McKeown Daniel J, McNeil Chris J, Simmonds Michael J, Kavanagh Justin J
Neural Control of Movement Laboratory, Menzies Health Institute Queensland, Griffith University, Gold Coast, Queensland, Australia.
Integrated Neuromuscular Physiology Laboratory, Centre for Heart, Lung, and Vascular Health, School of Health and Exercise Sciences, University of British Columbia, Kelowna, British Columbia, Canada.
Exp Physiol. 2020 Nov;105(11):1855-1868. doi: 10.1113/EP088887. Epub 2020 Sep 13.
What is the central question of this study? How does acute hypoxia alter central and peripheral fatigue during brief and sustained maximal voluntary muscle contractions? What is the main finding and its importance? Perception of fatigue during muscle contractions was increased progressively for 2 h after hypoxic exposure. However, an increase in motor cortex excitability and a decrease in voluntary activation of skeletal muscle were observed across the entire protocol when performing brief (3 s) maximal contractions. These adaptations were abolished if the brief contraction was held for a duration of 20 s, which was presumably attributable to a successful redistribution of blood to overcome the reduced oxygen content.
Few studies have examined the time course of changes in the motor system after acute exposure to hypoxia. Thus, the purpose of this study was to examine how acute hypoxia affects corticospinal excitability, voluntary activation (VA) and the perception of fatigue during brief (3 s) and sustained (20 s) maximal voluntary contractions (MVCs). Fourteen healthy individuals (23 ± 2.2 years of age; four female) were exposed to hypoxia and sham conditions. During hypoxia, peripheral blood oxygen saturation was titrated over a 15 min period and remained at 80% during testing. Corticospinal excitability and VA were assessed before titration (Pre), 0, 1 and 2 h after. At each time point, the brief and sustained elbow flexion MVCs were performed. Motor evoked potentials (MEPs) were obtained using transcranial magnetic stimulation. Superimposed and resting twitches were obtained from motor point stimulation of biceps brachii to calculate the level of VA, and ratings of perceived fatigue were obtained with a modified CR-10 Borg scale. A condition-by-time interaction was detected for the CR-10 Borg scale, whereby perception of fatigue increased progressively throughout the hypoxia protocol. However, main effects of MEP area and VA indicated that corticospinal excitability increased, and VA of the biceps brachii decreased, throughout the hypoxia protocol. Given that these changes in MEP area and VA were seen only when performing the brief MVCs (and not during the sustained MVCs), performing longer contractions might overcome reduced oxygen content by redirecting blood flow to active areas of the motor system.
本研究的核心问题是什么？急性低氧如何在短暂和持续的最大自主肌肉收缩过程中改变中枢和外周疲劳？主要发现及其重要性是什么？低氧暴露后2小时内，肌肉收缩期间的疲劳感逐渐增加。然而，在进行短暂（3秒）最大收缩时，在整个实验过程中观察到运动皮层兴奋性增加以及骨骼肌自主激活减少。如果将短暂收缩持续20秒，这些适应性变化就会消失，这可能归因于血液成功重新分配以克服氧含量降低的情况。
很少有研究考察急性低氧暴露后运动系统变化的时间进程。因此，本研究的目的是考察急性低氧如何影响短暂（3秒）和持续（20秒）最大自主收缩（MVC）期间的皮质脊髓兴奋性、自主激活（VA）和疲劳感。14名健康个体（年龄23±2.2岁；4名女性）接受低氧和假对照条件。在低氧期间，外周血氧饱和度在15分钟内进行滴定，并在测试期间保持在80%。在滴定前（Pre）、滴定后0、1和2小时评估皮质脊髓兴奋性和VA。在每个时间点，进行短暂和持续的肘部屈曲MVC。使用经颅磁刺激获得运动诱发电位（MEP）。从肱二头肌运动点刺激获得叠加和静息抽搐以计算VA水平，并使用改良的CR-10博格量表获得疲劳感知评分。在CR-10博格量表上检测到条件×时间交互作用，即在整个低氧实验过程中疲劳感逐渐增加。然而，MEP面积和VA的主效应表明，在整个低氧实验过程中，皮质脊髓兴奋性增加，肱二头肌的VA降低。鉴于MEP面积和VA的这些变化仅在进行短暂MVC时出现（而不是在持续MVC期间），进行更长时间的收缩可能通过将血流重新导向运动系统的活跃区域来克服氧含量降低的情况。
