A simple method for assessing the energy cost of running during incremental tests

The energy cost of running (Cr) is classically determined from steady-state oxygen consumption (Vo2) at constant speed, divided by running speed. In the present study, Cr was determined during incremental treadmill tests in the course of the assessment of Vo(2max) and related parameters as follows. Assume that the running speed is increased by a constant amount (Deltav) at regular short intervals (T) and that, during each intensity transient below the gas exchange threshold, Vo2 increases exponentially, without time delay, toward the steady state. If Vo2 is averaged over homologous times within each speed step, neglecting the initial 10 s, the Vo2 difference between corresponding time values becomes constant and equal to the difference between the appropriate steady states. Thus Cr was obtained from the ratio of the difference between the Vo2 averages for any two homologous times, within subsequent periods, to the corresponding speed difference. Since in aerobic conditions, Cr on the treadmill is independent of the speed, and since Deltav and T were constant, the relationship between Vo2 and speed is described by straight lines, where the slope yields Cr above resting. This was indeed experimentally observed, the slopes of the linear regressions (R2 range: 0.78 to 0.97 n = 9 to 16) within the three time windows being essentially equal. In six subjects, the grand-average of Cr amounted to 0.177 +/- 0.011 ml O(2)/(kg.m) [3.70 +/- 0.23 J/(kg.m)]. This value is essentially equal to that obtained for the same subjects by applying the "classical" procedure [0.177 +/- 0.015 ml O2/(kg.m); 3.70 +/- 0.31 J/(kg.m)], so confirming the validity of the incremental approach for assessing the energy cost of treadmill running.