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This study aimed to examine the tracking of physical activity (PA) during a 10-yr period and to investigate whether sex differences in PA trajectories are altered after aligning by maturity instead of age.The Iowa Bone Development Study collected accelerometer data on a cohort of 140 girls and 128 boys at ages 5, 9, 11, 13, and 15 yr. Logistic regression determined the odds ratio of being in lowest PA tertile at 15 yr if in lowest tertile at age 5 yr. Spearman correlation coefficients examined PA tracking from age 5 to 15 yr. Using additional Iowa Bone Development participants at ages 9, 11, 13, and 15 yr (N = 482, 457, 416, and 316, respectively), we examined the stability of PA for chronological and biological age in 2-yr intervals surrounding peak height velocity (PHV) age. Year from PHV was estimated using the Mirwald prediction equation.Girls had significant 10-yr correlations (r = 0.27); boys did not. Girls, not boys, were more likely to be in the lowest tertile for PA at age 15 yr if in the lowest tertile at age 5 yr (moderate to vigorous PA: odds ratio = 3.1, 95% confidence interval = 1.4–6.9; vigorous PA: odds ratio = 2.8, 95% confidence interval = 1.2–6.1). Girls’ 2-yr intervals for PA showed moderate associations (r = 0.31–0.56) when aligned chronologically but were moderate to high (r = 0.41–0.63) when aligned biologically. Boys’ associations were low to moderate (r = 0.26–0.56) chronologically. When aligned biologically, the associations were low for the −1- to 1-yr interval (1 yr before the 1-yr post-PHV) but moderate to high for the −2- to 0-yr and 0- to 2-yr intervals (r = 0.50–0.61).Physical activity tracking from childhood to adolescence is stronger in girls than that in boys. Intervention programs targeting boys should focus on time surrounding puberty in boys during which activity patterns change.