Reliability of low-power cycling efficiency in energy expenditure phenotyping of inactive men and women

Standardized approaches to assess human energy expenditure (EE) are well defined at rest and at moderate to high-intensity exercise, but not at light intensity physical activities energetically comparable with those of daily life (i.e., 1.5-4 times the resting EE, i.e., 1.5-4 METs). Our aim was to validate a graded exercise test for assessing the energy cost of low-intensity dynamic work in physically inactive humans, that is, those who habitually do not meet the guidelines for moderate-to-vigorous aerobic physical activity levels. In healthy and inactive young men and women (n = 55; aged 18-32years), EE was assessed in the overnight-fasted state by indirect calorimetry at rest and during graded cycling between 5 and 50W for 5min at each power output on a bicycle ergometer. Repeatability was investigated on three separate days, and the effect of cadence was investigated in the range of 40-90rpm. Within the low power range of cycling, all subjects perceived the exercise test as light on the Borg scale, the preferred cadence being 60rpm. A strong linearity of the EE-power relationship was observed between 10 and 50W for each individual (r > 0.98), and the calculation of delta efficiency (DE) from the regression slope indicated that DE was similar in men and women (similar to 29%). DE showed modest inter-individual variability with a coefficient of variation (CV) of 11%, and a low intra-individual variability with a CV of similar to 5%. No habituation or learning effect was observed in DE across days. In conclusion, the assessment of the efficiency of low power cycling by linear regression - and conducted within the range of EE observed for low-intensity movements of everyday life (1.5-4 METs) - extends the capacity for metabolic phenotyping in the inactive population.