Oxygen uptake kinetics during treadmill running across exercise intensity domains

Abstract.

The purpose of the present study was to examine comprehensively the kinetics of oxygen uptake ($\dot V{\rm O}_{\rm 2}$ ) during treadmill running across the moderate, heavy and severe exercise intensity domains. Nine subjects [mean (SD age, 27 (7) years; mass, 69.8 (9.0) kg; maximum $\dot V{\rm O}_{\rm 2}$ , $\dot V{\rm O}_{{\rm 2max}}$ , 4,137 (697) ml·min^–1] performed a series of "square-wave" rest-to-exercise transitions of 6 min duration at running speeds equivalent to 80% and 100% of the $\dot V{\rm O}_{\rm 2}$ at lactate threshold (LT; moderate exercise); and at 20%, 40%, 60%, 80% and 100% of the difference between the $\dot V{\rm O}_{\rm 2}$ at LT and $\dot V{\rm O}_{{\rm 2max}}$ (Δ, heavy and severe exercise). Critical velocity (CV) was also determined using four maximal treadmill runs designed to result in exhaustion in 2–15 min. The $\dot V{\rm O}_{\rm 2}$ response was modelled using non-linear regression techniques. As expected, the amplitude of the $\dot V{\rm O}_{\rm 2}$ primary component increased with exercise intensity [from 1,868 (136) ml·min^–1 at 80% LT to 3,296 (218) ml·min^–1 at 100% Δ, P<0.05]. However, there was a non-significant trend for the "gain" of the primary component to decrease as exercise intensity increased [181 (7) ml·kg^–1·km^–1 at 80% LT to 160 (6) ml·kg^–1·km^–1 at 100% Δ]. The time constant of the primary component was not different between supra-LT running speeds (mean value range = 17.9–19.1 s), but was significantly shorter during the 80% LT trial [12.7 (1.4) s, P<0.05]. The $\dot V{\rm O}_{\rm 2}$ slow component increased with exercise intensity from 139 (39) ml·min^–1 at 20% Δ to 487 (57) ml·min^–1 at 80% Δ (P<0.05), but decreased to 317 (84) ml·min^–1 during the 100% Δ trial (P<0.05). During both the 80% Δ and 100% Δ trials, the $\dot V{\rm O}_{\rm 2}$ at the end of exercise reached $\dot V{\rm O}_{{\rm 2max}}$ [4,152 (242) ml·min^–1 and 4,154 (114) ml·min^–1, respectively]. Our results suggest that the "gain" of the primary component is not constant as exercise intensity increases across the moderate, heavy and severe domains of treadmill running. These intensity-dependent changes in the amplitudes and kinetics of the $\dot V{\rm O}_{\rm 2}$ response profiles may be associated with the changing patterns of muscle fibre recruitment that occur as exercise intensity increases.