Sex-related differences in peripheral and central fatigue

The overall aim of this dissertation was to advance our understanding of the healthy human neuromuscular system in the context of isometric fatigue by examining sex- and age-related differences. The established objectives included: 1) to determine whether sex-related differences in central fatigue and motoneuronal excitability exist for elbow flexor (EF) vs. elbow extensor (EE) muscles; 2) to determine whether sex-related differences in group III and IV muscle afferent feedback impact the aforementioned measures; and 3) to determine whether sex- and age-related differences exist for peripheral fatigue elicited by electrically-evoked contractions at a stimulus frequency that matches voluntary motor unit firing rate. Study #1 addressed objective 1 and found that males exhibited greater central fatigue than females for the EF, but not EE, after a sustained 2min maximal contraction. Assessed for the first time between sexes, fatigue-related changes to motoneuronal excitability were similar between sexes for both muscle groups. This was the second study to investigate sex-related differences in EE fatigue (first to assess central fatigue). Study #2 re-examined EE muscles during a submaximal fatigue task, and found no sex-related differences in central fatigue or motoneuronal excitability. These studies suggest group III/IV muscle afferent feedback is similar between sexes during isometric EE tasks. Finally, to address aim 3, Study #3 and #4 assessed the intrinsic fatigability of quadriceps muscles of females and males during an electrically-evoked intermittent fatigue task. Study #3 also considered the influence of age, whereas Study #4 investigated if variability of the time between pulses of a stimulus train attenuated peripheral fatigue development. Results from both studies indicate there are minimal sex- and age-related differences in fatigue when stimulus frequency matches the activation rates during voluntary contractions. These studies highlight the need to carefully consider stimulation parameters before developing protocols for rehabilitation or experimental paradigms. Overall, this dissertation provides novel understanding of the mechanisms underlying sex- and age-related differences (or lack thereof) in the neuromuscular physiology of performance fatigue. Furthermore, they provide insight into methodological advancements that can improve future fatigue paradigms assessing central and peripheral mechanisms.Health and Social Development, Faculty of (Okanagan)Health and Exercise Sciences, School of (Okanagan)Graduat