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 1102 uptake (V-O2) kinetics and a greater overall 'gain' in V-O2 per unit increase in work rate. Seven healthy mates 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 (tau) was significantly (P < 0.01) greater in the M -> H condition (48 +/- 11s) 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 mLmin(-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. (c) 2006 Elsevier B.V. All rights reserved.