Abstract
Exercises for the inner muscles of the shoulder have been developed using elastic bands for postinjury rehabilitation and injury prevention. It is important to maintain a careful balance between the inner muscles (rotator cuff muscles) and the outer muscles when performing various movements of the shoulder joint. If this balance is not maintained, that is if the function of the inner muscles is at a lower level than that of the outer muscles, pain or damage can arise in the shoulder. Although low loads are generally used to improve such imbalances in inner muscle exercises, little is known about appropriate low-level exercise loads for selective conditioning of the inner muscles. The purpose of our study was to determine effective ranges of loads and motion angles during low-level shoulder exercises by analyzing electromyographic (EMG) data from outer muscles. Twelve normal male subjects without any history of shoulder injuries participated in the experiments (mean age 23.3). Using a Cybex 770 dynamometer to control the exercise load and the angle of motion, the subjects performed external rotations and abduction movements for the shoulder. The range of loads was 0 to 20 N・m, the range of motion was 0 to 120 degrees, and the angular velocity for the movements was fixed at 15 deg/sec. Dynamic EMGs were recorded with surface electrodes over the ten shoulder muscles. The raw EMG signals for each load, which were separated into eight equal parts (each covering 15 degrees), were rectified and integrated for each part. The integrated EMG data, which were normalized with the EMG data for a maximum voluntary contraction, were compared for each load and each motion angle condition. For external rotation of the shoulder, loads of less than 10 N・m for motion angles below 60 degrees and of less than 8 N・m for angles below 90 degrees were found not to produce strong electromyographic activity in the outer muscles. The ranges which did not produce significant activity in the outer muscles, as compared with no-load condition, were loads of less than 6 N・m for motion angles below 30 degrees and of less than 4 N・m for angles below 45 degrees. For shoulder abduction, loads of less than 10 N・m for motion angles below 30 degrees, of less than 8 N・m for angles below 60 degrees, and of less than 4 N・m for angles below 75 degrees were found not to produce strong activity in the outer muscles. The range which did not produce significant activity in the outer muscles, compared with no-load condition, was that of loads less than 4 N・m for motion angles below 30 degrees. The results of our experiments clearly show that there are upper limits for loads and motion angles in performing external rotation and abduction movements of the shoulder without involving the outer muscles. The exercise condition under this upper limit could be recommended for the selective training of the inner muscles. We suggest, in addition, that there is a shift in the range of loads and motion angles from levels that mainly activate the inner muscles to levels that mainly activate the outer muscles.
