Maximal power, but not fatigability, is greater during repeated sprints performed in the afternoon

The present study was designed to investigate if the suggested greater fatigability during repeated exercise in the afternoon, compared to the morning, represents a true time-of-day effect on fatigability or a consequence of a higher initial power. In a counterbalanced order, eight subjects performed a repeated-sprint test [10 x (6 s of maximal cycling sprint + 30 s of rest)] on three different occasions between: 08:00-10:00, 17:00-19:00, and 17:00-19:00 h controlled (17:00-19:00 h(cont), i.e., initial power controlled to be the same as the two first sprints of the 08:00-10:00 h trial). Power output was significantly (p < 0.05) higher for sprints 1, 2, and 3 in the afternoon than in the morning (e.g., sprint 1: 23.3 +/-1 versus 21.2 +/-1 W.kg(-1)), but power decrement for the 10 sprints was also higher in the afternoon. Based on the following observations, we conclude that this higher power decrement is a consequence of the higher initial power output in the afternoon. First, there was no difference in power during the final five sprints (e.g., 20.4 +/-1 versus 19.7 +/-1 W.kg(-1) for sprint 10 in the afternoon and morning, respectively). Second, the greater decrement in the afternoon was no longer present when participants were producing the same initial power output in the afternoon as in the morning. Third, electromyographic activity of the vastus lateralis decreased during the exercise (p < 0.05), but without a time-of-day effect.