Activation of the Rotator Cuff in Generating Isometric Shoulder Rotation Torque

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Abstract
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 were tested in 29 shoulder positions. Using a Cybex II dy namometer synchronously with electromyography, surface electrodes were placed over the pectoralis major muscle and three parts of the deltoid muscle. Intramuscular wire electrodes were inserted into the four rotator cuff muscles. We found that the greatest external rotation isometric force is generated in the frontal and scapular planes in the neutral or full internal rotation positions. The sagittal, dependent, and the scapular plane with 45° of elevation in rotational posi tions of either full or half external rotation generated the greatest torques for internal rotation isometric force. The rotator cuff muscles generated greatest electro myographic activity in neutral to midrotational posi tions. The scapular plane with 90° of shoulder eleva tion in neutral rotation best isolated the subscapularis muscle. The infraspinatus-teres minor muscles were isolated in the sagittal plane with 90° of shoulder ele vation in a half externally rotated position. We were unable to isolate the supraspinatus muscle in any of these tested positions. These positions are recom mended for manual muscle testing and for strengthen ing these muscles.

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