Purpose: Muscle imbalance between upper trapezius (UT) and serratus anterior (SA) during arm elevation is a factor causing shoulder dysfunction. However, there is no study to compare the muscle activities of the UT and middle deltoid (MD). The purpose of this study was to compare the muscle activities of the UT and MD between with and without elevation of shoulder girdle (ESG) during shoulder abduction. Methods: The subjects without (control group=9) or with (ESG group=8) participated in this study. The muscle activities of the UT and MD were measured using a electromyography during $90^{circ}$ shoulder abducted position in both group. The data in middle of 3-second of the 5-second periods were used. The mean value of three trials was used in the data analysis. For each muscle, independent t-tests were performed to compare for group differences. Results: The muscle activity of UT was significantly greater in ESG group, compared to that of the control group (p<0.05). The muscle activity of MD was significantly smaller in ESG group, compared to that of the control group (p<0.05). Conclusion: These findings showed that low muscle activation of MD as well as SA may contribute to hyperactivity of UT during arm elevation.