Modulation of Corticospinal Excitability with Contralateral Arm Cycling

This is the first study to examine the influence of activity in one limb on corticospinal excitability to the contralateral limb during a locomotor output. Corticospinal and spinal excitability to the biceps brachii of the ipsilateral arm were assessed using transcranial magnetic stimulation (TMS) of the motor cortex and transmastoid electrical stimulation (TMES) of corticospinal axons, respectively. Responses were evoked during the mid-elbow extension position of arm cycling across three different cycling tasks: (1) bilateral arm cycling (BL), (2) unilateral, contralateral cycling with the ipsilateral arm moving passively (IP), and (3) unilateral, contralateral cycling with the ipsilateral arm at rest (IR). Each of these three tasks were performed at two cadences: 60 and 90 rpm. TMS-induced motor evoked potential (MEPs) amplitudes were significantly smaller during BL compared to the IP and IR conditions; however, MEP amplitudes were not significantly different between IP and IR. TMES-evoked cervicomedullary MEP (CMEPs) amplitudes followed a similar pattern of task-dependent modulation, with BL having the smallest CMEPs and IR having the largest. In line with our previous findings, MEP amplitudes increased and CMEP amplitudes decreased as the cadence increased from 60 to 90 rpm. We suggest that the higher corticospinal excitability to the ipsilateral limb during the IP and IR conditions was predominantly due to disinhibition at both the cortical and spinal levels.