Influence of the oxygen uptake slow component on the aerobic energy cost of high-intensity submaximal treadmill running in humans

During high-intensity running, the oxygen uptake (V˙O₂) kinetics is characterised by a slow component which delays the attainment of the steady-state beyond the 3rd min of exercise. To assess if the aerobic energy cost of running measured at the 3rd min (C ₃) adequately reflects the variability of the true aerobic energy cost measured during the steady-state (C _ss), 13 highly-trained runners completed sessions of square-wave running at intensities above 80% maximal oxygen uptake (V˙O_2max) on a level treadmill. To evaluate the time at which the steady-state V˙O₂ was attained (t _ss), the V˙O₂ responses were described using a general double-exponential equation and t _ss was defined as the time at which V˙O₂ was less than 1% below the asymptotic value given by the model. All the subjects achieved a steady state for intensities equal to or greater than 92% V˙O_2max, and 8 out of 13 achieved it at 99% V˙O_2max. In all cases, t _ss was less than 13 min. For intensities greater than 85% V˙O_2max, C _ss was significantly higher than C ₃ and was positively related to %V˙O_2max (r= 0.44; P < 0.001) while C ₃ remained constant. The C ₃ only explained moderately the variability of C _ss (0.39 < r ² < 0.72, depending on the velocity or the (relative intensity at which the relationship was calculated). Moreover, the excess aerobic energy cost of running the (difference between C _ss and C ₃) was well predicted by age (0.90 < r ² < 0.93). Therefore, when the aerobic profile of runners is evaluated, it is recommended that their running efficiencies at velocities which reflect their race intensities should be determined, with V˙O₂ data being measured at the true steady-state.