Federico Schena

Transcranial magnetic stimulation (TMS)-induced muscle relaxation reflects intrinsic muscle contractile properties by interrupting the drive from the central nervous system during voluntary muscle contractions. We showed that fiber type I proportional area influences the TMS-induced muscle relaxation, suggesting that TMS could be used for the noninvasive estimation of muscle relaxation in unfatigued and fatigued human muscles when the feasibility of more direct method to study relaxation properties (i.e., muscle biopsy) is restricted.

The neural drive to the muscle is the primary determinant of the rate of force development (RFD) in the first 50 ms of a rapid contraction. It is still unproven if repetitive rapid contractions specifically impair the net neural drive to the muscles. To isolate the fatiguing effect of contraction rapidity, 17 male adult volunteers performed 100 burst‐like (i.e., brief force pulses) isometric contractions of the knee extensors. The response to electrically‐evoked single and octet femoral nerve stimulation was measured with high‐density surface electromyography (HD‐sEMG) from the vastus lateralis and medialis muscles. Root mean square (RMS) of each channel of HD‐sEMG was normalized to the corresponding M‐wave peak‐to‐peak amplitude, while muscle fiber conduction velocity (MFCV) was normalized to M‐wave conduction velocity to compensate for changes in sarcolemma properties. Voluntary RFD 0–50 ms decreased ( d = −0.56, p < 0.001) while time to peak force ( d = 0.90, p < 0.001) and time to RFD peak increased ( d = 0.56, p = 0.034). Relative RMS ( d = −1.10, p = 0.006) and MFCV ( d = −0.53, p = 0.007) also decreased in the first 50 ms of voluntary contractions. Evoked octet RFD 0–50 ms ( d = 0.60, p = 0.020), M‐wave amplitude ( d = 0.77, p = 0.009) and conduction velocity ( d = 1.75, p < 0.001) all increased. Neural efficacy, i.e., voluntary/octet force ratio, largely decreased ( d = −1.50, p < 0.001). We isolated the fatiguing impact of contraction rapidity and found that the decrement in RFD, particularly when calculated in the first 50 ms of muscle contraction, can mainly be explained by a decrease in the net neural drive.