Effects of Trunk Postures and Shoulder Flexion Angles on the Muscular Activity of Scapular Upward Rotators

Abstract

The forward bending posture is a risk factor that can cause musculoskeletal injury for workers working in industry. Previous studies have been largely concerned with examining the effects of turtle neck, slouched posture, and kyphosis on the scapular upward rotators. However, no studies have investigated the effect of forward bending of the trunk on the scapular upward rotators to date. The purpose of this study was to investigate the effect of forward tilting of the trunk and the shoulder flexion angle on the muscular activity of the Upper trapezius. The participant population comprised of 10 male adults and 20 shoulders in total were assessed. To begin with, EMG electrodes were attached to the upper trapezius muscles. The participants were instructed to perform isometric shoulder flexion at 120° angles in upright standing and forward bending postures. The muscle activity was measured by surface electromyography to obtain a root mean square value and was expressed as a percentage of the mean maximal voluntary isometric contraction (%MVIC). A paired t-test was used to determine the changes in muscle activity according to trunk posture and a shoulder flexion angle. A paired t-test was performed at α=.05. The result of Significant main effect for trunk posture was found for the upper trapezius muscles(p<.05). Additionally, a significant main effect for shoulder flexion angle was revealed in the upper trapezius muscles(p<.05). The results demonstrate that forward bending of the trunk may increase the activity of the upper trapezius. Therefore, industrial workers who manifest hyperactivation of the upper trapezius should maintain upright trunk posture while working.