The purpose of this study was to investigate (1) the activities of shoulder muscle under varying positions, loads, and motion angles during scapular plane abduction and (2) the effectiveness of exercise for an abductor, examined from muscular activity of each shoulder muscle.
Seven healthy male subjects (age 23.4±1.4 years) without any history of shoulder pathology participated in the experiment. The three conditions of scapula plane abduction, 1) with humeral external rotated (Full can position), 2) with humeral internal rotated (Empty can position), and 3) both middle (Neutral position), were performed from 0 to 90 degrees of abduction angle. During the scapular plane abduction, the applied load was 0-16 Nm. A Cybex 770-NORM dynamometer was used to control the angular velocity at 15 deg/sec. The electromyogram (EMG) was recorded from the supraspinatus with a fine-wire intramuscular electrode, the anterior, middle and posterior deltoid, and the upper trapezius with bipolar surface. The raw EMG signals for each muscle and load were divided at each 5 deg, then rectified, and the RMS (root mean square) calculated. The RMS of each muscle and load, which was normalized with the RMS in the manual muscle test used to elicit maximum activity, was compared for each load and muscle. Additionally, the increase aspect of %RMS with the change of the abduction angle of the supraspinatus, the middle deltoid, and the upper trapezius was compared.
For loads lower than 4 Nm with motion angle below 30 degrees, %RMS of the supraspinatus showed significant activity compared with no-load conditions in Full can and Neutral positions. In contrast, %RMS of the supraspinatus and the middle deltoid showed significant activity compared with no-load condition in Empty can position. For loads of 8 to 16 Nm and abduction angles of 30 to 90 degrees, %RMS of the supraspinatus and the middle deltoid demonstrated equivalent activity in Full can position and significant activity, in which the middle deltoid was comparable with the supraspinatus in Neutral and Empty can positions. The activity aspect of the upper trapezius in Empty can position showed differences between Full can and Neutral positions.
In conclusion, these findings suggest that abduction position (Full can position, Empty can position, and Neutral position), load, and motion angle are closely related to shoulder muscle activity and coordination. In addition, the Full can position and Neutral position seem to strengthen the functionality of the supraspinatus below 4 Nm for motion angles below 30 degrees.
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