Despite efforts to improve adolescent physical activity participation and adherence, as many as 80% of youth fail to achieve the recommended 60 minutes of moderate-to-vigorous intensity physical activity (MVPA) per day. Whilst physical activity guidelines are beginning to recognise the importance of vigorous intensity activity as an alternative to long duration moderate intensity activity, there are no specific recommendations regarding specific session parameters such as the duration of each vigorous bout. The primary focus of this PhD was to conduct a cohesive investigation into the metabolic effects of vigorous intensity physical activity, using both an epidemiological level study, and a detailed exercise intervention on a sub sample of adolescents. This thesis is constructed in two parts to reflect the flow of investigation into vigorous intensity physical activity and adolescent metabolic health. Part 1 (Chapters 2-4) investigates the measurement techniques most appropriate to quantify physical activity in adolescents, and thereafter determines the contribution of activity intensity to adolescent health from a large cross sectional study. Part 2 (Chapters 5-7) forms an investigation into high-intensity interval training (HIIT) as a strategy to deliver the health benefits of vigorous intensity physical activity in adolescents. The aim of Part 2 was to investigate the effect of HIIT on the metabolic health of low-active male adolescents, and explore the perceptual responses that may influence and determine long-term exercise adherence. Through Parts 1 and 2 of this thesis, a cohesive investigation into the metabolic effects of vigorous intensity physical activity was performed.

Outcomes from Part 1 showed that vigorous intensity physical activity was associated with key positive health outcomes in adolescents, supporting the concept of emphasising the importance of vigorous activity in youth physical activity guidelines. Data obtained by accurately measuring physical activity using accelerometry in 694 adolescents showed that vigorous physical activity was the only activity intensity associated with reduced body mass index (BMI) and waist circumference-to-height ratio (WCHt) in youth. Data also showed that an increase of 4.2% in daily duration of vigorous intensity activity was associated with a reduction in WCHt from the 75th centile to the 25th centile in adolescents. Hence, the clinical implication of these results suggest that an increase in daily vigorous activity by an average of 1.1 minutes each day is associated with a 0.05 reduction in WCHt in the adolescent cohort.

Furthermore, in Part 2, the investigation into the dose-response relationship of HIIT with metabolic health showed that low-active adolescent males, performing a single HIIT set twice-weekly (consisting of 4 repeated bouts of 20 seconds of near-maximal exertion interspersed with 10 seconds of passive recovery), in addition to one resistance training session, gained meaningful improvements in fitness and body composition. Performing additional HIIT sets provided few additional health improvements to those of the lowest dose studied. In support for HIIT uptake and long-term adherence, participants also gained meaningful improvements in several sub-domains of physical self-perception and expressed high levels of enjoyment towards the exercise intervention. This research indicated an effect of incremental HIIT dose on the physical self-concept subdomains; thus, adolescents and practitioners can assume that performing even low doses of HIIT in addition to resistance training improves physical self-perception. This body of work provides strong support for the promotion of vigorous intensity physical activity in adolescents, specifically endorsing the utilisation of HIIT as a potent and practical alternative method to deliver health benefits of exercise in youth. Through addressing the overarching research question, ‘What are the effects of vigorous intensity physical activity on the metabolic heath of adolescents?’ this thesis, as a whole, contributes new knowledge to the science of adolescent physical activity and health. The wider implications of this research may help to improve the lifelong health and wellbeing of adolescents by suggesting novel physical activity strategies that incorporate the promotion of vigorous intensity physical activity. Through implementing this research in the adolescent cohort, policy makers and exercise practitioners may be able to reduce the burden of chronic lifestyle related diseases to public health.