The purpose of this research is to examine the effect of the direction of neck rotation on the muscle activity and the rate of upper trapezius and lower trapezius during shoulder external rotation. Based on three different neck rotation directions (ipsilateral direction, neutral direction, and contralateral direction), 31 adults' muscle activity and rate of upper trapezius and lower trapezius are analyzed. By following the tester' s commands performed on a therapeutic table, the subjects of this research externally rotated their shoulder with shoulder abduction 90° and elbow flexion 90° in three different neck directions. The posture was maintained for five seconds and repeated three times for each different neck rotation direction. To minimize muscle fatigue, 60-seconds of rest was provided before the repetitive measurements in same task, and 10-minutes of rest was offered before starting a new test. Repeated measures ANOVA is used for statistical analysis. The results are as follows. In upper trapezius, rotating the neck in the ipsilateral direction reduces the muscle activity of upper trapezius during external shoulder rotation in a prone position. Also, compared with neutral direction, the contralateral direction exhibits significantly increased muscle activity of upper trapezius. In lower trapezius, the muscle activity of lower trapezius increases more for the ipsilateral direction than the neutral direction during the external shoulder rotation in a prone position, but the difference is not significant whereas it is lower for the contralateral direction with a significance level of difference. As a result of comparing the rate of muscle activity of upper trapezius for lower trapezius, the rate of muscle activity of upper trapezius for lower trapezius significantly increases more for the ipsilateral direction than the neutral direction during external shoulder rotation. Compared with the neutral direction, the contralateral direction demonstrates a lower level of rate of muscle activity of upper trapezius for lower trapezius. Therefore, the rate of muscle activity of upper trapezius for lower trapezius significantly increased in the ipsilateral direction than the neutral and contralateral direction. To resolve the muscle imbalance caused by the over-activation of upper trapezius and the weakening of lower trapezius, this research indicates that the muscle activity of upper trapezius should be limited while selectively reinforcing lower trapezius. An effective intervention is to rotate neck in the ipsilateral direction during external shoulder rotation.
Conclusionally, when a scapular is a mal-alignment caused by over-activation of upper trapezius and weakness of lower trapezius or when it is difficult to raise their arm in a prone position due to other pathological problems, an alternative solution is to rotate their neck in the ipsilateral direction during external shoulder rotation.