Oxygen uptake kinetics during treadmill running across exercise intensity domains

The purpose of the present study was to examine comprehensively the kinetics of oxygen uptake ((V) over dot O-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 (V) over dot O-2, (V) over dot O-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 (V) over dot O-2 at lactate threshold (LT; moderate exercise); and at 20%, 40%, 60%, 80% and 100% of the difference between the (V)over dot O-2 at LT and (V) over dot O-2max (Delta, 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 (V) over dot O-2 response was modelled using non-linear regression techniques. As expected, the amplitude of the (V) over dot O-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% Delta, 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% A]. 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 (V) over dot O-2 slow component increased with exercise intensity from 139 (39) ml.min(-1) at 20% Delta to 487 (57) ml.min(-1) at 80% Delta (P < 0.05), but decreased to 317 (84) ml.min(-1) during the 100% Delta trial (P < 0.05). During both the 80% Delta and 100% Delta trials, the (V) over dot O-2 at the end of exercise reached (V) over dot O-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 (V) over dot O-2 response profiles may be associated with the changing patterns of muscle fibre recruitment that occur as exercise intensity increases.