Aerobic exercise
Generally, there are two types of exercise regimes, aerobic and anaerobic. Aerobic exercise (e.g. marathons) involves low intensity, but long duration activities during which, the muscles used are below their maximal contraction strength. Aerobic activities rely on the aerobic respiration (i.e. citric acid cycle and electron transport chain) for metabolic energy by consuming fat, protein carbohydrates, and oxygen. Muscles involved in aerobic exercises contain a higher percentage of Type I (or slow-twitch) muscle fibers, which primarily contain mitochondrial and oxidation enzymes associated with aerobic respiration. On the contrary, anaerobic exercise is associated with short duration, but high intensity exercise (e.g. sprinting and weight lifting). The anaerobic activities predominately use Type II, fast-twitch, muscle fibers. Type II muscle fibers rely on glucogenesis for energy during anaerobic exercise. During anaerobic exercise, type II fibers consume little oxygen, protein and fat, produces large amounts of lactic acid and are fatigable. Many exercises are partially aerobic and anaerobic; for example, soccer and rock climbing.