Transient improvement induced by motor fatigue in focal occupational dystonia: the handgrip test.

Muscle fatigue induced by a previous sustained contraction temporarily decreases the motor output, transiently worsening motor performance. Whether muscle fatigue alters motor performance also in dystonia-a disorder whose main pathophysiological abnormality is motor overflow-remains unknown. To assess the effects of muscle fatigue in patients with focal occupational upper limb dystonia, we studied the effect of a previous maximum fatiguing voluntary contraction on motor performance in 10 musicians with focal occupational dystonia, in 3 musicians with hand motor impairment due to non-dystonic disorders, and in 5 normal musicians. The fatiguing task consisted of grasping a spring handgrip as long as possible until the task failed. In dystonic musicians, a fatiguing contraction significantly improved motor performance. The improvement lasted less than 5 minutes and appeared only after fatigue of the affected upper limb. In contrast, in musicians with non-dystonic motor impairment, motor performance remained unchanged or worsened, and normal musician performance consistently worsened.