The effect of interdian and diurnal variation on oxygen uptake kinetics during treadmill running

The aim of this study was to examine the variability of the oxygen uptake ((V) over dot O-2) kinetic response during moderate- and high-intensity treadmill exercise within the same day (at 06:00, 12:00 and 18:00 h) and across days (on five occasions). Nine participants (age 25 +/- 8 years, mass 70.2 +/- 4.7 kg, (V) over dot O-2max 4137 +/- 697 ml . min(-1); mean +/- s) took part in the study. Six of the participants performed replicate 'square-wave' rest-to-exercise transitions of 6 min duration at running speeds calculated to require 80% (V) over dot O-2 at the ventilatory threshold (moderate-intensity exercise) and 50% of the difference between (V) over dot O-2 at the ventilatory threshold and (V) over dot O-2max (50% Delta; high-intensity exercise) on 5 different days. Although the amplitudes of the (V) over dot O-2 response were relatively constant (coefficient of variation similar to6%) from day to day, the time-based parameters were more variable (coefficient of variation similar to15 to 30%). All nine participants performed replicate square-waves for each time of day. There was no diurnal effect on the time-based parameters of (V) over dot O-2 kinetics during either moderate- or high-intensity exercise. However, for high-intensity exercise, the amplitude of the primary component was significantly lower during the 12:00 h trial (2859 +/- 142 ml . min(-1) vs 2955 +/- 135 ml . min(-1) at 06:00 h and 2937 +/- 137 ml . min(-1) at 18:00 h; P < 0.05), but this effect was eliminated when expressed relative to body mass. The results of this study indicate that the amplitudes of the (V) over dot O-2 kinetic responses to moderate- and high-intensity treadmill exercise are similar within and across test days. The time-based parameters, however, are more variable from day to day and multiple transitions are, therefore, recommended to increase confidence in the data.