Investigating premotor corticospinal excitability in fast and slow voluntary contractions of the elbow flexors
Abstract
Corticospinal excitability (CSE) increases prior to a voluntary contraction; however the relative contributions of cortical and spinal mechanisms are not well understood. It is also unknown whether the intended voluntary contractile rate affects the change in CSE. Therefore, the purpose was to assess cortical and spinal contributions to premotor CSE prior to fast (ballistic) and slow (ramp) contractions.
Eighteen young, healthy participants (9F) completed isometric elbow flexion contractions targeting 50% maximal voluntary contraction (MVC) force, at fast (as fast as possible) and 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 evoked potentials (CMEPs), and M-waves, respectively, 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) following the “GO” signal and compared to a resting baseline.
MEP amplitude demonstrated a 45% increase from baseline at 75% RT (p = 0.009). CMEP amplitude was already 49% larger than baseline at 0% RT (p closer to EMG onset of the contraction. However, there were no differences in MEP and CMEP amplitudes when compared between contractile conditions (all p>0.05). Normalized to the CMEP, there was no difference in MEP amplitude from baseline in either contractile condition (all p>0.05).
These results indicate that increases in premotor CSE are predominantly spinally mediated. Furthermore, the increase in premotor CSE is not influenced by the intended voluntary contractile rate.