Energy expended during oxygen deficit period of submaximal exercise in man.

Aerobic (MR) and anaerobic (Man) metabolism was determined during the transition from mild (50 W external work) to heavier exercise (50% VO2max) in six subjects. The overall exercise efficiency was calculated during the oxygen deficit period and during steady-state exercise. MR was obtained by indirect calorimetry and Man by solving the heat balance equation: MR + Man - S = (R + C + E) + W, where radiative, convective, and evaporative heat losses (R + C + E) were measured by direct calorimetry, work output (W) by ergometry, and heat storage (S) by thermometry. (MR + Man) during the oxygen deficit period was found to be lower than MR during steady state. In the 1st min of exercise, mean mixed work efficiency (aerobic + anaerobic) was 33%, which was greater than aerobic efficiency (26.6%) during steady state. The mean anaerobic efficiency efficiency was 41%. This difference reflects the fact that the energy released by splitting of preformed high-energy bonds (i.e., creatine phosphate) is less than the energy released when high-energy bonds expended during mechanical work are continuously regenerated through oxidative phosphorylation. The reported measurements of overall energy metabolism in man provide means for estimating in vivo the coupling efficiency of physical work (i.e., 41%) as well as the efficiency with which energy released by substrate oxidations is recovered in the form of high-energy bonds (i.e., phosphorylation efficiency = 64%).