Pulmonary O₂uptake kinetics as a determinant of high-intensity exercise tolerance in humans

<jats:p>Tolerance to high-intensity constant-power (P) exercise is well described by a hyperbola with two parameters: a curvature constant (W′) and power asymptote termed “critical power” (CP). Since the ability to sustain exercise is closely related to the ability to meet the ATP demand in a steady state, we reasoned that pulmonary O<jats:sub>2</jats:sub>uptake (V̇o<jats:sub>2</jats:sub>) kinetics would relate to the P-tolerable duration (t<jats:sub>lim</jats:sub>) parameters. We hypothesized that 1) the fundamental time constant (τV̇o<jats:sub>2</jats:sub>) would relate inversely to CP; and 2) the slow-component magnitude (ΔV̇o<jats:sub>2sc</jats:sub>) would relate directly to W′. Fourteen healthy men performed cycle ergometry protocols to the limit of tolerance: 1) an incremental ramp test; 2) a series of constant-P tests to determine V̇o<jats:sub>2max</jats:sub>, CP, and W′; and 3) repeated constant-P tests (WR<jats:sub>6</jats:sub>) normalized to a 6 min t<jats:sub>lim</jats:sub>for τV̇o<jats:sub>2</jats:sub>and ΔV̇o<jats:sub>2sc</jats:sub>estimation. The WR<jats:sub>6</jats:sub>t<jats:sub>lim</jats:sub>averaged 365 ± 16 s, and V̇o<jats:sub>2max</jats:sub>(4.18 ± 0.49 l/min) was achieved in every case. CP (range: 171–294 W) was inversely correlated with τV̇o<jats:sub>2</jats:sub>(18–38 s; R<jats:sup>2</jats:sup>= 0.90), and W′ (12.8–29.9 kJ) was directly correlated with ΔV̇o<jats:sub>2sc</jats:sub>(0.42–0.96 l/min; R<jats:sup>2</jats:sup>= 0.76). These findings support the notions that 1) rapid V̇o<jats:sub>2</jats:sub>adaptation at exercise onset allows a steady state to be achieved at higher work rates compared with when V̇o<jats:sub>2</jats:sub>kinetics are slower; and 2) exercise exceeding this limit initiates a “fatigue cascade” linking W′ to a progressive increase in the O<jats:sub>2</jats:sub>cost of power production (V̇o<jats:sub>2sc</jats:sub>), which, if continued, results in attainment of V̇o<jats:sub>2max</jats:sub>and exercise intolerance. Collectively, these data implicate V̇o<jats:sub>2</jats:sub>kinetics as a key determinant of high-intensity exercise tolerance in humans.</jats:p>