Infraspinatus Isolation During External Rotation Exercise at Varying Degrees of Abduction

External rotation (ER) strengthening exercises are a common component of shoulder injury prevention and rehabilitation programs. They are primarily intended to target the infraspinatus muscle, based on its role in glenohumeral stabilization and inferior humeral glide. ER also recruits the posterior deltoid, which can be undesirable due to its role in subacromial space narrowing.

To determine the angle of humeral abduction that maximizes the infraspinatus to posterior deltoid activation ratio (INFRA/PD) during ER.

Within-subjects repeated-measures controlled lab trial.

A total of 10 healthy participants (5 males, 5 females) aged 21 (0.67) years participated in the study.

7 consecutive repetitions of ER at 7 different abduction angles ranging from 0° to 90°, with resistance normalized to 3% body mass.

Surface electromyography was performed on the infraspinatus, middle deltoid, and posterior deltoid. Surface electromyography data were processed to determine absolute muscle activation as well as INFRA/PD at each abduction angle. Group means were compared between abduction angles using 1-way analysis of variance.

Abduction significantly reduced overall infraspinatus activity but increased posterior deltoid activity (P < .01). Average and peak INFRA/PD decreased as the angle of abduction increased (P < .001 and P < .01, respectively).

Our findings suggest that ER should be performed in 0° of abduction to maximize infraspinatus isolation. Slight abduction, such as placing a towel under the humerus, as recommended by some clinicians, may improve patient comfort, but did not increase infraspinatus isolation in this study.