Rotator Cuff Muscle Activity Research Articles

Construct Validity of Muscle Force Tests of the Rotator Cuff Muscles: An Electromyographic Investigation

Manual muscle tests (MMTs) are used in clinical settings to evaluate the function and strength (force-generating capacity) of a specific muscle in a position at which the muscle is believed to be most isolated from other synergists and antagonists. Despite frequent use of MMTs, few electromyographic evaluations exist to confirm the ability of MMTs to isolate rotator cuff muscles. This study examined rotator cuff isolation during 29 shoulder muscle force tests (9 clinical and 20 generic tests). An experimental design was used in this study. Electromyographic data were recorded from 4 rotator cuff muscles and 10 additional shoulder muscles of 12 male participants. Maximal isolation ratios (mean specific rotator cuff muscle activation to mean activation of the other 13 recorded muscles) defined which of these tests most isolated the rotator cuff muscles. Three rotator cuff muscles were maximally isolated (obtained highest isolation ratios) within their respective clinical test groups (lateral rotator test group for the infraspinatus and teres minor muscles and abduction test group for the supraspinatus muscle). The subscapularis muscle was maximally isolated equally as effectively within the generic ulnar force and clinical medial rotation groups. Similarly, the supraspinatus and teres minor muscles were isolated equally as effectively in some generic test groups as they were in their respective clinical test groups. Postural artifact in the wire electrodes caused exclusion of some channels from calculations. The grouping of muscle force tests based on test criteria (clinical or generic tests and muscle action) may have influenced which groups most isolated the muscle of interest. The results confirmed the appropriateness of 9 commonly used clinical tests for isolating rotator cuff muscles, but suggested that several other muscle force tests were equally appropriate for isolating these muscles.

Shoulder Muscle Recruitment Patterns and Related Biomechanics during Upper Extremity Sports

Understanding when and how much shoulder muscles are active during upper extremity sports is helpful to physicians, therapists, trainers and coaches in providing appropriate treatment, training and rehabilitation protocols to these athletes. This review focuses on shoulder muscle activity (rotator cuff, deltoids, pectoralis major, latissimus dorsi, triceps and biceps brachii, and scapular muscles) during the baseball pitch, the American football throw, the windmill softball pitch, the volleyball serve and spike, the tennis serve and volley, baseball hitting, and the golf swing. Because shoulder electromyography (EMG) data are far more extensive for overhead throwing activities compared with non-throwing upper extremity sports, much of this review focuses on shoulder EMG during the overhead throwing motion. Throughout this review shoulder kinematic and kinetic data (when available) are integrated with shoulder EMG data to help better understand why certain muscles are active during different phases of an activity, what type of muscle action (eccentric or concentric) occurs, and to provide insight into the shoulder injury mechanism. Kinematic, kinetic and EMG data have been reported extensively during overhead throwing, such as baseball pitching and football passing. Because shoulder forces, torques and muscle activity are generally greatest during the arm cocking and arm deceleration phases of overhead throwing, it is believed that most shoulder injuries occur during these phases. During overhead throwing, high rotator cuff muscle activity is generated to help resist the high shoulder distractive forces approximately 80-120% bodyweight during the arm cocking and deceleration phases. During arm cocking, peak rotator cuff activity is 49-99% of a maximum voluntary isometric contraction (MVIC) in baseball pitching and 41-67% MVIC in football throwing. During arm deceleration, peak rotator cuff activity is 37-84% MVIC in baseball pitching and 86-95% MVIC in football throwing. Peak rotator cuff activity is also high is the windmill softball pitch (75-93% MVIC), the volleyball serve and spike (54-71% MVIC), the tennis serve and volley (40-113% MVIC), baseball hitting (28-39% MVIC), and the golf swing (28-68% MVIC). Peak scapular muscle activity is also high during the arm cocking and arm deceleration phases of baseball pitching, with peak serratus anterior activity 69-106% MVIC, peak upper, middle and lower trapezius activity 51-78% MVIC, peak rhomboids activity 41-45% MVIC, and peak levator scapulae activity 33-72% MVIC. Moreover, peak serratus anterior activity was approximately 60% MVIC during the windmill softball pitch, approximately 75% MVIC during the tennis serve and forehand and backhand volley, approximately 30-40% MVIC during baseball hitting, and approximately 70% MVIC during the golf swing. In addition, during the golf swing, peak upper, middle and lower trapezius activity was 42-52% MVIC, peak rhomboids activity was approximately 60% MVIC, and peak levator scapulae activity was approximately 60% MVIC.

