Asymmetries of bilateral isometric force matching with movement intention and unilateral fatigue.

During bilateral coordination, some level of inter-hemispheric remapping (i.e., the congruency between afferent and efferent force signals from both hemispheres) is required. In this case, sensory-motor information is exchanged between the two hemispheres, but it remains unclear whether this information exchange is always equivalent or not, especially in a bilateral isometric force-matching task. We used unilateral fatigue applied to one arm in order to determine whether inter-hemispheric remapping can vary asymmetrically during a bilateral isometric matching task. Because fatigue is considered to bias the sensory-motor system, we hypothesized that if bimanual coordination is modulated solely in function of the state of the sensory-motor system (motor efferences, inter-hemispheric inhibitions, and sensory reafferences), we should not observe any asymmetric effect of fatigue with movement intention (leading vs. matching arm). However, if any other process could participate in the modulation of inter-hemispheric remapping, we should observe an interaction between movement intentions and fatigue on the force produced. We found that, when the leading arm was the non-fatigued arm, participants succeeded in reproducing the force level with their fatigued arm. By contrast, when the leading arm was fatigued, subjects over-estimated the force level produced with their non-fatigued arm. Hence, lateralized fatigue exacerbates an asymmetric behavior that seems modulated by movement intention (leading vs. matching). In other words, when unilateral fatigue is introduced in a bilateral isometric force-matching task, inter-hemispheric remapping is asymmetrical. Intensity levels of motor commands sent to both arms (directly or modulated through inter-hemispheric inhibitions) and sensory reafferences alone cannot explain these observations. Some attentional focus may be not balanced continuously between both arms but may be mainly directed toward the matching arm.