Morning-to-Evening Differences in Oxygen Uptake Kinetics in Short-Duration Cycling Exercise

This study analyzed diurnal variations in oxygen (O2) uptake kinetics and efficiency during a moderate cycle ergometer exercise. Fourteen physically active diurnally active male subjects (age 23±5 yrs) not specifically trained at cycling first completed a test to determine their ventilatory threshold (Tvent) and maximal oxygen consumption (VO2max); one week later, they completed four bouts of testing in the morning and evening in a random order, each separated by at least 24 h. For each period of the day (07:00-08:30 h and 19:00-20:30 h), subjects performed two bouts. Each bout was composed of a 5 min cycling exercise at 45 W, followed after 5 min rest by a 10 min cycling exercise at 80% of the power output associated with Tvent. Gas exchanges were analyzed breath-by-breath and fitted using a mono-exponential function. During moderate exercise, the time constant and amplitude of VO2 kinetics were significantly higher in the morning compared to the evening. The net efficiency increased from the morning to evening (17.3±4 vs. 20.5±2%; p