The anaerobic threshold is an O2-related threshold of metabolic acidemia of which the chief metabolic acid is lactic acid. As such, it is a crucial parameter of aerobic function. For power outputs that are below the anaerobic threshold, the dynamics of O2 uptake (VO2) is well characterized as a linear first-order exponential process. The system time constant for leg exercise in humans has been shown to be congruent to 25-35 s with a "delay" of 15-20 s. Steady states are therefore normally achieved within 3 min at this work intensity. Above the anaerobic threshold a second, slower component of VO2 becomes evident that delays the steady state (if attainable). Consequently, the difference in VO2 between the third and the sixth minute of exercise is zero if the work rate is subthreshold and becomes progressively greater, the higher the increment above this parameter; this also correlates highly with the increment of arterial blood lactate, [L-]. This slow phase of the VO2 kinetics results in "excess" VO2, in that the VO2 rises to values above those attained by fitter subjects. This excess VO2 correlates highly with the increased [L-] (and possibly other factors), although its magnitude increases even more rapidly at work rates for which the increase in [L-] exceeds 4-5 meq/liter.