We reported previously that concentric or eccentric exercise can lead to errors in human limb position sense. Our data led us to conclude that the errors, post-exercise, were not due to an altered responsiveness of the proprioceptive afferents, and we proposed that they resulted from central changes in the processing of the afferent input. However, it remained uncertain what was responsible for triggering those changes, the volume of afferent traffic during the exercise or the developing fatigue. The afferent traffic hypothesis was tested by subjects carrying out a series of 250 lightly loaded concentric contractions of elbow flexors that produced little fatigue (6 ). This did not lead to significant position errors. In a second experiment, a series of fatiguing isometric contractions, which kept movements of the muscle to a minimum, led to a 24 fall in force and significant position errors (3 degrees , direction of extension). In the third experiment, at 24 h after eccentric exercise, when the short-term effects of fatigue and accumulated metabolites were gone, but force was still 28 below control values, this was accompanied by significant position errors in the direction of extension, 3.2 degrees in the relaxed arm and 3.3 degrees in the self-supported arm. It is concluded that it is the fall in force accompanying exercise which is responsible for disturbing limb position sense. It is suggested that the exercise effects are generated in the brain, perhaps as a result of an alteration of the body map, triggered by the fall in force.