Two weeks of arm cycling sprint interval training enhances spinal excitability to the biceps brachii.

The present study aimed to investigate whether a 2-week arm cycling sprint interval training (SIT) program modulated corticospinal pathway excitability in healthy, neurologically intact participants. We employed a pre-post study design with two groups: 1) an experimental SIT group and 2) a non-exercising control group. Transcranial magnetic stimulation (TMS) of the motor cortex and transmastoid electrical stimulation (TMES) of corticospinal axons were used at baseline and post-training to provide indices of corticospinal and spinal excitability, respectively. Stimulus-response curves (SRCs) recorded from the biceps brachii were elicited for each stimulation type during two submaximal arm cycling conditions ((25 watts (W) and 30% peak power output (PPO)). All stimulations were delivered during the mid-elbow flexion phase of cycling. Compared to baseline, performance on the time-to-exhaustion (TTE) test at post-testing was improved for members of the SIT group but was not altered for controls, suggesting that SIT improved exercise performance. There were no changes in the area under the curve (AUC) for TMS-elicited SRCs for either group. However, the AUC for TMES-elicited cervicomedullary motor evoked potential SRCs were significantly larger at post-testing in the SIT group only (25W: =0.012, =0.870; 30% PPO: =0.016, =0.825). This data shows that overall corticospinal excitability is unchanged following SIT, while spinal excitability is enhanced. While the precise mechanisms underlying these findings during arm cycling at post-SIT are unknown, it is suggested that the enhanced spinal excitability may represent a neural adaptation to training.