Measurement of voluntary activation of fresh and fatigued human muscles using transcranial magnetic stimulation
Measurement of voluntary activation of fresh and fatigued human muscles using transcranial magnetic stimulation
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DOI：
10.1113/jphysiol.2003.044099
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发表时间：
2003-09-01
影响因子：
5.5
通讯作者：
Gandevia, SC
中科院分区：
文献类型：
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作者：
Todd, G;Taylor, JL;Gandevia, SC
最近，运动皮层（TMS）的经颅磁刺激显示，在最大努力期间肌肉的自主激活受损。因此，我们评估了它作为新鲜肌肉和疲劳肌肉在一系列收缩强度上的自愿激活测量的用途，并将其与使用神经刺激的标准抽搐插值进行比较。受试者等长收缩肘屈肌，同时记录二头肌和三头肌的力和肌电图。在一项研究中，八名受试者在休息的情况下进行了次最大和最大测试收缩，以尽量减少疲劳。在第二项研究中，八名受试者进行了持续的最大收缩，将力量减少到初始值的 60%，然后进行短暂的测试收缩。在 TMS 或电刺激二头肌运动神经后记录力反应。在其他收缩中，记录肌电图对 TMS（运动诱发电位，MEP）或臂丛神经刺激（最大 M 波，M-max）的反应。在最大 50% 的收缩期间，TMS 引起二头肌中较大的 MEN（> 90% M-max），在最大努力下其尺寸减小（类似于 70% M-max）。这表明更快的放电率使一些运动单位有效地难以控制。疲劳时，MEP 也变小，但在最大收缩 50-100% 时仍保持 > 70% M-max。对于新鲜和疲劳的肌肉，运动神经和运动皮层刺激引起的叠加抽搐随着收缩强度的增加而减少。对于神经刺激，关系是曲线的，而对于 TMS，对于最大 50-100% 的收缩，关系是线性的 (r(2) = 1.00)。随意激活使用以下表达式得出：(1 - 叠加抽搐/静息抽搐) x 100。静息抽搐是直接测量神经刺激和 TMS 的，通过抽搐和随意力之间的线性回归外推来估计。对于皮质刺激，这导致自愿激活和力之间存在高度线性关系。此外，估计的激活与收缩强度很好地对应。使用 TMS 或神经刺激，在新鲜肌肉的最大努力期间，自主激活程度很高。对于疲劳，这两项测量都显示在最大努力期间自主激活（即中枢疲劳）减少。通过 TMS 测量，这种中枢疲劳占最大随意力下降的四分之一。我们得出的结论是，TMS 可以量化新鲜或疲劳肌肉在最大力量 50-100% 时的自愿激活。与肘部屈肌的标准抽搐插值不同，通过 TMS 测量的随意激活与随意力成比例变化，它揭示了运动皮层何时可提供额外输出来增加力量，并且它从所有相关的增效肌肉中引出力量。
Recently, transcranial magnetic stimulation of the motor cortex (TMS) revealed impaired voluntary activation of muscles during maximal efforts. Hence, we evaluated its use as a measure of voluntary activation over a range of contraction strengths in both fresh and fatigued muscles, and compared it with standard twitch interpolation using nerve stimulation. Subjects contracted the elbow flexors isometrically while force and EMG from biceps and triceps were recorded. In one study, eight subjects made submaximal and maximal test contractions with rests to minimise fatigue. In the second study, eight subjects made sustained maximal contractions to reduce force to 60 % of the initial value, followed by brief test contractions. Force responses were recorded following TMS or electrical stimulation of the biceps motor nerve. In other contractions, EMG responses to TMS (motor evoked potentials, MEPs) or to stimulation at the brachial plexus (maximal M waves, M-max) were recorded. During contractions of 50 % maximum, TMS elicited large MEN in biceps (> 90 % M-max) which decreased in size (to similar to70 % M-max) with maximal efforts. This suggests that faster firing rates made some motor units effectively refractory. With fatigue, MEPs were also smaller but remained > 70 % M-max for contractions of 50- 100 % maximum. For fresh and fatigued muscle, the superimposed twitch evoked by motor nerve and motor cortex stimulation decreased with increasing contraction strength. For nerve stimulation the relation was curvilinear, and for TMS it was linear for contractions of 50-100% maximum (r(2) = 1.00). Voluntary activation was derived using the expression: (1 - superimposed twitch/resting twitch) x 100. The resting twitch was measured directly for nerve stimulation and for TMS, it was estimated by extrapolation of the linear regression between the twitch and voluntary force. For cortical stimulation, this resulted in a highly linear relation between voluntary activation and force. Furthermore, the estimated activation corresponded well with contraction strength. Using TMS or nerve stimulation, voluntary activation was high during maximal efforts of fresh muscle. With fatigue, both measures revealed reduced voluntary activation (i.e. central fatigue) during maximal efforts. Measured with TMS, this central fatigue accounted for one-quarter of the fall in maximal voluntary force. We conclude that TMS can quantify voluntary activation for fresh or fatigued muscles at forces of 50-100 % maximum. Unlike standard twitch interpolation of the elbow flexors, voluntary activation measured with TMS varies in proportion to voluntary force, it reveals when extra output is available from the motor cortex to increase force, and it elicits force from all relevant synergist muscles.