The Physiology of Training Practice Test

Endurance training boosts the muscle’s ability to produce ATP aerobically, increasing mitochondrial density and oxidative capacity. With more ATP being generated through oxidative phosphorylation at a given submaximal workload, cytosolic ADP accumulates less. ADP (and AMP) strongly stimulate glycolysis via regulators like phosphofructokinase, so when ADP is lower, the glycolytic flux is reduced. In other words, the body can meet energy demands more efficiently through oxidative pathways, so reliance on glycolysis decreases. This fits with the idea that trained muscles rely more on fat oxidation and aerobic metabolism at submaximal intensities, rather than on glycolysis. At very high intensities, glycolysis can still ramp up, but the typical endurance adaptation is a lower glycolytic contribution for the same submaximal effort.