Dynamic exercise is defined as skeletal muscle contractions at changing lengths and with rhythmic episodes of relaxation. Fundamental to any discussion of dynamic exercise is a description of its intensity. Since dynamically exercising muscle primarily generates energy from oxidative metabolism, a traditional standard is to measure, by mouth, the oxygen uptake (Vo2) of an exercising subject. This measurement is limited to dynamic exercise and usually to the steady state, when exercise intensity and oxygen consumption are stable and no net energy is provided from nonox-idative sources. Three implications of the original oxygen consumption measurements deserve mention. First, the centrality of oxygen usage to work output gave rise to the now-standard term "aerobic" exercise. Second, the apparent excess in oxygen consumption during the first minutes of recovery has been termed the oxygen debt (Fig. 30.1). The "excess" oxygen consumption of recovery results from a multitude of physiological processes and little usable information is obtained from its measurement. Third, and more
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