The purpose of this study was to investigate the effect of fatigued external rotator muscles of the shoulder on the shoulder position sense of abduction，internal rotation，and external rotation． Ten healthy male college students participated in this study．All subjects had played baseball in the past．The shoulder position sense of the subjects were tested under 2 conditions：fatigue and control．Measurements in each condition were conducted in 2 days apart．ln the fatigue condition，first，shoulder joint position sense was measured．Subsequently，a fatigue task of the shoulder’s external rotator muscles was perforrned．After the fatigue task，shoulder position sense was measured again．ln the control condition，the fatigue task was not performed． In the fatigue condition，fatigue of the infraspinatus muscle was observed，and shoulder position sense of internal and extemal rotation decreased after the fatigue task was performed．This result suggested that it fatigue caused a reduction in the sensitivity of muscle spindles．However，there was no significant difference in the position sense of abduction before and after the fatigue task was performed. This is partly because the extemal rotator muscles did not act as prime movers in shoulder abduction．These results suggested that fatigue of the external rotator muscles decreased position sense in the direction of motion when the external rotator muscles functioned as prime movers．

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