During the immediate onset of exercise (030 s at >100%Wmax), energy provision is muscle-centric and involves the rapid utilization of skeletal muscle glycogen stores, the production of allosteric regulators such as ADP, AMP, Pi, and Ca2+as well as an increased production of lactate. As such, the ATP production is predominantly supplied by PCr hydrolysis and glycolysis at the onset (06 s); however, as exercise duration continues (1530 s), the contribution of oxidative phosphorylation begins to increase. During prolonged exercise (0240 min) at submaximal intensities (<100%Wmax), energy provision involves the coordination of multiple tissues. During the first half of the exercise bout (0120 min), energy provision is maintained via the utilization of plasma glucose, plasma free fatty acids (FFAs), and intramuscular glycogen and intramuscular triglyceride (IMTG) stores. However, as the exercise bout continues (120240 min), a shift in substrate oxidation from carbohydrate to fat oxidation is observed, which is a result of an increased utilization of plasma FFA. Representation of ATP turnover rate adapted with permission from the American Physiological Society.8Representation of energy expenditure and oxidation rate adapted with permission from the Physiological Society.16
