Adenosine triphosphate (ATP) provides the energy for muscle contraction, just as for other cellular functions. ATP can be generated most efficiently by oxidative phosphorylation, but it can also be generated by anaerobic glycolysis. Anaerobic glycolysis uses glucose or glycogen stores in muscle to generate ATP and is useful at the immediate onset of exercise and for short periods of time. The major muscle store of high-energy phosphate necessary to generate ATP from adenosine diphosphate (ADP) is phosphocreatine. Dietary supplementation with creatine phosphate may slightly enhance short bursts of high-intensity exercise, but sustained levels of exercise depend on oxidative phosphorylation. Hence, maximal oxygen consumption, or VO2 max, is the best measure of absolute exercise capacity.

VO2 max is measured by having an individual exercise at increasing workloads while measuring VO2. Above a certain workload, VO2 will stop increasing, and any additional increases in work are fueled by anaerobic glycolysis. This will increase lactate production, but it is important to note that lactate levels in the blood are controlled by factors other than simply anaerobic production. The term anaerobic threshold has been used to describe the point at which arterial lactate levels increase, but this may not correlate perfectly with VO2 max, and a better term for this phenomenon is the lactate threshold. VO2 max is essentially the same for exercise of the legs only or both legs and arms, although it is significantly less when exercise is limited to only the arms; hence, a bicycle ergometer is a useful way to measure VO2 max in most people. Fitness level and the type of exercise determine how long one can maintain VO2 max. For example, a world-class middle-distance runner might sustain VO2 max during a 4-minute mile, while an elite longdistance runner might sustain just over 80% VO2max during a marathon. Exercise lasting more than 20 minutes usually occurs at less than 90% of VO2 max.

If exercise is started abruptly, VO2 will not increase as rapidly as the work level. The difference between the actual work performed and the caloric equivalent of oxygen consumption during the onset of exercise defines the oxygen debt or deficit; this represents work done during anaerobic glycolysis. Similarly, VO2 does not drop immediately to resting levels at the end of exercise but decreases more slowly. However, the extra work being fueled by this oxygen does more than just synthesize glucose from lactate, so many exercise physiologists refer to excess post-exercise oxygen consumption (EPOC) instead of oxygen debt. Excess VO2 following exercise also results from increased temperature and hormonal stimulation of metabolism, in addition to regenerating phosphocreatine.

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