Metabolic rate during submaximal exercise

Exercise increases the energy requirement well in excess of RMR. Metabolism increases in direct proportion to the increase in exercise intensity, as shown in the figure below. The subject exercised on a cycle ergometer for 5 min at 50 watts(W); oxygen consumption(VO₂) increased from its resting value to a steady-state value within 1 to 2 minute. The same subject then cycled on the same or the next day for 5 min at 150W, 200W, 250W, and 300W, respectively, and steady-state values were achieved at each power output. The steady-state VO₂ values were plotted against their respective power outputs(right part of figure below), showing clearly that there is a linear increase in the VO₂ with increases in power output. The steady-state VO₂ value represents the energy cost for that specific power output.

From more recent studies, it is clear that the VO₂ response at higher rates of work does not follow the steady-state response pattern shown in the figure a, but rather follows the pattern b. It appears that at power outputs above the lactate threshold(lactate response is indicated by the dashed line in the right half of figures a and b), the oxygen consumption continues to increase beyond the typical 1 to 2 min needed to reach a steady-state value. This increase has been called the slow component of oxygen uptake kinetics. The most likely mechanism for this slow component is an alteration in muscle fiber recruitment patterns, with the recruitment of more type II muscle fibers, which are less efficient( i.e. , they require a higher VO₂ to achieve the same power output).

A similar, but unrelated, phenomenon is reffered to as VO₂ drift. VO₂ drift is defined as slow increase in VO₂ during prolonged, submaximal, constant power output exercise. Unlike the slow component, VO₂ drift is observed at power outputs well below lactate threshold, and the magnitude of the increase in VO₂ drift is much less. Although not understood completely, VO₂ drift is likely attributable to an increase in ventilation and effects of increased circulating catecholamines.