Effects of baseline metabolic rate on pulmonary O2 uptake on-kinetics during heavy-intensity exercise in humans

We hypothesised that initiating heavy-intensity exercise from an elevated baseline metabolic rate would result in slower Phase II O2 uptake (V˙O2) kinetics and a greater overall ‘gain’ in V˙O2 per unit increase in work rate. Seven healthy males performed a series of like-transitions on a cycle ergometer: (1) from light exercise to ‘moderate’ exercise (80% of the gas exchange threshold, GET; L → M); (2) from light exercise to ‘heavy’ exercise (40% of the difference between GET and V˙O2 peak; L → H); (3) from moderate exercise to heavy exercise (M → H). The Phase II time constant (τ) was significantly (P < 0.01) greater in the M → H condition (48 ± 11 s) compared to the L → M and L → H conditions (26 ± 6 s versus 27 ± 4 s, respectively). Moreover, the end-exercise ‘gain’ values were significantly different between the three conditions (L → M, 8.1 ± 0.7 mL min−1 W−1; L → H, 9.7 ± 0.4 mL min−1 W−1; M → H, 10.7 ± 0.7 mL min−1 W−1; P < 0.05). This ‘non-linearity’ in the pulmonary V˙O2 response to exercise might be attributed, at least in part, to differences in the metabolic properties of the muscle fibres recruited in the abrupt transition from a lower to a higher work rate.