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Electromyographic muscle activities of the shoulder muscles during shoulder external rotation with reference to load magnitude were studied in 8 healthy male subjects without history of shoulder injuries. In addition, we discussed the relationship between rotator cuff muscles and superficial muscles. The subjects performed shoulder external rotation from 60•K internal rotation to 45•K external rotation at 20•K of shoulder abduction and 20•K of flexion. A Cybex dynamometer used to prescribe angle and velocity of the shoulder movement. At the same time, intramuscular wire electrodes were inserted into the supraspinous, infraspinous and teres minor muscles, and then surface electrodes were placed over the posterior deltoid and middle trapezius muscles. The load magnitude ranged 2 •` 18 Nm and the angle velocity was set at 15 deg/sec. The rotator cuff muscles became significantly to be activated from 2 Nm (the supraspinous and infraspinous m.) and from 4•`5 Nm (the teres minor m.) compared with the muscle activity during non-load. The superficial muscles became significantly to be activated from 4•`7 Nm (the posterior deltoid m.) and from 3•`6 Nm (the middle trapezius m.) compared with the muscle activity during non-load. Within the load range (18 Nm 46.8% MVC), %iEMG of the rotator cuff muscles was always larger than that of the superficial muscles. Therefore, we concluded that the contribution of the rotator cuff muscles was larger than that of the superficial muscles up to 3•`4 Nm, and activities of the superficial muscles increased gradually from 3•`4 Nm during shoulder external rotation. (Jpn. J. Phys. Fitness Sports Med. 2000, 49 : 481•`494) key word : shoulder external rotation, load magnitude, electromyography, rotator cuff muscles, superficial muscles

ELECTROMYOGRAPHIC ANALYSIS OF SHOULDER MUSCLES DURING SHOULDER EXTERNAL ROTATION WITH REFERENCE TO LOAD MAGNITUDE