Short-interval intracortical inhibition in knee extensors during locomotor cycling.

AIM: Short-interval intracortical inhibition (SICI) can provide information on changes in cortical responsiveness during voluntary contractions. It is, however, unknown whether the magnitude of SICI changes throughout the cycle of rhythmic movements such as leg cycling.
METHODS: The effects of four conditioning stimulus (CS) intensities, 70, 80, 90 and 95% of active motor threshold (AMT), on the magnitude of SICI were tested during three conditions: (1) activation phase of the electromyography (EMG) burst, (2) deactivation phase of the EMG burst and (3) static contractions. The three conditions were matched for EMG amplitude and test motor-evoked potential (MEP) size with reference to the vastus lateralis muscle. Responses were also recorded from rectus femoris and vastus medialis.
RESULTS: short-interval cortical inhibition was weak during static knee contractions (15% reduction in control MEP) relative to previous reports during contractions in other muscle groups. SICI was abolished during the activation phase of the knee extensor EMG burst (P > 0.05), but present (approx. 90% of control MEP size) during the deactivation phase of EMG (P < 0.05). Furthermore, inhibition was elicited at a lower CS intensity during the deactivation phase of EMG during cycling than during static contractions (70 AMT vs. 90% AMT).
CONCLUSION: The results suggest that the efficacy of intracortical inhibitory projections to knee extensor corticomotoneurons is particularly weak during muscle activation. A lower threshold of activation for inhibitory cells during deactivation phase of cycling EMG was evident, and there was a phasic modulation of intracortical inhibition affecting corticospinal projections to the working muscles.