Faculty of Health Sciences, School of Kinesiology, The University of Western Ontario, London, Ontario, Canada.
Faculty of Health, School of Kinesiology and Health Science, York University, Toronto, Ontario, Canada.
J Neurophysiol. 2024 Dec 1;132(6):1670-1679. doi: 10.1152/jn.00330.2024. Epub 2024 Oct 9.
Corticospinal excitability (CSE) increases before a voluntary contraction; however, the relative contributions of premotor cortical and spinal mechanisms are poorly understood. It is unknown whether the intended voluntary contractile rate affects CSE. Eighteen young, healthy participants (9 females) completed isometric elbow flexion contractions targeting 50% maximal voluntary contraction (MVC) torque, at either fast (fast as possible) or 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 motor evoked potentials (CMEPs), and M-waves, 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 - defined as the time between the "GO" cue and onset of biceps brachii EMG) and compared with a resting baseline. MEP amplitude was greater than baseline at 75% RT ( = 0.009), and CMEP amplitude was significantly increased at all RT points relative to baseline ( ≤ 0.001). However, there were no differences in MEP and CMEP amplitudes when compared between fast and slow conditions ( ≥ 0.097). Normalized to the CMEP, there was no difference in MEP amplitude from baseline in either contractile condition ( ≥ 0.264). These results indicate that increased premotor CSE is a spinally mediated response. Furthermore, premotor CSE is not influenced by the intended voluntary contractile rate. CMEP amplitudes were larger for females than for males within the premotor RT period ( = 0.038), demonstrating that premotor spinal excitability responses may be influenced by sex. The present study demonstrated that increased premotor (corticospinal excitability) CSE before initiating voluntary contractions is a spinally mediated response. The increase of premotor CSE was not different for fast and slow contractions, indicating that modulation of premotor CSE from a resting baseline is not specific to the intended contractile rate. Results also demonstrated a sex-based difference in premotor spinal excitability, potentially indicating greater neuromodulatory drive in females before initiating voluntary contractions.
皮质脊髓兴奋性 (CSE) 在自愿收缩前增加；然而，运动前皮质和脊髓机制的相对贡献尚不清楚。目前尚不清楚预期的自愿收缩率是否会影响 CSE。18 名年轻健康的参与者（9 名女性）完成了以 50%最大自主收缩 (MVC) 扭矩为目标的等长肘部屈曲收缩，收缩速度分别为快速（尽可能快）或慢速（25% MVC/s）。参与者被提示使用警告（红色）和“GO”（绿色）视觉信号进行收缩。在肱二头肌记录的表面肌电图 (EMG) 中，应用磁刺激和电刺激来引出运动诱发电位 (MEP)、颈髓运动诱发电位 (CMEP) 和 M 波。MEP 和 CMEP 分别在运动前反应时间的 0、25、50 和 75%（定义为“GO”提示和肱二头肌 EMG 起始之间的时间）采集，并与休息基线进行比较。MEP 振幅在 75% RT 时大于基线（=0.009），CMEP 振幅在所有 RT 点均显著高于基线（≤0.001）。然而，在快速和慢速条件之间比较时，MEP 和 CMEP 振幅没有差异（≥0.097）。与 CMEP 相比，在任何收缩条件下，MEP 振幅从基线开始均无差异（≥0.264）。这些结果表明，运动前 CSE 的增加是一种脊髓介导的反应。此外，预期的自愿收缩率不会影响运动前 CSE。在运动前 RT 期间，女性的 CMEP 振幅大于男性（=0.038），表明运动前脊髓兴奋性反应可能受性别影响。本研究表明，在开始自愿收缩之前，增加的运动前（皮质脊髓兴奋性）CSE 是一种脊髓介导的反应。快速和慢速收缩之间的运动前 CSE 增加没有差异，这表明从休息基线调制运动前 CSE 不是特定于预期的收缩率。结果还表明，运动前脊髓兴奋性存在基于性别的差异，这可能表明女性在开始自愿收缩前有更大的神经调制驱动。
