Corticospinal Excitability During Explosive Voluntary Contractions and Its Association With Rapid Torque Production

We investigated relationships between rapid torque and corticospinal excitability (denoted by motor-evoked potential; MEP) and inhibition (denoted by silent period duration; SPD) during explosive voluntary contractions, as well as differences in MEP and SPD between different phases of explosive contractions and at maximum voluntary contraction (MVC) plateau. In 14 adults, and across multiple repeated trials, quadriceps muscle MEP and SPD were measured at the early, middle and late phases of knee-extensor isometric explosive contractions, and at the MVC plateau, using transcranial magnetic stimulation (TMS). Torque at equivalent time points was also measured on trials without TMS. Using repeated measures correlation applied to early phase data from TMS trials, we found MEP and torque (measured just prior to MEP) were correlated across trials within participants (r = 0.43, p < 0.001). Using Spearman rho correlations to investigate correlations across participants for each phase, we found MEP (averaged across phases up to the phase of interest) and torque (measured on non-TMS trials) to be significantly correlated for the middle phase only (rho = 0.73, p = 0.004). Linear mixed effects models were used to investigate the effect of phase (three explosive phases and MVC plateau) on MEP and SPD. Absolute MEP, MEP normalised to maximal M-wave and SPD all increased across the phases of explosive contraction and up to MVC plateau (fixed effects of phase, p < 0.025). Our results suggest corticospinal excitability may be an important determinant of rapid torque. Further, corticospinal inhibition and excitability both increase throughout the rising torque-time curve and up to MVC plateau.