Scapular and rotator cuff muscle activity during arm elevation: A review of normal function and alterations with shoulder impingement.

OBJECTIVE: The purpose of this manuscript is to review current knowledge of how muscle activation and force production contribute to shoulder kinematics in healthy subjects and persons with shoulder impingement. RESULTS: The middle and lower serratus anterior muscles produce scapular upward rotation, posterior tilting, and external rotation. Upper trapezius produces clavicular elevation and retraction. The middle trapezius is primarily a medial stabilizer of the scapula. The lower trapezius assists in medial stabilization and upward rotation of the scapula. The pectoralis minor is aligned to resist normal rotations of the scapula during arm elevation. The rotator cuff is critical to stabilization and prevention of excess superior translation of the humeral head, as well as production of glenohumeral external rotation during arm elevation. Alterations in activation amplitude or timing have been identified across various investigations of subjects with shoulder impingement as compared to healthy controls. These include decreased activation of the middle or lower serratus anterior and rotator cuff, delayed activation of middle and lower trapezius, and increased activation of the upper trapezius and middle deltoid in impingement subjects. In addition, subjects with a short resting length of the pectoralis minor exhibit altered scapular kinematic patterns similar to those found in persons with shoulder impingement. CONCLUSION: These normal muscle functional capabilities and alterations in patient populations should be considered when planning exercise approaches for the rehabilitation of these patients.

Use of 2-[ 18f]-fluoro-2-deoxy- d-glucose positron emission tomography (FDG PET) imaging for the evaluation of muscle metabolic activity in ruptured rotator cuffs: identification of shoulder muscles by fusion imaging studies involving both FDG PET and magnetic resonance imaging

The uptake of 2-[ 18F]-fluoro-2-deoxy- d-glucose (FDG) into shoulder muscles was studied in 11 patients with rotator cuff tears. FDG was injected under resting conditions, and cuff muscle exercises were performed followed by evaluation of the standardized uptake value (SUV) in shoulder muscles bilaterally. Analysis was performed with a subtracted SUV, obtained by subtracting the SUV of the muscle at rest from that after exercise. Fusion imaging studies involving FDG positron emission tomography (PET) and magnetic resonance imaging would provide a novel strategy with the potential of improving on the limited resolution of PET images. In this study this relatively new strategy revealed a similar decrease in the supraspinatus SUV in cases involving both complete and partial tears. The SUV in the infraspinatus and subscapularis also decreased, regardless of whether these muscles were intact or ruptured; the SUV in the deltoid remained basically unchanged. With the use of this exercise protocol, FDG PET might prove useful for objective evaluation of rotator cuff muscle activity.

Assessment of the Infraspinatus Spinal Stretch Reflex in the Normal, Athletic, and Multidirectionally Unstable Shoulder

To examine neural aspects of motor control in the glenohumeral joint, this study evaluates utilization of an innate spinal segmental pathway, the spinal stretch reflex, as an investigational tool that reflects neural circuitry. The purpose of this study was to determine if this reflex could be evoked from the infraspinatus muscle, if the testing apparatus and protocol for elicitation were reliable, and if the reflex response varies between groups of subjects and therefore could be useful clinically. These reflex characteristics were evaluated in the infraspinatus muscle, since rotator cuff muscle activity in subjects with glenohumeral instability exhibits differences in electromyographic activity and coordination patterns, implicating its role in dynamic stability. Normal shoulders were compared with athletic shoulders and shoulders with multidirectional instability. The spinal stretch reflex was elicited in a controlled and reliable manner. Shoulders with multidirectional instability exhibited a more-prominent spinal stretch reflex response than normal shoulders, whereas athletic shoulders exhibited a more-quiescent spinal stretch reflex response. As the spinal stretch reflex probably plays a role in motor control, variation in this reflex profile may reflect some differences in development that contribute to the variable expression of dynamic glenohumeral stability. This study suggests that the spinal stretch reflex profile may be a useful clinical tool to assist in discriminating between the normal and pathologic state. This information may also be useful in the evaluation of new treatment approaches exploiting spinal cord plasticity and spinal stretch reflex mutability through neuromuscular training.

