The purpose of the present study was to examine both the within-day and between-day reliability of sprint start variables. The reaction time and the premotor time (PMT) were used as markers of the reactive function. Additionally, 3-m time, first-step flight time, first-step length and first-step velocity were utilized as spatiotemporal markers of the sprint start. Prior to the reliability study, an extensive number of sprint start trials were performed to familiarize the participants. A repeated measures analysis of variance, intra-class correlation coefficient (ICC) and Cronbach`s alpha reliability coefficient were used to assess the within-day and between-day reliability. The coefficient of variation (CV%) was evaluated to determine the within-participant variation. Both the within-day and between-day spatiotemporal markers showed higher reliability with lower variability from the reactive function markers. This study shows that well-familiarized participants can perform a reliable sprint start performance both within a day and between days. We conclude that the reactive markers have a high variability within a day and between days and require further research to determine their dynamic role in the sprint start action. Therefore, spatiotemporal markers provide a more valid method of sprint start assessment.
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