Article Dynamic Parameters Variability: Time Interval Interference on Ground Reaction Force during Running Juliana Pennone, Bruno Meze ˆ ncio, Alberto C. Amadio, and Ju ´lio C. Serra ˜o Laboratory of Biomechanics – School of Physical Education and Sports (EEFE-USP), Sa ˜o Paulo, Brazil Abstract The aim of this study was to determine the effect of the time between measures on ground reaction force running variability; 15 healthy men (age ¼ 23.8  3.7 years; weight ¼ 72.8  7.7 kg; height 174.3  8.4 cm) performed two trials of running 45 minutes at 9 km/hr at intervals of seven days. The ground reaction forces were recorded every 5 minutes. The coefficients of variation of indicative parameters of the ground reaction forces for each condition were compared. The coefficients of variations of the ground reaction forces curve analyzed between intervals and ses- sions were 21.9% and 21.48%, respectively. There was no significant difference for the ground reaction forces parameters Fy1, tFy1, TC1, Imp50, Fy2, and tFy2 between intervals and sessions. Although the ground reaction forces variables present a nat- ural variability, this variability in intervals and in sessions remained consistent, ensur- ing a high reliability in repeated measures designs. Keywords running, variability, ground reaction force Perceptual and Motor Skills 0(0) 1–9 ! The Author(s) 2016 Reprints and permissions: sagepub.com/journalsPermissions.nav DOI: 10.1177/0031512516633346 pms.sagepub.com Corresponding Author: Juliana Pennone, Laboratory of Biomechanics – School of Physical Education and Sports (EEFE-USP), Av. Prof. Melo Moraes, 65 – Sa ˜o Paulo, Brazil. Email: juliana.pennone@usp.br