ACTIVATION OF THE ROTATOR CUFF IN GENERATING ISOMETRIC SHOULDER ROTATION TORQUE

This study compared and quantified electromyographic muscle activation of the rotator cuff with the isometric torque generated by performing shoulder rotation in various positions. Twenty healthy volunteers wer e tested in 29 shoulder positions. Using a Cybex II dynamometer synchr onously with electromyography, surface electrodes were placed over the pectoralis major muscle and three parts of the deltoid muscle. Intram uscular wire electrodes were inserted into the four rotator cuff muscl es. We found that the greatest external rotation isometric force is ge nerated in the frontal and scapular planes in the neutral or full inte rnal rotation positions. The sagittal, dependent, and the scapular pla ne with 45 degrees of elevation in rotational positions of either full or half external rotation generated the greatest torques for internal rotation isometric force. The rotator cuff muscles generated greatest electromyographic activity in neutral to midrotational positions. The scapular plane with 90 degrees of shoulder elevation in neutral rotat ion best isolated the subscapularis muscle. The infraspinatus-teres mi nor muscles were isolated in the sagittal plane with 90 degrees of sho ulder elevation in a half externally rotated position. We were unable to isolate the supraspinatus muscle in any of these tested positions. These positions are recommended for manual muscle testing and for stre ngthening these muscles.