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To assess the validity of the new Actical accelerometer step count function.Actical step counts were compared according to two criterion standards. 1) Eight Acticals were assessed using a mechanical shaker table under six different testing conditions. 2) Thirty-eight volunteers (aged 9-59 yr) wore eight Acticals and eight Actigraphs during treadmill walking (50 and 83 m·min−1) and running (133 m·min−1) for 6 min at each speed. Steps were counted during the second and fourth minutes of each speed by a trained observer.The correlation between Actical step counts and the mechanical shaker step counts was excellent (r = 1.0). Compared with visually counted steps, both the Actical and Actigraph step counts were significantly different at 50 m·min−1; however, no significant differences were evident at 83 and 133 m·min−1. The criterion-related validity correlations (r) for the Actical and Actigraph, respectively, were 0.73 and 0.52 at the slow walk condition and 0.99 and 0.99 at the normal walk and run conditions.The new step count function of the Actical accelerometer provides valid estimates of step counts at 83 and 133 m·min−1 on a range of healthy participants.