The purpose of this study was to elucidate the effects of “shoulder girdle training” that is used to correct scapular muscle imbalance. We investigated 100 shoulders in 50 healthy volunteers free of any complaints related to the shoulder joint. In the posture for performing the infraspinatus test, male volunteers held a 5-kg load and female volunteers held a 3-kg load for 3 s thrice, and the muscular activity of the trapezius muscle was simultaneously measured. Next, shoulder girdle training was performed for 5 s twice, followed by muscle activity measurement in the same manner described above. Muscle activity levels before and after this training and also between the dominant and non-dominant sides were compared. The shoulder girdle training significantly reduced the level of muscle activity of the upper fibers of the trapezius muscle and significantly increased the level of muscle activity of the middle and lower fibers of the trapezius muscle (both p<0.05). Moreover, the levels of muscle activity of the upper, middle, and lower fibers of the trapezius muscle were significantly higher on the dominant side than on the non-dominant side (p<0.05).