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ROEMMICH, J. N., C. M. GURGOL, and L. H. EPSTEIN. Open-Loop Feedback Increases Physical Activity of Youth. Med. Sci. Sports Exerc., Vol. 36, No. 4, pp. 668–673, 2004.The number of youth that meet activity guidelines is decreasing and easy access to reinforcing sedentary behaviors competes with increasing physical activity. In the laboratory, open-loop feedback that used pedometer activity counts to gain access to sedentary alternatives doubled physical activity. This study evaluated the influence of open-loop feedback and reinforcement on physical activity and television (TV) time in a small clinical trial.Children (8–12 yr old) were randomized to an open-loop feedback plus reinforcement intervention (N = 11) or no feedback, no reinforcement control (N = 7). Subjects wore an accelerometer for 6 wk and attended meetings to download the accelerometer. Accumulating physical activity counts gave subjects in the open-loop group access to TV time, controlled by a TV Allowance™ device, with 400 counts = 1 h of TV. The control group had no feedback for activity and free access to TV.The open-loop group had a 24% increase in physical activity, which was greater (P = 0.02) than the control group. TV time of the open-loop group was reduced by 18% or 20 min·d−1 whereas the control group increased by 13 min·d−1, but these were not significant changes. The change in time spent watching television was directly related to the change in BMI z-score (r = 0.69, P = 0.002).Open-loop feedback increases physical activity in children, thus helping children to achieve physical activity recommendations. Reductions in TV watching may reduce or minimize gains in body weight.