Electromyographic activity of selected shoulder muscles in commonly used therapeutic exercises

The purpose of this study was to evaluate and compare the muscle activity of the supraspinatus, infraspinatus, teres minor, and lower trapezius muscles during commonly prescribed therapeutic exercises in subjects with and without shoulder pathology.

Twenty healthy subjects (9 male, 11 female) and 20 subjects with recurrent unilateral shoulder pain and weakness (14 male, 6 female), aged 18 to 40 years (mean = 28, SD = 5.8), participated in this study.

Subjects performed each of the following exercises using a hand-held weight: prone lateral (external) rotation, sidelying lateral rotation, and arm elevation in the scapular plane. Indwelling fine-wire electrodes recorded electromyographic (EMG) activity during each exercise. The EMG activity in five phases of concentric contraction of each exercise was averaged and divided into three equal time intervals. Mean EMG values normalized to maximal activity for the entire phase of concentric contraction and for each of the three intervals were used in subsequent analyses.

Two-way repeated-measures analyses of variance (ANOVAs) revealed between-group differences only in the prone lateral rotation exercise. Compared with subjects without shoulder pathology, subjects with shoulder pain showed significantly greater EMG activity in the infraspinatus muscle and less activity in the supraspinatus muscle during this exercise.

These results suggest that the pattern of muscle activation during specific shoulder movements in patients with shoulder pain may be related to pathology. Future studies are needed to determine whether an imbalance in neuromuscular control is a factor contributing directly to shoulder dysfunction or whether such an imbalance is secondary to some pathology.