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Tracking of physical activity through childhood and adolescence tends to be low. Variation in the timing of biological maturation within youth of the same chronological age (CA) might affect participation in physical activity and may partially explain the low tracking.To examine the stability of physical activity over time from childhood to late adolescence when aligned on CA and biological age (BA).A total of 91 males and 96 females aged 8-15 yr from the Saskatchewan Pediatric Bone Mineral Accrual Study (PBMAS) were assessed annually for 8 yr. BA was calculated as years from age at peak height velocity. Physical activity was assessed using the Physical Activity Questionnaire for Children/Adolescents. Tracking was analyzed using intraclass correlations for both CA and BA (2-yr groupings). To be included in the analysis, an individual required a measure at both time points within an interval; however, not all individuals were present at all tracking intervals.Physical activity tracking by CA 2-yr intervals were, in general, moderate in males (r = 0.42-0.59) and females (r = 0.43-0.44). However, the 9- to 11-yr CA interval was low and nonsignificant (r = 0.23-0.30). Likewise, tracking of physical activity by BA 2-yr intervals was moderate to high in males (r = 0.44-0.60) and females (r = 0.39-0.62).Accounting for differences in the timing of biological maturity had little effect on tracking physical activity. However, point estimates for tracking are higher in early adolescence in males and to a greater extent in females when aligned by BA versus CA. This suggests that maturity may be more important in physical activity participation in females than males.