Muscle is capable of operating over a wide range of metabolic rates. Most of the metabolic energy for work (ATP), and essentially all of the oxygen consumption of muscle, is due to mitochondrial oxidative phosphorylation. The rate of mitochondrial oxidative phosphorylation is determined by the rate of ATP consumption by the cells (demand) while the metabolic energy level at each metabolic rate is determined by the supply of oxidizable substrate and oxygen (supply). The maximal rate of metabolism is limited by several factors, including the supply of oxidizable substrates (mitochondrial dehydrogenases), of oxygen (blood supply and oxygen diffusion) and the respiratory enzyme capacity (tissue content of mitochondria).
(C)1995The American College of Sports Medicine