The goal of our studies is to examine the mechanism of the muscle output of the shoulder joint. In this paper we compared and quantified electromyographic muscle activities and the onset order of the rotator cuff and shoulder superficial muscles in relation to the change in concentric torque generated by performing shoulder external rotation.
Twenty-four healthy male subjects (mean age 23.3) without any history of shoulder injury participated in the experiments. A Cybex 770 dynamometer was used to control the load and the angle of motion; the range of loads was 0-18 Nm, the range of motion was 75-0 degrees of internal rotation, and the angular velocity of the movements was fixed at 15 deg/sec. Electromyographs (EMGs) were recorded using surface electrodes placed on the shoulder superficial muscles and intramuscular wire electrodes from the rotator cuff. The raw EMG signals for each load were separated into five equal parts (each covering 15 degrees), which were then separately rectified and integrated. The integrated EMG data, which was normalized against data for maximum voluntary contraction, was compared for each load and each angle condition.
The results of this experiment clearly showed that the supraspinatus (SSP), middle deltoid (MD), and middle trapezius (MT), as well as infraspinatus (ISP), teres minor (TM), and posterior deltoid (PD) muscles took part in external rotational movements. The EMG-onsets of the rotator cuff were generally faster, and their muscle activities were greater, than in the superficial muscles. For each muscle, however, the degree of participation was markedly dependent on the load and angle of motion.
The EMG-onset of the ISP was the fastest, and the muscle activity of the ISP was the highest of all muscles, irrespective of load or angle of motion. It is thus certain that the ISP plays an important role as both a stabilizer and a mover during external rotational movement. At low loads, the TM demonstrated a pattern of muscle activity similar to that of the ISP, but it was notable that muscle activity did not increase for loads greater than 30-40% MVC. On the other hand, the muscle activity of the SSP was lower than that of the TM in the area approaching full internal rotation and at low loads. As the load increased, however, muscle activity of the SSP became higher than that of the TM. The TM and the SSP, therefore, may have different functional roles depending on the situation; both muscles may contribute to external rotational movements as the supporting mover.
The muscle activity of the MD was higher than that of the PD in the area approaching full internal rotation. Clearly, not only the PD but the MD also takes part in external rotational movements, depending on the angle of motion. The EMG-on-set of the MT was faster, and the muscle activity of the MT at low loads was higher, than seen in other superficial muscles. Moreover, the activity of the MT increased with increasing load and degree of external rotation. These results suggest that the MT assists the mover muscles by anchoring the scapula at low loads but adducting it at high loads and in the area of external rotation.
