The effect of fatigued internal rotator and external rotator muscles of the shoulder on the shoulder position sense

The purpose of this study was to investigate which muscle group, the agonist or antagonist, contributes most to the shoulder position sense (SPS). The SPS was tested under 2 conditions: fatigued shoulder internal rotator (IR) muscles (pectoralis major and latissimus dorsi) and fatigued external rotator (ER) muscles (infraspinatus). In each condition, the SPS was measured before and after a fatiguing task involving the IR or ER muscles by repeating shoulder joint rotation. SPS was measured using a method in which subjects reproduced a memorized shoulder joint rotation angle. The position error values in all conditions (fatigued IR and ER muscles) and measurement periods (before- and after-fatigue task) were compared using 2-way analysis of variance with repeated measures (IR/ER × before/after). Position error increased significantly after both fatigue tasks (before- vs. after-fatigue: IR muscle, 2.68° vs. 4.19°; ER muscle, 2.32° vs. 4.05°). In other words, SPS accuracy decreased when either the agonist or antagonist muscle was fatigued. This finding indicated that SPS may be affected by an integrated information of the afferent signals in the agonist and antagonist muscles.