2.3. Data Analysis

Performance. A force-time integral including the sum of the data points, multiplied by the sample interval during the 60 s sustained MVCs (performed by the tested leg) was calculated in experiments B and C.

Neuromuscular function. The maximal force output and the corresponding root mean square EMG [rmsEMG of the VL and RF muscles] was quantified during the first and last 2 s intervals of the sustained 60 s left and right leg MVCs, as well as over 500 ms prior to each TMS evoked during brief right leg MVCs. The VL and RF rmsEMG were normalized to the amplitude of the corresponding muscle compound action potential (Mmax) recorded during the same contraction to calculate the rmsEMG∙Mmax⁻¹ ratio (rmsEMG₁₀₀).

The central voluntary activation (VA_PNS) was measured using the twitch interpolation technique [23] with the following formula, where Fb is the voluntary force output just before the PNS, D is the difference between Fb and the maximum SIT force evoked by the stimulus, F_MAX is the highest force output during MVC, and FPt is the size of Pt force evoked at rest [24].

The voluntary cortical activation (VA_TMS) was assessed using the amplitude of SITs evoked by TMS during 5 s contractions at 100%, 75%, and 50% of MVC [25,26]. The y-intercept of the linear regression (r² > 0.9) between the SITs was computed to identify the estimated resting twitch (ERT). VA_TMS was then calculated using the following equation:

In order to explore the excitability of the corticospinal pathway, the areas under the MEP and the Mmax signals were measured for both VL and RF muscles. The onset of MEP and Mmax were defined as the point at which the voltage trace became tangential to baseline in either the positive or negative direction. The responses to VL and RF MEP were normalized to the subsequent Mmax recorded during the same contraction to calculate MEP∙Mmax^-1 ratios (MEP₁₀₀, MEP₇₅, MEP₅₀). The duration (ms) of the silent period (i.e., indicative of corticospinal inhibition) was assessed for VL and RF MEPs as the interval from the stimulus artifact to the return of the continuous EMG by visual inspection during 100%, 75%, and 50% of MVC contractions (SP₁₀₀, SP₇₅, SP₅₀). The end of the SP for two participants in experiments A and B was not distinguishable, so the values were removed from the data pool.