Investigating Premotor Cortical And Motoneuronal Excitability In Fast And Slow Voluntary Contractions Of The Elbow Flexors

Corticospinal excitability (CSE) increases prior to a voluntary contraction; however the contributions of cortical and spinal mechanisms are not well understood. It is also unknown whether the intended contractile rate affects the change in CSE. PURPOSE: To assess premotor cortical and motoneuronal excitability prior to fast (ballistic) vs. slow (ramp) contractions. METHODS: Eighteen young, healthy participants (9 M; 9F) were positioned supine with the non-dominant arm flexed to ∼90° at the elbow joint. They completed 72 isometric elbow flexion contractions targeting 50% maximal voluntary contraction (MVC) torque. In a randomized order, contractions were completed at a fast (as fast as possible) and slow (25% MVC/s) contractile rate. Participants were also cued to contract with warning (red) and “GO” (green) visual signals. Stimulation was applied over the motor cortex, corticospinal tract, and brachial plexus to elicit motor evoked potentials (MEPs), cervicomedullary evoked potentials (CMEPs), and M-waves, respectively, in the surface electromyogram (EMG) recorded over the biceps brachii. With the muscle relaxed, stimulation intensities elicited MEPs and CMEPs of ~10% maximal M-wave (Mmax). MEPs and CMEPs were collected at 0, 25, 50 and 75% of premotor reaction time (RT) following the “GO” signal. Baseline MEPs and CMEPs were elicited at rest between trials, without being preceded by the visual signals. Cortical and motoneuronal excitability were evaluated by ratios of MEP/CMEP and CMEP/Mmax amplitude at 0, 25, 50, 75% RT compared to resting baseline. RESULTS: CMEP/Mmax amplitude was 45% greater in the ballistic condition (p < 0.001) and 42% greater in the ramp condition at 0% RT (p < 0.001) compared to baseline. CMEP/Mmax amplitude increased progressively closer to EMG onset, reaching 63% greater than baseline in the ballistic condition (p < 0.001) and 64% greater in the ramp condition at 75% RT (p < 0.001). Normalized to the CMEP, there were no differences in MEP amplitudes from baseline in either contractile condition across all RT points (all p > 0.05). CONCLUSIONS: These data indicate that increases in premotor CSE are mediated by spinal mechanisms rather than cortical mechanisms. Furthermore, the increase in premotor CSE does not appear to be influenced by the intended contractile rate. Supported by NSERC. NSERC