Motor cortex contribution to dynamic force regulation in the index finger revealed by TMS interference.

Lifting a paper cup without crushing it requires precise regulation of finger force. In humans, such control is thought to rely primarily on the contralateral corticospinal tract (CST), but the cortical mechanisms underlying graded force modulation remain unclear. To address this, we used online repetitive transcranial magnetic stimulation (rTMS) as a transient interference during an isometric index-finger force-tracking task. Eleven healthy adults increased and decreased index-finger isometric force at low (0-20% maximal voluntary contraction, MVC) and high (0-60% MVC) levels, while 2-s trains of 10 Hz rTMS were delivered over contralateral or ipsilateral primary motor cortex (M1; cM1, iM1) or a midline control site (Pz). Accuracy, consistency, and force smoothness were quantified using RMSE, SDAE, and normalized YankRMS within the 2-s stimulation windows, and outcomes were normalized to Pz. Factorial mixed-effects analysis showed a significant main effect of stimulation-site (Condition) across all three measures (all p < 0.0022). Holm-adjusted comparisons showed greater disruption during cM1 than iM1 stimulation and no-TMS, whereas iM1 did not differ from no-TMS. For YankRMS, the Condition × Intensity interaction was significant (p=0.0031), with a larger cM1-iM1 difference at the higher force level (60% MVC) than at the lower force level (20% MVC; interaction contrast, p=0.0033). No other task-dependent interactions were significant. These findings provide time-locked causal evidence that ongoing cM1 activity is essential for dynamic index-finger force regulation, with the contralateral-ipsilateral asymmetry becoming more pronounced at higher force demands.