Probing the Link Between Cortical Inhibitory and Excitatory Processes and Muscle Fascicle Dynamics

Abstract During movements, neural signals are translated into shortening of muscle fibers. This study investigated cortical excitatory and inhibitory processes in relation to muscle fascicle dynamics during isometric explosive contractions. Fourteen adults performed submaximal and maximal dorsiflexions. Single and paired pulse transcranial magnetic stimulation of the tibialis anterior (TA) was applied during rest, the activation and deactivation phase of contractions to test for short- (SICI) and long-interval intracortical inhibition (LICI) and intracortical facilitation (ICF). Ultrasound images were taken to measure muscle fascicle dynamics of the superficial (TASF) and deep (TADP) TA compartments. The results show significantly greater maximal shortening velocities (p = 0.003) and greater maximal fascicle shortening (p = 0.003) in TASF than TADP during submaximal dorsiflexions. Significantly lower SICI levels during activation compared to deactivation (p = 0.019) and at rest (p<0.0001) were observed. ICF was significantly greater during activation (p = 0.03) than during rest while LICI did hardly modulate. Maximal TASF but not TADP shortening velocity correlated with SICI levels at activation (p = 0.06) and with the rate of torque development (p = 0.02). The results suggest that SICI influences muscle fascicle behavior and that intracortical inhibition and excitation are phase-dependently modulated.