Metabolic and respiratory profile of the upper limit for prolonged exercise in man

For high-intensity cycling, power (P) can be well described as a hyperbolic function of tolerable work duration (t): P=(W'/t) + P LL W' is a constant and P LL is the lower limit (asymptote) for P which is shown to occur at an O2 uptake (VdotO2) lying above the estimated threshold for sustained blood lactate increase (ΘIac) but below the maximum VdotO2 (VdotO2max) obtained during incremental cycling. This relation suggests that, above P LL, only a certain amount of work (W') can be accomplished regardless of its rate of performance, with VdotO2 max being attained at fatigue. Hence, P LL defines a point of discontinuity in the VdotO2-P relation for supra-ΘIac exercise. In order to determine the factors responsible for the continued increase in VdotO2 (to the maximum fatiguing value) at power outputs >P LL, we documented the temporal profiles of metabolic (rectal temperature; blood lactate, pyruvate, norepinephrine, epinephrine) and respiratory (VdotE; VdotO2; VdotCO2; blood pH, PCO2, HCO3 −) responses to constant-load cycling in eight healthy males at P LL (24 min) and slightly above P LL (to exhaustion, i.e. P LL and attained VdotO2max. The response patterns at P LL, and > P LL suggest that the slow phase of the VdotO2 response is best correlated with the temporal profile of blood lactate, and hence the site and route of metabolism of this variable may play a major role in the VdotO2 kinetics for high-intensity exercise.