Faculty of Health and Life Sciences, Northumbria University, Newcastle Upon Tyne, UK.
Université Lyon, UJM-Saint-Etienne, Inter-university Laboratory of Human Movement Biology, Saint-Etienne, France.
J Physiol. 2019 Dec;597(23):5577-5595. doi: 10.1113/JP278699. Epub 2019 Oct 30.
Females demonstrate greater fatigue resistance than males during contractions at intensities relative to maximum force. However, previous studies have not accounted for the influence of metabolic thresholds on fatigability. This study is the first to test whether sex differences in fatigability exist when exercise intensity is normalised relative to a metabolic threshold: the critical intensity derived from assessment of the intensity-duration relationship during intermittent, isometric knee extensor contractions. We show that critical intensity in females occurred at a higher percentage of maximum force compared to males. Furthermore, females demonstrated greater fatigue resistance at exercise intensities above and below this metabolic threshold. Our data suggest that the sex difference was mediated by lesser deoxygenation of the knee extensors during exercise. These data highlight the importance of accounting for metabolic thresholds when comparing fatigability between sexes, whilst emphasising the notion that male data are not generalisable to female populations.
Females are less fatigable than males during isometric exercise at intensities relative to maximal voluntary contraction (MVC); however, whether a sex difference in fatigability exists when exercise is prescribed relative to a critical intensity is unknown. This study established the intensity-duration relationship, and compared fatigability and recovery between sexes following intermittent isometric contractions normalised to critical intensity. Twenty participants (10 females) completed four intermittent isometric knee extension trials to task failure to determine critical intensity and the curvature constant (W'), followed by fatiguing tasks at +10% and -10% relative to critical intensity. Neuromuscular assessments were completed at baseline and for 45 min post-exercise. Non-invasive neurostimulation, near-infrared spectroscopy, and non-invasive haemodynamic monitoring were used to elucidate the physiological mechanisms responsible for sex differences. Females demonstrated a greater critical intensity relative to MVC than males (25 ± 3 vs. 21 ± 2% MVC, P = 0.003), with no sex difference for W' (18,206 ± 6331 vs. 18,756 ± 5762 N s, P = 0.850). Time to task failure was greater for females (62.37 ± 17.25 vs. 30.43 ± 12.75 min, P < 0.001) during the +10% trial, and contractile function recovered faster post-exercise (P = 0.034). During the -10% trial females experienced less contractile dysfunction (P = 0.011). Throughout the +10% trial, females demonstrated lesser decreases in deoxyhaemoglobin (P = 0.007) and an attenuated exercise pressor reflex. These data show that a sex difference in fatigability exists even when exercise is matched for critical intensity. We propose that greater oxygen availability during exercise permits females to sustain a higher relative intensity than males, and is an explanatory factor for the sex difference in fatigability during intermittent, isometric contractions.
在与最大力相关的强度下，女性在收缩期间比男性表现出更高的疲劳抵抗力。然而，之前的研究并没有考虑代谢阈值对疲劳性的影响。这项研究首次测试了当运动强度相对于代谢阈值（从间歇性等长膝关节伸展收缩强度持续时间关系评估得出的临界强度）正常化时，疲劳性是否存在性别差异。我们表明，女性的临界强度比男性高，占最大力的百分比更高。此外，女性在高于和低于该代谢阈值的运动强度下表现出更强的疲劳抵抗力。我们的数据表明，这种性别差异是由运动过程中膝关节伸肌脱氧程度较小介导的。这些数据强调了在比较性别之间的疲劳性时考虑代谢阈值的重要性，同时强调了男性数据不能推广到女性人群的观点。
在与最大自主收缩（MVC）相关的等长运动强度下，女性比男性更不易疲劳；然而，当运动强度根据临界强度来规定时，疲劳性是否存在性别差异尚不清楚。本研究建立了强度-持续时间关系，并比较了间歇性等长膝关节伸展收缩后男女之间的疲劳性和恢复情况，这些收缩都根据临界强度进行了归一化。20 名参与者（10 名女性）完成了四项间歇性等长膝关节伸展试验，直至任务失败，以确定临界强度和曲率常数（W'），然后在相对于临界强度+10%和-10%的疲劳任务。在基线和运动后 45 分钟进行了神经肌肉评估。使用非侵入性神经刺激、近红外光谱和非侵入性血液动力学监测来阐明导致性别差异的生理机制。女性的临界强度比男性高（25±3%比 21±2%MVC，P=0.003），而 W'没有性别差异（18,206±6331 比 18,756±5762 N s，P=0.850）。女性在+10%试验中达到任务失败的时间更长（62.37±17.25 比 30.43±12.75 分钟，P<0.001），并且运动后收缩功能恢复更快（P=0.034）。在-10%试验中，女性经历的收缩功能障碍较小（P=0.011）。在整个+10%试验中，女性的脱氧血红蛋白（P=0.007）减少幅度较小，运动压力反射减弱。这些数据表明，即使在临界强度匹配的情况下，疲劳性也存在性别差异。我们提出，运动期间更高的氧可用性允许女性维持比男性更高的相对强度，这是间歇性等长收缩期间疲劳性性别差异的一个解释因素。
