Creatine (Cr) is an endogenous as well as an exogenous compound related with high-energy metabolism [1, 2] . During muscular exercise, ATP is directly used for mechanical contractions of actin and myosin filaments. The fastest anaerobic metabolic pathway to regenerate ATP is the utilization of phosphocreatine (PCr). However, the total pool of PCr is limited. The ingestion of exogenous Cr consequently increases intracellular Cr and PCr concentrations [3, 4] . This increase could therefore

more »

... ically improve muscular performance during anaerobic exercises. In parallel, an increase in body weight is also usually reported [1, 5] . The exact origin of the ergogenic effect of Cr supplementation is unclear; in particular, the mechanism behind improvement in running performance. Experimentally, many studies have highlighted an ergogenic effect of Cr during anaerobic exercises, but some authors have shown no effect on anaerobic performance and even a negative effect [1] . Sprint running is the archetype of anaerobic effort. The aim of sprinting exercise is to maximize external work to reach the maximal speed and to try to maintain it until the finishing line. Sprinting is therefore a good model to study the effective effect of Cr intake in anaerobic sports. Running speed is the product of stride length and stride frequency. In order to reach a maximal speed, the athlete tries to improve both parameters. Different studies suggested that stride length is of major importance in determining the speed of the sprinter [6, 7] . The purpose of this study was to clarify how 1 week of exogenous Cr (20 g/d) could affect average running performance during short-distance sprinting. The hypothesis was that biomechanically, speed enhancement after Cr intake can only be the result of an increase of stride frequency, stride length, or both of them [6] . The issue was to determine which of these three possibilities plays a role in the improvement of speed performance. Repeated sprint bouts were performed to highlight the effect of fatigue on average sprinting speed [1]. The biomechanical factors improving speed performance were determined using accelerometric measurements [8] . Seven healthy male volunteers were selected to participate in our study. They gave and signed a written informed consent form in accordance with institutional guidelines of the Ethical Commission of the University of Lausanne and the Ethical Standards of