Sex differences in the fatigability of locomotor muscles

Ansdell, Paul (2020) Sex differences in the fatigability of locomotor muscles. Doctoral thesis, Northumbria University.

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Abstract

Fatigability during exercise is determined by a myriad of factors including characteristics of the task being performed and the individual performer. When the latter is considered, the sex of the individual can influence the physiological responses, and therefore the underpinnings of fatigue during a wide range of tasks. Typically, fatigability of males and females has been compared following tasks normalised to maximum capacity (e.g. maximum voluntary contraction). Whilst these approaches have identified that females might experience less fatigue, they cannot provide further insight into the differences between the two sexes, as they do not account for potential differences in metabolic thresholds. The aim of this thesis was to therefore compare fatigability of males and females during exercise normalised to the intensity-duration relationship. It was hypothesised that due to anatomical and physiological differences between males and females, greater critical intensities would be observed in females, however, when exercise was subsequently normalised to these thresholds fatigability would be similar. In Study 1, the neuromuscular function and fatigability of females was compared across the eumenorrheic menstrual cycle. Alterations in nervous system function were observed in line with changes in neuro-excitatory and inhibitory hormone concentrations, meaning that in subsequent Chapters, hormonal status had to be controlled. Study 2 then demonstrated that assessments of neuromuscular function and fatigability were repeatable in a hormonally-constant population of monophasic oral contraceptive pill users. This finding indicated the suitability of this population for comparison with males in studies involving repeated visits. In Study 3, the intensity-duration relationship was compared between males and females for intermittent, isometric exercise, then fatigability and physiological responses were observed for exercise normalised to the critical intensity. Females demonstrated a greater relative critical intensity, however contrary to the original hypothesis, still demonstrated greater fatigue-resistance during metabolically-matched intensities. Near-infrared spectroscopy and neurostimulation data showed lesser deoxygenation and contractile dysfunction, respectively, within female knee-extensors during these normalised exercise trials, implying that the locus of the sex difference resided in the musculature. Study 4 then developed a novel method for assessment of subcortical excitability of descending tracts for the knee-extensor muscles. This study confirmed that lumbar stimulation was capable of activating the corticospinal tract and evoking responses at rest and during contractions, for use in subsequent study. In a similar study design to Study 3, Study 5 compared the intensity-duration relationship during cycling exercise between males and females, and assessed physiological responses and fatigability during metabolically-matched severe and heavy intensity exercise. In contrast to the original hypothesis, critical power was not different between sexes, however during exercise at 110 and 90% of critical power, time to task failure was the same between sexes. Despite similar exercise time at metabolically-matched intensities, females again demonstrated lesser deoxygenation and contractile dysfunction of the knee-extensors following exercise. Collectively, the work in this thesis extends the understanding of the sex difference in fatigability during exercise, offering insight into the difference in metabolic and neuromuscular consequences of single-limb and locomotor exercise which can be used to explain previous observations. Furthermore, the data implies that for the same relative volume of exercise, female skeletal muscle experiences less disruption compared to males, which has consequences for acute and chronic exercise prescription in a range of populations.

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