MAXIMAL POWER, BUT NOT FATIGABILITY, IS GREATER DURING REPEATED SPRINTS PERFORMED IN THE AFTERNOON Sebastien Racinais, 1,2 Stephane Perrey, 2 Romain Denis, 2,3 and David Bishop 4 1 ASPETAR, Qatar Orthopaedic and Sports Medicine Hospital, Research and Education Centre, Doha, Qatar 2 Motor Efficiency and Deficiency Laboratory, EA 2991, UFR STAPS, University Montpellier-1, Montpellier, France 3 Institute for Sport and Health, School of Physiotherapy and Performance Science, University College Dublin, Dublin, Ireland 4 Institute of Sports, Exercise and Active Living (ISEAL), Victoria University, Melbourne, Australia 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 × (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 ±1W·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 conse- quence of the higher initial power output in the afternoon. First, there was no differ- ence in power during the final five sprints (e.g., 20.4 ±1 versus 19.7 ±1W·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. (Author correspondence: sebastien.racinais@aspetar.com) Keywords Circadian variations; Exercise; Muscle electrical activity; Time-of-day 855 Address correspondence to Sébastien Racinais, ASPETAR, Qatar Orthopaedic and Sports Medicine Hospital, Research and Education Centre, P.O. Box 29222, Doha, Qatar. Tel.: +974.413.2570; Fax: +974.413.2020; E-mail: sebastien.racinais@aspetar.com Submitted September 27, 2009, Returned for revision October 20, 2009, Accepted December 23, 2009 Chronobiology International, 27(4): 855–864, (2010) Copyright © Informa UK Ltd. ISSN 0742-0528 print/1525-6073 online DOI: 10.3109/07420521003668412 Chronobiol Int Downloaded from informahealthcare.com by 89.211.165.17 on 06/18/10 For personal use only.