It is demonstrated, that the bioenergetic model combined with the mathematical constraints determined by the experimental knowledge of the aerobic metabolism and the Lohmann reaction dictates the exact lactate (La)-time relationship during exercise. The theory predicts that La is necessarily produced (above the resting baseline), even during extremely low work loads, where the metabolism was usually considered in the past to be "pure" aerobic. The La rate of production increases linearly as a function of the work load. The anaerobic threshold is strictly determined by the saturation of the La clearance mechanisms of the body different from the "La shuttle" and not by the involvement of a sudden increased La production at the cellular level. These results imply that the half time of the PCr breakdown kinetics at the onset of a constant load exercise can be expressed as a function of the onset speed of the aerobic and of the anaerobic metabolism, even in the case of a very low mechanical power. The PCr half-time does not depend on the workload and represents a physiological invariant. The bioenergetic model was created during a long historical period, when it was believed that the La production was not present at all for very low exercise levels but, actually, the bioenergetic model predicts exactly the opposite result!