Diurnal Increase In Muscle Power Is Not Sustained For Repeated Sprints.

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Maximal muscle power during a cycling sprint is know to be higher in the afternoon than in the morning. This diurnal variation still exists after an active warm-up but disappears if maximal sprints have to be repeated, suggesting a higher fatigability in the afternoon. PURPOSE: To investigate if the higher power decrement observed in the afternoon is the due to higher fatigability. METHODS: 8 subjects randomly performed a repeated-sprint test [10 x (6 s of maximal cycling sprint + 30 s of rest)] on three occasions [8-10 am, 5-7 pm and 5-7 pm but with a submaximal initial power for the 2 first sprints]. Muscle power, oxygen consumption, muscle oxygenation and muscle electrical activity were recorded. RESULTS: Power production was significantly higher for sprints 1, 2 and 3 in the afternoon as compared to the morning (e.g. 21.2 ±1 vs 23.3 ±1 W.Kg-1 for sprint 1 in the morning and afternoon respectively, P < 0.01). However, power decrement throughout the repetition of sprint was also higher in the afternoon and there were no differences during the five final sprints (e.g. 19.7 ±1 vs 20.4 ±1 W.Kg-1 for sprint 10 in the morning and afternoon respectively). When controlling for the diurnal difference between the two first sprints, subject were still able to produce higher muscle power in the afternoon than in the morning for sprints 3 to 6 (all P < 0.05) and power didn't decrease (e.g., 20.7 ±1 vs 20.5 ±1 W.Kg-1 for sprint 1 vs sprint 10). Both the muscle deoxygenation ([HHb]) and the systemic oxygen consumption increased during the exercise (P < 0.05), but without effect of time of day. The RMS activity of the Vastus Lateralis decreased during the exercise (P < 0.05), but without effect of time-of-day. CONCLUSIONS: Maximal initial power was higher in the afternoon than the morning and power decrement was higher in the afternoon if the sprints are to be repeated. In addition: 1) if the initial power production is matched in the afternoon with the morning value, there were no power decrement with repeated sprints; and 2) systemic oxygen consumption, muscle deoxygenation and muscle electrical activity were sensitive to fatigue but without effect of time-of-day. Collectively, these data showed that fatigability is not higher in the afternoon but that the beneficial effect of time-of day on muscle power is not sustained for repeated sprints.