Smith Janette L, Martin Peter G, Gandevia Simon C, Taylor Janet L
Prince of Wales Medical Research Institute and University of New South Wales, Randwick, New South Wales, Australia.
J Appl Physiol (1985). 2007 Aug;103(2):560-8. doi: 10.1152/japplphysiol.00220.2007. Epub 2007 Apr 26.
During sustained maximal voluntary contractions (MVCs), most fatigue occurs within the muscle, but some occurs because voluntary activation of the muscle declines (central fatigue), and some of this reflects suboptimal output from the motor cortex (supraspinal fatigue). This study examines whether supraspinal fatigue occurs during a sustained submaximal contraction of 5% MVC. Eight subjects sustained an isometric elbow flexion of 5% MVC for 70 min. Brief MVCs were performed every 3 min, with stimulation of the motor point, motor cortex, and brachial plexus. Perceived effort and pain, elbow flexion torque, and surface EMGs from biceps and brachioradialis were recorded. During the sustained 5% contraction, perceived effort increased from 0.5 to 3.9 (out of 10), and elbow flexor EMG increased steadily by approximately 60-80%. Torque during brief MVCs fell to 72% of control values, while both the resting twitch and EMG declined progressively. Thus the sustained weak contraction caused fatigue, some of which was due to peripheral mechanisms. Voluntary activation measured by motor point and motor cortex stimulation methods fell to 90% and 80%, respectively. Thus some of the fatigue was central. Calculations based on the fall in voluntary activation measured with cortical stimulation indicate that about two-thirds of the fatigue was due to supraspinal mechanisms. Therefore, sustained performance of a very low-force contraction produces a progressive inability to drive the motor cortex optimally during brief MVCs. The effect of central fatigue on performance of the weak contraction is less clear, but it may contribute to the increase in perceived effort.
在持续的最大自主收缩（MVC）过程中，大部分疲劳发生在肌肉内部，但也有一些是由于肌肉的自主激活下降（中枢性疲劳）导致的，其中部分反映了运动皮层的输出不理想（脊髓上疲劳）。本研究探讨在5%MVC的持续次最大收缩过程中是否会发生脊髓上疲劳。8名受试者以5%MVC进行等长肘屈曲70分钟。每3分钟进行一次短暂的MVC，并对运动点、运动皮层和臂丛神经进行刺激。记录主观用力和疼痛程度、肘屈曲扭矩以及肱二头肌和肱桡肌的表面肌电图。在持续5%收缩过程中，主观用力程度从0.5增加到3.9（满分10分），肘屈肌肌电图稳步增加约60 - 80%。短暂MVC期间的扭矩降至对照值的72%，同时静息抽搐和肌电图均逐渐下降。因此，持续的弱收缩会导致疲劳，其中一些是由外周机制引起的。通过运动点和运动皮层刺激方法测量的自主激活分别降至90%和80%。因此，部分疲劳是中枢性的。基于皮层刺激测量的自主激活下降进行的计算表明，约三分之二的疲劳是由脊髓上机制引起的。因此，持续进行极低强度收缩会导致在短暂MVC期间逐渐无法最佳驱动运动皮层。中枢性疲劳对弱收缩表现的影响尚不清楚，但可能导致主观用力程度增加。
