Identification of lactate kinetics system during exercise

The relationship between blood lactate concentration (LA) and oxygen uptake (VO2) during incremental exercise remains controversial: Does LA increase continuously as a function of VO2 or does it begin to increase above a particular threshold? These competing models are investigated here. In this work, a time series model of LA-VO2 kinetics, with work rate as input and VO2 and LA as outputs, was fitted to published data. The time course of the estimated model parameters suggests an alternative to the continuous and threshold models. Two major transitions were found to occur at intensity levels equivalent to 53 ± 8 % VO2 max and 77 ± 9 % VO2 max. Thus, the metabolic rate domain is divided into three main regions, each with distinct dynamics.