The effect of sustained low‐intensity contractions on supraspinal fatigue in human elbow flexor muscles
The effect of sustained low‐intensity contractions on supraspinal fatigue in human elbow flexor muscles
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DOI：
10.1113/jphysiol.2005.103598
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发表时间：
2006-06
期刊：
影响因子：
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通讯作者：
K. Søgaard;S. Gandevia;G. Todd;N. Petersen;Janet L. Taylor
中科院分区：
文献类型：
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作者：
K. Søgaard;S. Gandevia;G. Todd;N. Petersen;Janet L. Taylor
当受试者进行最大自主收缩(MVC)时，他们很快就会疲劳。大部分力量的丧失来自肌肉内部的过程(外周疲劳)，但也有一些是因为肌肉的自愿激活下降(中枢性疲劳)。中枢性疲劳在次极值收缩中的作用尚不清楚。这项研究调查了在长时间的低力自主收缩过程中是否会出现中枢疲劳。受试者(n=9)保持15%MVC的等长屈肘43min。每3min测量一次短暂MVCs的自主激活。在每一次MVC期间，经颅磁刺激(TMS)后刺激臂丛或臂二头肌运动神经。神经刺激后，受试者在恢复15%的MVC之前也会诱发一次静息性抽动。记录受试者感觉用力、肘关节屈曲扭矩和二头肌、肱二头肌、三头肌的表面肌电信号。在维持15%MVC的同时给予TMS。在持续收缩期间，感觉到的努力从∼2上升到∼8(满分10分)，而持续的二头肌肌电从最初最大值的6.9±2.1%增加到20.0±7.8%。短暂MVC和静息抽动的扭矩分别降至对照组的58.6±14.5%和58.2±13.2%。MVCs的EMG也下降到最初最大值的62.2±15.3%，神经刺激和TMS诱发的抽动呈进行性增加。由这些抽动计算出的自主激活分别从∼的98%下降到71.9±38.9%和76.9±18.3%。TMS后静默期在短暂的MVC中延长(∼40ms)，在持续的靶收缩中(∼延长18ms)。持续收缩结束后，静止期立即恢复，自主激活和自主肌电在几分钟内恢复，而MVC扭矩仅恢复到∼的85%基线。静息的抽搐没有恢复。因此，除了肌肉的疲劳，长期的低力收缩也会产生进行性的中枢疲劳，而受试者最大限度地驱动肌肉的能力受到的一些损害，是由于运动皮质的输出不是最理想的。虽然由低力收缩引起，但外周和中枢疲劳都损害了最大自愿力的产生。虽然中枢性疲劳只能在MVC期间表现出来，但它可能是导致在长期低力收缩期间感觉到的努力不成比例增加的原因之一。
Subjects quickly fatigue when they perform maximal voluntary contractions (MVCs). Much of the loss of force is from processes within muscle (peripheral fatigue) but some occurs because voluntary activation of the muscle declines (central fatigue). The role of central fatigue during submaximal contractions is not clear. This study investigated whether central fatigue developed during prolonged low‐force voluntary contractions. Subjects (n= 9) held isometric elbow flexions of 15% MVC for 43 min. Voluntary activation was measured during brief MVCs every 3 min. During each MVC, transcranial magnetic stimulation (TMS) was followed by stimulation of either brachial plexus or the motor nerve of biceps brachii. After nerve stimulation, a resting twitch was also evoked before subjects resumed the 15% MVC. Perceived effort, elbow flexion torque and surface EMG from biceps, brachioradialis and triceps were recorded. TMS was also given during the sustained 15% MVC. During the sustained contraction, perceived effort rose from ∼2 to ∼8 (out of 10) while ongoing biceps EMG increased from 6.9 ± 2.1% to 20.0 ± 7.8% of initial maximum. Torque in the brief MVCs and the resting twitch fell to 58.6 ± 14.5 and 58.2 ± 13.2% of control values, respectively. EMG in the MVCs also fell to 62.2 ± 15.3% of initial maximum, and twitches evoked by nerve stimulation and TMS grew progressively. Voluntary activation calculated from these twitches fell from ∼98% to 71.9 ± 38.9 and 76.9 ± 18.3%, respectively. The silent period following TMS lengthened both in the brief MVCs (by ∼40 ms) and in the sustained target contraction (by ∼18 ms). After the end of the sustained contraction, the silent period recovered immediately, voluntary activation and voluntary EMG recovered over several minutes while MVC torque only returned to ∼85% baseline. The resting twitch showed no recovery. Thus, as well as fatigue in the muscle, the prolonged low‐force contraction produced progressive central fatigue, and some of this impairment of the subjects' ability to drive the muscle maximally was due to suboptimal output from the motor cortex. Although caused by a low‐force contraction, both the peripheral and central fatigue impaired the production of maximal voluntary force. While central fatigue can only be demonstrated during MVCs, it may have contributed to the disproportionate increase in perceived effort reported during the prolonged low‐force contraction.