Muscle Power Predicts Adolescent Bone Strength: Iowa Bone Development Study

Purpose To assess association between lower body muscle power and bone strength as well as the mediating effect of muscle cross-sectional area (MCSA) on that association. Methods Participants (141 males and 162 females) were approximately 17 yr. Muscle power was predicted using vertical jump and the Sayers equation. Using peripheral quantitative computed tomography (pQCT), bone strength indices were obtained at two locations of the tibia, corresponding to primary stressors acting upon each site: bone strength index for compression (BSI) at the distal 4% site; density-weighted polar section modulus strength-strain index (SSIp) and cortical bone area (CoA) at the 66% midshaft site for torsion. Muscle cross-sectional area was measured at the 66% site. Pearson bivariate and partial correlation coefficients were estimated to quantify the strength of the associations among variables. Direct and indirect mediation model effects were estimated, and 95% bootstrap confidence intervals were constructed to test the causal hypothesis. Height and maturity were examined as covariates. Results Pearson correlation coefficients among muscle power, MCSA, and bone strength were statistically significant (P < 0.01) and ranged from r = 0.54 to r = 0.78. After adjustment for covariates, associations were reduced (r = 0.37 to 0.69) (P < 0.01). Mediation models for males for BSI, SSIp, and CoA accounted for 38%, 66%, and 54% of the variance in bone strength, respectively. Models for females for BSI, SSIp, and CoA accounted for 46%, 77%, and 66% of the variance, respectively. Conclusions We found strong and consistent associations as well as direct and indirect pathways, among muscle power, MCSA, and tibia strength. These results support the use of muscle power as a component of health-related fitness in bone health interventions for older adolescents.