Magnuson Justine R, Bruce Christina D, Dalton Brian H, McNeil Chris J
School of Health and Exercise Sciences and Centre for Heart, Lung and Vascular Health, The University of British Columbia, Kelowna, British Columbia, Canada.
J Neurophysiol. 2024 Dec 1;132(6):1907-1916. doi: 10.1152/jn.00309.2024. Epub 2024 Nov 6.
The increasing descending drive needed to sustain submaximal isometric torque makes it difficult to isolate fatigue-related changes to neural excitability because evoked electromyography (EMG) responses are influenced by the relative activation of the motoneuron pool. Hence, it is becoming increasingly common to investigate fatigue using a sustained contraction with maintained output from the motoneuron pool; i.e., matched-surface EMG. Although this approach controls motoneuron pool output, it is unknown how cortical contributions to ongoing muscle activity or common modulation between muscles are altered during a matched-EMG contraction. During separate visits, 16 participants performed a sustained 10-min isometric elbow flexion contraction at 20% maximal voluntary contraction (MVC) torque or the level of integrated biceps brachii EMG recorded at 20% MVC torque. Electroencephalographic and surface EMG recordings were obtained from the sensorimotor area and biceps and triceps brachii, respectively. The matched-torque contraction caused increased corticomuscular coherence for biceps brachii (∼75%) and intermuscular coherence (∼97%), but reduced MVC torque (∼33%), voluntary activation (∼9%), and torque steadiness (∼83%). In contrast, the matched-EMG contraction caused reduced MVC torque (∼21%), with no change in coherence, voluntary activation, or EMG steadiness. Furthermore, participants reported higher ratings of perceived effort scores by 6 min into the matched-torque compared with matched-EMG contraction. These findings indicate that, during a matched-torque contraction, the nervous system enhanced common oscillatory activity to continue the task, but this did not prevent degradation of performance (torque steadiness). In contrast, when motoneuron pool output was maintained, other neural strategies were used to preserve muscle output. Sustained electromyography (EMG) tasks offer valuable insights into fatigue-related changes to neural excitability, yet nothing is known about communication between the cortex and active muscles. A 10-min matched-EMG contraction reduced maximal torque-generating capacity but did not alter corticomuscular coherence (CMC), intermuscular coherence (IMC), or task performance (EMG steadiness). In contrast, a matched-torque contraction increased CMC and IMC yet further impaired maximal torque and compromised torque steadiness, which reveals unique neural strategies for the two tasks.
维持次最大等长扭矩所需的下行驱动增加，使得难以将与疲劳相关的变化分离为神经兴奋性变化，因为诱发肌电图（EMG）反应受运动神经元池的相对激活影响。因此，使用运动神经元池保持输出的持续收缩来研究疲劳变得越来越普遍；即匹配表面肌电图。虽然这种方法控制了运动神经元池的输出，但在匹配肌电图收缩期间，皮质对持续肌肉活动的贡献或肌肉之间的共同调制如何改变尚不清楚。在不同的访视中，16名参与者以最大自主收缩（MVC）扭矩的20%或在20%MVC扭矩下记录的肱二头肌综合肌电图水平进行了10分钟的持续等长屈肘收缩。分别从感觉运动区以及肱二头肌和肱三头肌获得脑电图和表面肌电图记录。匹配扭矩收缩导致肱二头肌的皮质-肌肉相干性增加（约75%）和肌肉间相干性增加（约97%），但最大自主收缩扭矩降低（约33%）、自主激活降低（约9%）和扭矩稳定性降低（约83%）。相比之下，匹配肌电图收缩导致最大自主收缩扭矩降低（约21%），相干性、自主激活或肌电图稳定性无变化。此外，与匹配肌电图收缩相比，参与者在匹配扭矩收缩6分钟时报告的主观用力评分更高。这些发现表明，在匹配扭矩收缩期间，神经系统增强了共同振荡活动以继续任务，但这并未阻止性能下降（扭矩稳定性）。相比之下，当运动神经元池输出保持时，会使用其他神经策略来维持肌肉输出。持续肌电图（EMG）任务为与疲劳相关的神经兴奋性变化提供了有价值的见解，但对于皮质与活跃肌肉之间的通信却一无所知。10分钟的匹配肌电图收缩降低了最大扭矩产生能力，但未改变皮质-肌肉相干性（CMC）、肌肉间相干性（IMC）或任务性能（肌电图稳定性）。相比之下，匹配扭矩收缩增加了CMC和IMC，但进一步损害了最大扭矩并损害了扭矩稳定性，这揭示了这两项任务独特的神经策略。
