Differential patterns of muscle activation in patients with symptomatic and asymptomatic rotator cuff tears

Patients with rotator cuff tears have varying degrees of symptom expression. Our purpose was to evaluate the differential firing patterns of the rotator cuff, deltoid, and scapular stabilizer muscle groups in normal control subjects and in patients with symptomatic and asymptomatic 2-tendon rotator cuff tears. Eighteen subjects were evaluated: six normal subjects and twelve with 2-tendon cuff tears (six asymptomatic and six symptomatic). All cuff tear patients had magnetic resonance imaging (MRI) scans documenting superoposterior tear configurations involving the supraspinatus and infraspinatus tendons; all normal subjects had an ultrasound examination confirming the absence of cuff pathology. Subjects were grouped based on shoulder examination and outcomes questionnaires. Asymptomatic patients had minimal pain (<3 on the visual analog scale and no loss of active range of motion compared with the contralateral side); symptomatic patients had pain greater than 3 on the visual analog scale and decreased range of motion compared with the contralateral side (>10 degrees of motion loss). Electromyographic activity from 12 muscles and kinematic data were collected simultaneously during 10 functional tasks. Both symptomatic and asymptomatic cuff subjects demonstrated a trend toward increased muscle activation during all tasks compared with normal subjects. During the internal rotation tasks, asymptomatic patients had significantly greater (P<.05) subscapularis activity than symptomatic patients (65% maximal voluntary contraction [MVC] vs 42% MVC). During the carrying task, asymptomatic patients demonstrated significantly less (P<.03) upper trapezius muscle activation than symptomatic patients (16% MVC vs 50% MVC). During shoulder elevation tasks, symptomatic patients had significantly greater supraspinatus (52% MVC vs 28% MVC, P<.03), infraspinatus (32% MVC vs 16% MVC, P<.05), and upper trapezius (39% MVC vs 20% MVC, P<.04) muscle activation compared with asymptomatic patients. During heavy elevation (8 lb), asymptomatic patients showed a trend toward increased activation (P<.06) of the subscapularis compared with symptomatic patients (34% MVC vs 21% MVC). Differential shoulder muscle firing patterns in patients with rotator cuff pathology may play a role in the presence or absence of symptoms. Asymptomatic subjects demonstrated increased firing of the intact subscapularis, whereas symptomatic subjects continued to rely on torn rotator cuff tendons and periscapular muscle substitution, resulting in compromised function.