Effect of prior metabolic rate on the kinetics of oxygen uptake during moderate-intensity exercise

Abstract.

Pulmonary oxygen uptake (\(\) \(\dot V{\rm O}_{\rm 2} \) ) dynamics during moderate-intensity exercise are often assumed to be dynamically linear (i.e. neither the gain nor the time constant (τ) of the response varies as a function of work rate). However, faster, slower and unchanged \(\) \(\dot V{\rm O}_{\rm 2} \) kinetics have been reported during work-to-work transitions compared to rest-to-work transitions, all within the moderate-intensity domain. In an attempt to resolve these discrepancies and to improve the confidence of the parameter estimation, we determined the \(\) \(\dot V{\rm O}_{\rm 2} \) response dynamics using the averaged response to repeated exercise bouts in seven healthy male volunteers. Each subject initially performed a ramp-incremental exercise test for the estimation of the lactate threshold (\(\) \(\hat \theta _{\rm L} \) ). They then performed an average of four repetitions of each of three constant-work-rate (WR) tests: (1) between 20 W and a work rate of 50% (WR50) between 20 W and 90% \(\) \(\hat \theta _{\rm L} \) (step 1→2), (2) between WR50 and 90% \(\) \(\hat \theta _{\rm L} \) (step 2→3), and (3) between 20 W and 90% \(\) \(\hat \theta _{\rm L} \) (step 1→3); 6 min was spent at each work rate increment and decrement. Parameters of the kinetic response of phase II \(\) \(\dot V{\rm O}_{\rm 2} \) were established by non-linear least-squares fitting techniques. The kinetics of \(\) \(\dot V{\rm O}_{\rm 2} \) were significantly slower at the upper reaches of the moderate-intensity domain (step 2→3) compared to steps 1→2 and 1→3 [group mean (SD) phase II τ: step 1→2 25.3 (4.9) s, step 2→3 40.0 (7.4) s and step 1→3 32.2 (6.9) s]. The off-transient values of τ were not significantly different from each other: 36.8 (16.3) s, 38.9 (11.6) s and 30.8 (5.7) s for steps 1→2, 2→3 and 1→3, respectively. Surprisingly, the on-transient gain (G, \(\) \(\Delta \dot V{\rm O}_{\rm 2} {\rm /}\Delta {\rm WR}\) ) was also found to vary among the three steps [G=10.56 (0.42) ml·min^–1·W^–1, 11.85 (0.64) ml·min^–1·W^–1 and 11.23 (0.52) ml·min^–1·W^–1 for steps 1→2, 2→3 and 1→3, respectively]; the off-transient G did not vary significantly and was close to that for the on-transient step 1→3 in all cases. Our results do not support a dynamically linear system model of \(\) \(\dot V{\rm O}_{\rm 2} \) during cycle ergometer exercise even in the moderate-intensity domain. The greater oxygen deficit per unit power increment in the higher reaches of the moderate-intensity domain necessitates a greater transient lactate contribution to the energy transfer, or a greater phosphocreatine breakdown, or possibly both.