• day-to-day variation;
  • energy expenditure;
  • female adolescents;
  • indirect calorimetry;
  • oxygen consumption

The day-to-day variation in oxygen consumption (O2) and energy expenditure (EE) during horizontal treadmill walking was measured using indirect calorimetry in 20 female adolescents (mean age 17·3 years). Two different walking speeds were used: 5 km h−1 and an individually convenient speed of 3·0 km h−1 (mean). The two sets of measurements were performed on 2 consecutive days, and great care was taken to minimize possible disturbing factors. The mean O2 was 919 ml min−1 at 5 km h−1 and 622 ml min−1 at the individual speed, and the mean values of EE were 4·5 kcal min−1 and 3·1 kcal min−1 respectively. The individual day-to-day variation in O2 (at 5 km h−1) was between −11·7% and +12·6% of the mean O2. The coefficient of variation (CV) was 6·4% when values were calculated in ml min−1 kg−1. The energy expenditure varied somewhat less between the 2 days (CV = 5·7%). The corresponding value for EE when walking at the individual speed was 7·2%, and the mean day-to day variation in O2 was 7·5% (CV). The rate of perceived exertion according to Borg's scale was lower on day 2 (11·9) compared with day 1 (13·0) when walking at 5 km h−1. There was no difference in heart rate between the 2 days. It is concluded that EE varies somewhat less than O2 on successive days, probably because of an interchangeable relationship between breathing gases, depending on which substrate is used for combustion. When using O2 and EE for evaluation of physical capacity, the day-to-day variation in the measurements must be taken into consideration.