Ipsilateral motor cortex regulates distinctly via interhemispheric inhibition force production versus force control in isometric unimanual contraction.

Ipsilateral primary motor cortex (M1) contributions to force production are recognized, whereas conflicting evidence persists regarding movement-modulated short-interval interhemispheric inhibition (SIHI) dynamics and their role in force control precision. This study systematically investigates how ipsilateral M1 regulates unimanual force production through SIHI, focusing on its dual role in force production and force control precision. Experiment 1 used short trains of 10-Hz transcranial magnetic stimulation (TMS) to evaluate the effects of disruption on the ongoing activity of ipsilateral M1 on force-tracking variability during 20% and 50% maximal voluntary contraction (MVC). Experiment 2 used dual-coil TMS to quantify SIHI dynamics during sustained isometric contractions. Behavioral performance was assessed using the coefficient of variation (CV). Disruptive repetitive TMS of the ipsilateral M1 significantly increased force-tracking CV at both 20% (P = 0.002) and 50% MVC (P < 0.001), confirming its facilitatory role. Sustained contractions reduced ipsilateral-to-active SIHI (P < 0.001), whereas persistent SIHI during isometric contraction positively correlated with force control precision at both 20% (r = 0.41, P = 0.045) and 50% MVC (r = 0.59, P = 0.003). This study demonstrates that ipsilateral M1 regulates unilateral hand force production and precision through dynamic bidirectional modulation of SIHI, highlighting its dual role in isometric unimanual contraction and potential implications for poststroke rehabilitation.NEW & NOTEWORTHY This study reveals a dual regulatory mechanism by which the ipsilateral M1 optimizes unimanual force production and control through dynamic modulation of SIHI. We demonstrate that ipsilateral M1 facilitates force generation by attenuating SIHI, whereas persistent SIHI during isometric contraction correlates with precision during sustained contractions.