Comparative electromyographic analysis of shoulder muscles during planar motions: anterior glenohumeral instability versus normal.

This study compared the electromyographic activity of rotator cuff and scapular muscles between subjects with anterior instability and subjects with normal shoulders. Thirty-eight patients were studied; 23 had anterior instability that was subsequently surgically confirmed, and 15 had normal shoulders. Fine wire electrodes were inserted into the subscapularis (upper and lower portions), supraspinatus, infraspinatus, rhomboid, serratus anterior, and trapezius (upper and lower portions) muscles. Abduction, scapular plane abduction (scaption), and forward flexion were performed over the range of motion and later divided into 30 degrees intervals. In both abduction and scaption, the supraspinatus demonstrated significantly less electromyographic activity from 30 degrees to 60 degrees in shoulders with anterior instability compared with normal shoulders (p < 0.05). During all three motions, shoulders with anterior instability demonstrated significantly less electromyographic activity in the serratus anterior when compared with normal shoulders (p < 0.05). This occurred at 30 degrees to 120 degrees of abduction and at 0 degree to 120 degrees of scaption and forward flexion. None of the other muscles demonstrated significant differences. These differences during planar motions were similar to those demonstrated during challenging overhead sport motions. Early rehabilitation efforts should focus both on the rotator cuff and scapular muscles to establish smooth, coordinated shoulder motion.

Kinesiology of the empty can test.

The "empty can test" has been described to isolate supraspinatus muscle activity from the activity of other rotator cuff muscles. The shoulder is positioned in 90 degrees of abduction, with full internal rotation, and 30 degrees of forward flexion and maintained against resistance. The purpose of this study was to determine if the supraspinatus muscle is isolated by the empty can test. Ten normal male subjects were studied (age 25-43/mean 32) with fine-wire electromyography recording from their nondominant arm middle deltoid, supraspinatus, infraspinatus, and teres minor. During the maneuver electromyogram activity was seen not only in the supraspinatus, but also in the infraspinatus and the middle head of the deltoid. Teres minor was inactive throughout the test; however, this is interesting because the teres minor and infraspinatus have previously been described as a functional unit. Our study found that the empty can test does not allow selective activation of the supraspinatus muscle.

The rotator cuff opposes superior translation of the humeral head.

To determine the influence of rotator cuff muscle activity on humeral head migration relative to the glenoid during active arm elevation we studied five fresh cadaveric shoulders. The shoulder girdles were mounted in an apparatus that simulated contraction of the deltoid and rotator cuff muscles while maintaining the normal scapulothoracic relationship. The arms were abducted using four different configurations of simulated muscle activity: deltoid alone; deltoid and supraspinatus; deltoid, infraspinatus, teres minor, and subscapularis; and deltoid, supraspinatus, infraspinatus, teres minor, and subscapularis. For each simulated muscle configuration the vertical position of the humeral head in relation to the glenoid was determined at 30 degrees, 60 degrees, 90 degrees, and 120 degrees of abduction using digitized anteroposterior radiographs. Both muscle activity and abduction angle significantly influenced the glenohumeral relationship. With simulated activity of the entire rotator cuff, the geometric center of the humeral head was centered in the glenoid at 30 degrees but had moved 1.5 mm superiorly by 120 degrees. Abduction without the subscapularis, infraspinatus, and teres minor muscles caused significant superiorly directed shifts in humeral head position as did abduction using only the deltoid muscle. These results support the possible use of selective strengthening exercises for the infraspinatus, teres minor, and subscapularis muscles in treatment of the impingement syndrome.