The impact of localized fatigue on contralateral tremor and muscle activity is exacerbated by standing posture.

Physiological tremor is an inherent feature of the motor system that is influenced by intrinsic (neuromuscular) and/or extrinsic (task) factors. Given that tremor must be accounted for during the performance of many fine motor skills; there is a requirement to clarify how different factors interact to influence tremor. This study was designed to assess the impact localized fatigue of a single arm and stance position had on bilateral physiological tremor and forearm muscle activity. Results demonstrated that unilateral fatigue produced bilateral increases in tremor and wrist extensor activity. For example, fatigue resulted in increases in extensor activity across both exercised (increased 8-10% MVC) and the non-exercised arm (increased 3-7% MVC). The impact of fatigue was not restricted to changes in tremor/EMG amplitude, with altered hand-finger coupling observed within both arms. Within the exercised arm, cross-correlation values decreased (pre-exercise r=0.62-0.64; post-exercise r=0.37-0.43) while coupling increased within the non-exercised arm (pre-exercise r=0.51-0.55; post-exercise r=0.62-0.67). While standing posture alone had no significant impact on tremor/EMG dynamics, the tremor and muscle increases seen with fatigue were more pronounced when standing. Together these results demonstrate that the combination of postural and fatigue factors can influence both tremor/EMG outputs and the underlying coordinative coupling dynamics.