Interlimb Coordination Selectively Modulates Short-Interval Intracortical Inhibition During Arm Cycling

Corticospinal excitability is greater during synchronous than asynchronous arm cycling; however, potential cortical mechanisms underlying this coordination-dependent modulation remain unclear. The purpose of this study was to examine whether short-interval intracortical inhibition (SICI) and long-interval intracortical inhibition (LICI) differ between synchronous and asynchronous arm cycling. We hypothesized that both forms of intracortical inhibition would be greater during asynchronous cycling. Paired-pulse transcranial magnetic stimulation (TMS) was used to assess SICI and LICI in healthy adults during arm cycling at a fixed cadence and workload. SICI was assessed using a conditioning stimulus at 80% active motor threshold and a test stimulus at 120% active motor threshold with a 3-ms interstimulus interval. For LICI, suprathreshold paired-pulses were delivered with a 100-ms interstimulus interval and timed to the ascending phase of biceps brachii activation. Motor evoked potentials were quantified using peak-to-peak amplitude and expressed as paired-pulse to single-pulse ratios. SICI was significantly greater during synchronous compared with asynchronous arm cycling, whereas LICI did not differ between cycling modes. Background muscle activity was comparable across conditions. These findings indicate that interlimb coordination selectively modulates fast-acting intracortical inhibitory mechanisms during rhythmic arm cycling and provide mechanistic insight into coordination-dependent modulation of corticospinal excitability during locomotor-like movement.