumeral elevation in the scapular plane. In model 2 the coracohumeral ligament was resected and the glenoid was positioned vertically at 30 degrees of scapular elevation whereas the humerus was positioned at 60 degrees of glenohumeral elevation in the scapular plane. Each specimen was sequentially tested under 3 conditions: intact, after nondestructive anterior capsular stretch, and after simulated posterior capsular contracture. Measurements included rotational range of motion and relative glenohumeral position from neutral to maximum external rotation of the humerus. In model 2 the glenohumeral forces were also measured by use of a 6-degree-of-freedom load cell. The stretching of the anterior capsule in model 1 was performed while the humeral head was constrained in the glenoid and in model 2 while the humeral head was unconstrained. Both models showed increased humeral external rotation and decreased humeral internal rotation similar to that seen in throwing athletes. Both models also showed an increased humeral shift inferiorly after anterior capsular stretching. Model 1 demonstrated a subsequent humeral shift superiorly after posterior capsule plication. Unconstrained specimens of model 2 dislocated posteriorly and inferiorly after posterior capsule plication when the humerus was rotated to maximum external rotation. On the basis of these findings, for future studies, constrained capsular stretching models will be further investigated.

Electromyography of pongid shoulder muscles II. Deltoid, rhomboid and "rotator cuff".

AbstractElectromyographic (EMG) recordings were taken of action potentials in the deltoid, rhomboid, supraspinatus, infraspinatus, subscapularis, and teres minor muscles of pongid apes as they spontaneously elevated their arms and engaged in suspensory activity on a trapeze.During brachial elevation, activities of pongid deltoid, rhomboid, supra‐spinatus, infraspinatus and subscapularis muscles basically resemble those of human subjects. The intermediate deltoid exhibited high potentials particularly near and at the zenith of brachial elevation. The anterior and, to a lesser extent, posterior deltoid were more variable than the intermediate deltoid during arm‐raising. The three heads often acted discretely. Rhomboid potentials also increased steadily with the highest ones at 130–150°. The supraspinatus sometimes acted as an initiator and accelerator of arm‐raising. Supraspinatus potentials commonly decreased markedly near the zenith of brachial elevation. When subjects reached overhead, it was impossible to discern whether the subscapularis acted fundamentally as an arm‐raising muscle or merely as a medial rotator to position the hand.In the orangutan, the teres minor was inconsequentially active during arm‐raising. This is strikingly different from the human activity pattern in which it is supposed to function in the inferior component of a glenohumeral force couple (Inman et al., '44).Remarkably little consequential EMG activity was recorded in the shoulder muscles of gorilla during hoisting activity. Per contra, in the orangutan some muscles were quite active as she hoisted herself on the trapeze. The teres minor seemed to be a prime retractor of the humerus in that context. In the chimpanzee, the rhomboid and especially the supraspinatus were significantly active during hoisting.All muscles tested in the three great apes were silent or inconsequentially active during pendent suspension from the trapeze.We conclude that at least during short‐term pendent suspension, osseoligamentous mechanisms are probably chiefly responsible for maintaining integrity of pongid glenohumeral joints. The important role of the supraspinatus muscle as a brachial elevator should be emphasized in future studies of pongid scapular shape. In view of the variable, contextually dependent, actions of the pongid “rotator cuff” muscles we suggest that they be termed the deep scapulohumeral muscles.