Rehabilitation Research

Differential Patterns of Muscle Activation in Patients with Symptomatic and Asymptomatic Massive Rotator Cuff Tears

Treatment of massive rotator cuff tears (operative versus non-operative) remains controversial.  Our purpose was to evaluate the differential firing patterns of rotator cuff, peri-scapular, and shoulder synergist muscles in normal controls and in patients with symptomatic and asymptomatic massive cuff tears.

Twelve subjects were evaluated: five normals, four asymptomatics, and three symptomatics. Subjects were grouped based upon shoulder examination, and outcomes questionnaires. All cuff tear patients had MRI scans documenting a two tendon tear (supraspinatus and infraspinatus); all normals had an ultrasound examination confirming the absence of cuff pathology. Electromyographic activity from twelve muscles and kinematic data were collected simultaneously during ten functional tasks.  The twelve muscles tested included: anterior, middle, and posterior deltoids; pectoralis major, latissimus dorsi, upper, middle, and lower trapezius; serratus anterior, supraspinatus, infraspinatus, and subscapularis. The ten functional tasks were based upon the L'Insalata and ASES Standardized questionnaires and included:  reaching to the small of the back, reaching to the middle of the back, reaching across to the opposite shoulder, raising a 1-lb weight to shoulder level and overhead, raising an 8-lb weight to shoulder level and overhead, walking with a 20-lb weight, tossing a ball underhand, and throwing a ball overhand.

On average, both symptomatic and asymptomatic cuff subjects demonstrated increased muscle activation during all tasks compared to normals. Compared to symptomatic subjects, asymptomatics had significantly greater anterior deltoid activation (p<0.03), and significantly less supraspinatus and infraspinatus activation (p<0.05) with 1-lb shoulder elevation.  During the initiation of elevation, asymptomatic subjects demonstrated increased activation of the subscapularis compared to symptomatic subjects. On average, symptomatic subjects demonstrated increased activation of the peri-scapular, supraspinatus and infraspinatus muscles and decreased activation of the subscapularis during all tasks compared to asymptomatic subjects. Differential shoulder muscle firing patterns in patients with massive rotator cuff pathology may play a role in the presence or absence of symptoms. Asymptomatic subjects demonstrated more efficient use of the intact subscapularis and greater activation of the anterior deltoid during elevation, while symptomatic subjects continued to rely on torn rotator cuff tendons and peri-scapular muscle substitution resulting in compromised function.