Efficacy of High-Definition Transcranial Alternating Current Stimulation (HD-tACS) at the M1 Hotspot Versus C3 Site in Modulating Corticospinal Tract Excitability.

Previous studies have shown that beta-band transcranial alternating current stimulation (tACS) applied at the M1 hotspot can modulate corticospinal excitability. However, it remains controversial whether tACS can influence motor unit activities at the spinal cord level. This study aims to compare the efficacy of applying tACS over the hotspot versus the conventional C3 site on motor unit activities and subsequent behavioral changes. This study used a randomized crossover trial design, where fifteen healthy participants performed a paced ball-squeezing exercise while receiving high-definition tACS (HD-tACS) at 21 Hz and 2 mA for 20 min. HD-tACS targeted either the flexor digitorum superficialis (FDS) hotspot or the C3 site, with the order of stimulation randomized for each participant and a 1-week washout period between sessions. Motor unit activities were recorded from the FDS. HD-tACS intervention significantly reduced the variability of motor unit firing rates and increased force variability during isometric force production. The significant modulation effects were seen only when the intervention was applied at the hotspot, but not at the C3 site. Our findings demonstrate that HD-tACS significantly modulates motor unit activities and force variability. The results indicate that cortical-level entrainment by tACS can lead to the modulation of spinal motor neuron activities. Additionally, this study provides further evidence that the C3 site may not be the optimal target for tACS intervention for hand muscles, highlighting the need for personalized neuromodulation strategies.