Age-Dependent Effects of Physical Activity on Lean Mass Accrual

Abstract

Physical activity may be one of the most important modifiable factors for the development of lean mass (an index of muscular growth) in childhood and adolescence. However, the effects of normal growth and maturation may mask the effects of physical activity. To distinguish the age-dependent effects of physical activity from those of growth and maturation, individual growth trajectories must be identified; this can only be achieved using longitudinal data. PURPOSE: To investigate the independent effects of physical activity at each measurement occasion on total body lean mass (TBLM) accrual. METHODS: 105 boys and 103 girls, participating in the Saskatchewan Pediatric Bone Mineral Accrual Study (PBMAS), were repeatedly assessed during the circumpubertal years. At study entry they were 8 and 15 years of age; a 13-year developmental pattern (8 to 21 years) was constructed based on seven years of annual data. Height and physical activity were assessed biannually. TBLM was assessed annually by dual energy X-ray absorptiometry (DXA) (Hologic 2000). Physical activity was determined using the physical activity questionnaires for children and adolescence (PAQ-C/A). Biological age, a measure of maturity, was denned as years from age of peak height velocity. Data were analyzed using random effects models. RESULTS: The fixed coefficients that significantly predicted TBLM were biological age (years) (−722g ± 127g, p>0.05), height (cm) (607g ± 18g, p>0.05), body mass (g) (213g ± 12g, p>0.05), sex (0=boy;1=girl) (−2410g ± 434g, p>0.05) and physical activity (1=low;5=high) (214g ± 81g, p>0.05). CONCLUSIONS: For boys of the same biological age, height and body mass TBLM is predicted to be 2.2% higher in the most active boy compared to the least active boy; a 3.2% difference was found in girls. The results indicate that physical activity has a significant independent effect on total body lean mass accrual during childhood and adolescence.