Investigating premotor corticospinal excitability in fast and slow voluntary contractions of the elbow flexors

Corticospinal excitability (CSE) increases before a voluntary contraction; however, the relative contributions of premotor cortical and spinal mechanisms are poorly understood. It is unknown whether the intended voluntary contractile rate affects CSE. Eighteen young, healthy participants (9 females) completed isometric elbow flexion contractions targeting 50% maximal voluntary contraction (MVC) torque, at either fast (fast as possible) or slow (25% MVC/s) contractile rates. Participants were cued to contract with warning (red) and "GO" (green) visual signals. Magnetic and electric stimulations were applied to elicit motor evoked potentials (MEPs), cervicomedullary motor evoked potentials (CMEPs), and M-waves, in the surface electromyogram (EMG) recorded over the biceps brachii. MEPs and CMEPs were collected at 0, 25, 50, and 75% of premotor reaction time (RT - defined as the time between the "GO" cue and onset of biceps brachii EMG) and compared with a resting baseline. MEP amplitude was greater than baseline at 75% RT (P = 0.009), and CMEP amplitude was significantly increased at all RT points relative to baseline (P ≤ 0.001). However, there were no differences in MEP and CMEP amplitudes when compared between fast and slow conditions (P ≥ 0.097). Normalized to the CMEP, there was no difference in MEP amplitude from baseline in either contractile condition (P ≥ 0.264). These results indicate that increased premotor CSE is a spinally mediated response. Furthermore, premotor CSE is not influenced by the intended voluntary contractile rate. CMEP amplitudes were larger for females than for males within the premotor RT period (P = 0.038), demonstrating that premotor spinal excitability responses may be influenced by sex.NEW & NOTEWORTHY The present study demonstrated that increased premotor (corticospinal excitability) CSE before initiating voluntary contractions is a spinally mediated response. The increase of premotor CSE was not different for fast and slow contractions, indicating that modulation of premotor CSE from a resting baseline is not specific to the intended contractile rate. Results also demonstrated a sex-based difference in premotor spinal excitability, potentially indicating greater neuromodulatory drive in females before initiating voluntary contractions.