The Muscle-Bone Unit in Children and Adolescents: Insights from the Canadian Health Measures Survey.

Muscle and bone are closely related during growth and development. We sought to identify determinants of the muscle-bone relationship using peripheral quantitative computed tomography (pQCT) at the distal lower extremity and jumping mechanography among Canadian children. Data were obtained from the Canadian Health Measures Survey (CHMS), a national, cross-sectional, population-based study. 3,225 participants (aged 6 to 19.9 years) were included. Calf muscle cross-sectional area (CSA; 66% site) z-score was used as a surrogate measure of muscle strength, bone mineral content (BMC) z-score of the tibia shaft cross-section (38% site) was used as a surrogate measure of bone strength. Maximum jumping power (Pmax) z-score from the single two-legged jump test was assessed as a mediator between muscle CSA and BMC z-scores using path analysis. Peripheral QCT and Pmax z-scores were calculated using CHMS data. Height, weight, and body mass index (BMI) z-scores were calculated using Centers for Disease Control reference data. We found a strong positive association between muscle CSA z-score and tibia BMC z-score (β = 0.64, 95% confidence interval [CI]: 0.62 to 0.67). Calf subcutaneous fat CSA (β = -0.04, 95% CI: -0.07 to -0.01) and muscle density (β = 0.08, 95% CI: 0.03 to 0.13) z-scores were weak determinants, and weight (β = 0.10, 95% CI: 0.05 to 0.15) and BMI (β = 0.14, 95% CI: 0.07 to 0.21) z-scores were strong determinants of the association between muscle CSA z-score and BMC z-score. Self-reported moderate-to-vigorous physical activity and serum total calcium, 25-hydroxy-vitamin D3, and parathyroid hormone were not determinants. Pmax z-score was a partial mediator of the association between muscle CSA and BMC z-scores. In conclusion, we found that the effect of muscle size on bone mass was in part mediated by muscle power. Subcutaneous fat, muscle density, weight, and BMI were determinants of the muscle-bone relationship.