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

TLDR

High‑intensity cycling power follows a hyperbolic relation with tolerable work duration, with a lower limit P_LL occurring at an O₂ uptake above the lactate‑threshold but below VO₂max, defining a discontinuity in the VO₂–power relationship beyond which only a fixed amount of work can be performed regardless of rate. The study aimed to identify the factors driving the continued rise in VO₂ to its maximum during exercise at power outputs above P_LL. To this end, the authors recorded temporal metabolic and respiratory responses—including rectal temperature, blood lactate, pyruvate, catecholamines, VE, VO₂, VCO₂, blood pH, PCO₂, and bicarbonate—during constant‑load cycling at P_LL (24 min) and slightly above P_LL until exhaustion in eight healthy males. They found that at P_LL VO₂ reached a delayed steady state while catecholamines and core temperature rose and lactate and pH plateaued, whereas at power > P_LL VO₂ continued to climb slowly to VO₂max, and the slow phase of VO₂ kinetics correlated best with the lactate profile, implicating lactate metabolism as a key determinant of high‑intensity VO₂ kinetics.

Abstract

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 ([Vdot]O2) lying above the estimated threshold for sustained blood [lactate] increase (ΘIac) but below the maximum [Vdot]O2 ([Vdot]O2max) 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 [Vdot]O2 max being attained at fatigue. Hence, P LL defines a point of discontinuity in the [Vdot]O2-P relation for supra-ΘIac exercise. In order to determine the factors responsible for the continued increase in [Vdot]O2 (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 ([Vdot]E; [Vdot]O2; [Vdot]CO2; 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. < 24 min). [Vdot]O2 manifested a delayed steady state at P LL, despite catecholamine levels and core temperature continuing to increase throughout; blood [lactate] and pH plateaued, however. In contrast, [Vdot]O2 continued to increase slowly for the duration of the exercise > P LL and attained [Vdot]O2max. The response patterns at P LL, and > P LL suggest that the slow phase of the [Vdot]O2 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 [Vdot]O2 kinetics for high-intensity exercise.