Second order modeling for the pulmonary oxygen uptake on-kinetics: a comprehensive solution for overshooting and non-overshooting responses to exercise.

The human oxygen uptake (VO2 ) response to step-like increases in work rate is currently modeled by a First Order System Multi-Exponential (FOME) arrangement. Due to their first order nature, none of FOME model's exponentials is able to model an overshoot in the oxygen uptake kinetics (OVO2 K). Nevertheless, OVO2 K phenomena are observed in the fundamental component of trained individuals' step responses. We hypothesized that a Mixed Multi-Exponential (MiME) model, where the fundamental component is modeled with a second instead of a first order system, would present a better overall performance than that of the traditional FOME model in fitting VO2 on-kinetics at all work rates, either presenting or not OVO2 K. Fourteen well-trained male cyclists performed three step on-transitions at each of three work rates below their individual lactate thresholds' work rate (WRLT ), and two step on-transitions at each of two exercise intensities above WRLT . Averaged responses for each WR were fitted with MiME and FOME models. Root mean standard errors were used for comparisons between fitting performances. Additionally, a methodology for detecting and quantifying OVO2 K phenomena is proposed. Second order solutions performed better (p<0.000) than the first order exponential when the OVO2 K was present, and did not differ statistically (p=0.973) in its absence. OVO2 K occurrences were observed below and, for the first time, above WRLT (88 and 7%, respectively). We concluded that the MiME model is more adequate and comprehensive than the FOME model in explaining VO2 step on-transient responses, considering cases with or without OVO2 K altogether.