Cerebellar neuromodulation synchronized to ankle-assist cycling yields prolonged post-assist dorsiflexion

Rhythmic lower-limb movements require precise coordination across multiple joints, a process in which the cerebellum is thought to play a critical role. This study investigated whether cerebellar neuromodulation synchronized with mechanical ankle assistance modulates use-dependent motor adaptation during cyclic movement in healthy young adults. A portable ankle-assist ergometer (AAE) delivered phase-specific dorsiflexion torque during cycling, and participants completed three within-subject conditions: Sham stimulation, synchronized transcranial alternating current stimulation (tACS), and unsynchronized tACS. Ankle dorsiflexion angle and tibialis anterior (TA) muscle activity were assessed before, during, and after a 6 min assistance period. Baseline-normalized data were analyzed using LMMs across assist and post-assist phases. Synchronized cerebellar stimulation significantly modulated the overall magnitude of the dorsiflexion aftereffect, whereas unsynchronized stimulation did not. In contrast, TA muscle activity exhibited robust phase-dependent changes but did not differ across stimulation conditions, indicating that the kinematic modulation was not driven by sustained changes in agonist muscle activation. These findings demonstrate that phase-aligned cerebellar neuromodulation biases adaptive motor output during cyclic lower-limb movement, likely by influencing multijoint coordination and predictive control rather than peripheral muscle recruitment. The combination of a compact AAE with precisely timed, non-invasive cerebellar stimulation provides an accessible platform for probing and biasing motor adaptation, with potential implications for future lower-limb rehabilitation strategies.