Motor cortical excitability is differentially modulated by HIIT and strength exercise.

High-intensity interval training (HIIT) and strength exercise (SE) induce corticospinal plasticity in exercised and non-exercised muscles. While HIIT's systemic neuromodulatory effects are well-established, SE's comparable remote responses remain unclear. This study investigated changes in corticospinal, intracortical, and spinal excitability, as well as motor performance, in non-exercised flexor carpi radialis following single-session HIIT and plantar flexion SE. Using a randomised crossover design, eighteen participants completed: (1) 20-minute high-intensity interval cycling, (2) high-intensity resistive plantar flexion, and (3) control. Corticospinal responses were assessed using transcranial magnetic stimulation and peripheral nerve stimulation; motor performance through reaction time tasks and maximum voluntary force at baseline and post-exercise. Only HIIT significantly increased motor-evoked potential amplitude (p = 0.002) and reduced short-interval intracortical inhibition (p = 0.04). HIIT reduced silent period duration compared to baseline and control (p <0.001), while SE showed reduction only versus control (p <0.001). Both HIIT and SE improved reaction time compared to baseline and control (p <0.05). These findings highlight differential corticospinal responses in a remote, non-exercised muscle, with HIIT inducing distributed excitability enhancements compared to SE. Unilateral, small-muscle SE may be insufficient to induce widespread corticospinal excitability changes, with implications for neurorehabilitation strategies targeting remote muscle groups.