This chapter describes energy metabolism during and following exercise, and carbohydrate and fat metabolism during exercise. The energy for muscle contraction arises from the hydrolysis of adenosine triphosphate (ATP) to adenosine diphosphate (ADP) and inorganic phosphate. Aerobic ATP production occurs via oxidative phosphorylation in mitochondria of muscle involving the consumption of oxygen and the production of carbon dioxide and water. Varying proportions of long-chain fatty acids, glycogen, glucose, ketones, and amino acids are oxidized. Long-chain fatty acid oxidation is essential for aerobic production of ATP in working muscle, and utilization arises from lipolysis of intramuscular and extramuscular stores of triglycerides in which one glycerol and three fatty acids are formed as product of a biochemical reaction. A large part of the lactate, which is metabolically removed, involves gluconeogenesis. This process requires energy which ultimately is derived from oxidative metabolism. The lactate threshold in blood signifies when net anaerobic glycolysis begins to contribute significantly to the energetics of exercise in the body as a whole. However, it does not represent the onset of net lactate production in working muscle.
