Biological Age and Sex-Related Declines in Physical Activity during Adolescence JOHN CAIRNEY 1 , SCOTT VELDHUIZEN 2 , MATTHEW KWAN 1 , JOHN HAY 3 , and BRENT E. FAUGHT 3 1 Department of Family Medicine, McMaster University, Hamilton, Ontario, CANADA; 2 Centre for Addictions and Mental Health, Toronto, Ontario, CANADA; and 3 Community Health Sciences Brock University, St. Catharines, Ontario, CANADA ABSTRACT CAIRNEY, J., S. VELDHUIZEN, M. KWAN, J. HAY, and B. E. FAUGHT. Biological Age and Sex-Related Declines in Physical Activity during Adolescence. Med. Sci. Sports Exerc., Vol. 46, No. 4, pp. 730–735, 2014. Introduction: Sex differences in the rate of decline in physical activity (PA) are most pronounced during adolescence. However, once boys and girls are aligned on biological age, sex differences in the patterns of PA become attenuated. The aim of this study was to test whether biological maturation can account for sex differences in participation in PA over time from late childhood to early adolescence. Methods: A prospective cohort of children (N = 2100; 1064 boys) was followed from ages 11 to 14 yr, with repeated assessments of PA and anthropometry. Self-reported participation in organized and free play activities was used to track participation in PA. Biological age was measured using an estimate of years to attainment of peak height velocity. Mixed-effects models were used to test whether controlling for biological age attenuates the effect of chronological age and sex on PA. Results: As expected, the rate of decline in participation in PA was greater for girls than for boys (B = j1.18, P G 0.01). In multivariable analyses, adjusting for biological age completely attenuated the effect of sex and chronological age for participation in free play activities, but not for participation in organized play. Overall, biological age was a stronger predictor of participation than chronological age. Conclusions: The effect of biological age on sex by chronological age differences may be specific to certain types of PA participation. Given the importance of maturation to participation in activity, it is suggested that public health strategies target biological not chronological age to prevent declines in PA during adolescence particularly when promoting habitual or lifestyle activity. Key Words: ADOLESCENCE, PEAK HEIGHT VELOCITY, BIOLOGICAL AGE, PHYSICAL ACTIVITY T wo of the most consistent findings in epidemiological studies of physical activity involve age and sex: physical activity has been shown to decline with age for both sexes (2,4,7,8,21,28), and boys are more active than girls throughout childhood and adolescence (9,10,18,22,23,29,30), particularly when aligned by chronological age. Work in these areas suggests that childhood and adolescence is a critical de- velopmental period for understanding declining physical ac- tivity and the emergence of sex differences. Given that physical inactivity is widely regarded as one of the most pressing public health problems (3), it is important that we understand both sex and age-related declines in physical activity. Although physical activity has been shown to decline from childhood through later adulthood, this decline is most rapid during adolescence (1,4,16,31). A recent meta-analysis of longitudinal studies of physical activity during adoles- cence reported a mean decline in physical activity of 7% per year from 10 to 19 yr of age (6). It is also during this period that patterns of physical activity begin to diverge for males and females. Studies have consistently shown that physical activity declines more rapidly among girls than boys during this period (10,26,28). Adolescence is a period of rapid and often dramatic changes not only in physical activity but also in the physi- ological, psychological, and social factors associated with it (12). Growth spurts and the onset of puberty, with associated changes in body shape and proportion, can have a profound impact on physical activity (25), both directly and through related changes in social behavior and gender roles. These factors may plausibly be linked to sex differences in activity patterns. Some have argued, however, that the more rapid decline in activity among girls simply reflects sex differ- ences in biological maturation; that is, activity declines in step with maturation, irrespective of sex, and that the more rapid decrease among girls is attributable to their earlier at- tainment of peak growth, full adult height, and sexual ma- turity (5,19,26,29). Peak height velocity, a commonly used Address for correspondence: John Cairney, Departments of Family Medicine, Psychiatry and Behavioural Neuroscience, and CanChild Centre for Childhood Disability Research, McMaster University, 175 Longwood Road South, Suite 201a, Hamilton, Ontario L8P 0A1, Canada; E-mail: cairnej@mcmaster.ca. Submitted for publication June 2013. Accepted for publication September 2013. 0195-9131/14/4604-0730/0 MEDICINE & SCIENCE IN SPORTS & EXERCISE Ò Copyright Ó 2014 by the American College of Sports Medicine DOI: 10.1249/MSS.0000000000000168 730 EPIDEMIOLOGY Copyright © 2014 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited.