Shoulder musculature activation during upper extremity weight-bearing exercise

Study Design: Repeated-measures design comparing 7 static weight-bearing shoulder exercises. Objective: The purpose of this study was to determine the demand on shoulder musculature during weight-bearing exercises and the relationship between increasing weight-bearing posture and shoulder muscle activation. Background: Weight-bearing shoulder exercises are commonly prescribed in the rehabilitation of shoulder injuries. Limited information is available as to the demands placed on shoulder musculature while these exercises are performed. Methods: Eighteen healthy college students volunteered for this study. Surface bipolar electrodes were applied over the infraspinatus, posterior deltoid, anterior deltoid, and pectoralis major muscles. Fine-wire bipolar intramuscular electrodes were inserted into the supraspinatus muscle. Electromyographic (EMG) root mean square signal intensity was normalized to 1 second of EMG obtained with a maximal voluntary isometric contraction (MVIC). Subjects were tested under 7 isometric exercise positions that progressively increased upper extremity weight-bearing posture. Results: There was a high correlation between increasing weight-bearing posture and muscular activity (r = 0.97, P<0.01). There was relatively little demand on the shoulder musculature for the prayer and quadruped positions (2%-10% MVIC). Muscular activation was greater for the infraspinatus than for other shoulder muscles throughout most of the exercise positions tested. Conclusion: These results indicate that alterations of weight-bearing exercises, by varying the amount of arm support and force, resulted in very different demands on the shoulder musculature. Specifically, the infraspinatus was particularly active during the weight-bearing exercises used in this study.