Is there a threshold for use-dependent plasticity? A study of repeated motor skill training in the human lower limbs.

Use-dependent plasticity, induced by voluntary movement, shares neural mechanisms with stimulation-based approaches but lacks a defined dose-response profile. We tested whether sequential bouts of lower-limb visuomotor training modulates corticospinal excitability, and whether a threshold exists beyond which further training triggers homeostatic downregulation. Fourteen healthy participants performed two bouts of a force tracking task targeting the knee extensors, separated by 60 min. A control condition matched the experimental timeline without active training. Corticospinal excitability (MEP amplitude), corticospinal inhibition (silent period), short interval intracortical inhibition (SICI), and spinal excitability (LEP) were measured at baseline and at four post-task time points. Motor performance was assessed via force steadiness. Force steadiness improved across the session (P < 0.05), indicating motor skill learning. No significant changes were observed in MEP amplitude, SICI, or LEP (all P > 0.05). A delayed increase in silent period duration was noted at the final time point (P = 0.032). No session*time interactions were found. Findings suggest that repeated bouts of visuomotor training do not facilitate corticospinal output in the knee extensors. Despite behavioural improvement, neurophysiological markers remained unchanged. The lack of neurophysiological effect may reflect a task-or muscle-specific threshold for inducing plasticity, or a ceiling imposed by homeostatic mechanisms. Contrasting upper-limb protocols, lower-limb motor tasks may require greater intensity/novelty to engage plastic processes. We conclude that, under the conditions tested, the dose of voluntary movement was insufficient to alter corticospinal excitability. Going forwards, researchers should assess whether adaptive responses can be enhanced through modified task parameters or multimodal stimulation.