Potential Factors Influencing the Acute and Chronic Response of Human Skeletal Muscle to Interval Exercise Training

Abstract

This thesis considered three potential factors that may influence skeletal muscle responses to interval exercise training in healthy adults, with a focus on mitochondrial remodelling: biological sex, fibre type and contraction pattern. Study 1 assessed the acute response to a sprint interval training (SIT) protocol that involved three, 20-second ‘all-out’ cycling efforts interspersed with 2 minutes of recovery. It found similar exercise-induced increases in the mRNA expression of genes linked to mitochondrial biogenesis in men and women. Study 2 showed that a single session of SIT and moderate-intensity continuous training (MICT) induced similar increases in the phosphorylation of signalling proteins linked to mitochondrial biogenesis in type I and IIa fibres when compared within the same individuals. It also assessed responses to 12 weeks of training in two different groups and found MICT elicited greater increases in markers of mitochondrial content, including cytochrome c oxidase subunit IV (COXIV) protein content, in type I fibres, whereas the increase in type IIa fibres was similar compared to SIT. Study 3 utilized single-leg cycling as a within-subject model to compare interval and continuous training protocols that were matched for exercise intensity and total training volume, but the pattern of contraction differed. Measurements of mitochondrial content, including subsarcolemmal mitochondrial area, the maximal activity of citrate synthase and COXIV protein content, increased after 4 weeks of continuous training but were unchanged after interval training. Overall, this thesis advances our understanding of the influence of biological sex, muscle fibre type and contraction pattern on skeletal muscle mitochondrial responses to exercise. Major findings include: the acute response to SIT was similar between sexes, acute responses to interval and continuous exercise were similar but training elicited some divergent fibre type-specific responses, and mitochondrial content increased following continuous training but was unchanged following work- and intensity-matched interval training.