Unilateral contractions modulate interhemispheric inhibition most strongly and most adaptively in the homologous muscle of the contralateral limb.

We investigated how volitional contractions affect interhemispheric inhibition (IHI) from the active to the passive hemisphere. Younger and older adults isometrically contracted their dominant thumb (abductor pollicis brevis, APB) to various force targets. In ballistic contraction trials, transcranial magnetic stimulation (TMS) was administered very shortly after the onset of APB activity. In tonic contraction trials, TMS was delivered while the target force was maintained. In control trials both thumbs remained quiescent. In all trials, a test stimulus (TS) was directed to the APB hotspot in the non-dominant hemisphere (130% left APB resting motor threshold, RMT). In half the trials, a conditioning stimulus (130% right APB RMT) was applied to the APB hotspot in the dominant hemisphere 10 ms prior to the TS. Targeted ballistic contractions of the right APB were found to modulate (increase) IHI measured in the left APB, as previously reported for tonic contractions. Furthermore, the extent of the IHI increase was found to scale with the strength of the contralateral ballistic or tonic contraction. Less pronounced, but statistically significant, IHI increases were also observed in the left abductor digiti minimi and extensor carpi radialis during right APB contraction. For these muscles, however, the extent of the IHI modulation was independent of APB contraction strength. The capacity to modulate inhibition during contractions was unaffected by advancing age. During volitional actions, the ability to modulate IHI most adaptively in the homologous muscle of the resting limb may contribute to the prevention of mirror movements.