Between-day variability of net and gross oxygen uptake during graded treadmill walking: effects of different walking intensities on the reliability of locomotion economy.

There have been few studies of clinical relevance conducted on the reliability of walking economy. This study was designed to determine if walking economy reproducibility increases as a function of walking intensity, and if there is any advantage in expressing walking economy as net oxygen uptake (VO2) rather than gross VO2 for reproducibility purposes. Sixteen participants (9 males, 7 females; mean age, 22.3 +/- 4.3 years) performed resting, submaximal, and maximal protocols on 2 different days, under identical circumstances, within a 7 day period. The submaximal protocol consisted of five 5 min walks (4 km.h-1) at treadmill grades of 0%, 2.5%, 5.0%, 7.5%, and 10%. Findings indicate that increments of 2.5% in treadmill grade effectively increased gross and net VO2 during walks. The reliability of net and gross measures increased as a function of walking relative intensity, reporting intraclass correlation coefficients ranging from 0.89-0.94 and 0.87-0.91, respectively, and mean coefficients of variation (CV) from 7.3%-3.6% and 8.8%-4.4%, respectively. There were no significant differences between the CV for gross and for net VO2 across the spectrum of walking relative intensities. In conclusion, there is no advantage of expressing walking economy as net VO2 instead of gross VO2 for reproducibility purposes, and a single treadmill testing session at a constant speed of 4 km.h-1 is reliable for estimating group and individual walking economy, particularly at higher percent grades.