Intensity matters: effects of cadence and power output on corticospinal excitability during arm cycling are phase- and muscle-dependent.

The present study investigated the effects of cadence and power output on corticospinal excitability to the biceps (BB) and triceps brachii (TB) during arm cycling. Supraspinal and spinal excitability were assessed using transcranial magnetic stimulation (TMS) of the motor cortex and transmastoid electrical stimulation (TMES) of the corticospinal tract, respectively. Motor evoked potentials (MEPs) elicited by TMS and cervicomedullary motor evoked potentials (CMEPs) elicited by TMES were recorded at two positions during arm cycling corresponding to mid-elbow flexion and mid-elbow extension (i.e. 6 and 12 o'clock made relative to a clock face, respectively). Arm cycling was performed at combinations of two cadences (60 and 90 rpm) at three relative power outputs (20, 40 and 60% peak power output). At the 6 o'clock position, BB MEPs increased ~11.5% as cadence increased and up to ~57.2% as power output increased ( p<.05). In the TB, MEPs increased ~15.2% with cadence ( p=.013) but were not affected by power output, while CMEPs increased with cadence (~16.3%) and power output (up to ~19.1%, p<.05). At the 12 o'clock position, BB MEPs increased ~26.8% as cadence increased and up to ~96.1% as power output increased ( p<.05), while CMEPs decreased ~29.7% with cadence ( p=.013) and did not change with power output ( p=.851). In contrast, TB MEPs were not different with cadence or power output, while CMEPs increased ~12.8% with cadence and up to ~23.1% with power output ( p<.05). These data suggest that the 'type' of intensity differentially modulates supraspinaland spinal excitability in a manner that is phase- and muscle-dependent.