D-37 Free Communication/Poster - Pulmonary Disease and Exercise

Abstract

The positive association between muscle mass and bone mass is well known. However, muscle mass and areal bone mineral density are surrogate measures of the more important constructs of muscle function, muscle cross-sectional area (MCSA), and bone strength (as posited in the Mechanostat Theory). PURPOSE: We examined the relationship between lower body muscle power (a measure of muscle function), MCSA, and tibial bone strength in a healthy cohort of 303 17-year-olds. METHODS: This cross-sectional study was part of the ongoing Iowa Bone Development Study. Using peripheral quantitative computed tomography (pQCT), MCSA (66% tibia, mm2), Polar Moment of Inertia (PMI, 38% and 66% tibia, mm4), and Bone Strength Index (BSI, 4% tibia, g/cm3/mm4) were obtained during a clinical visit. Anthropometric measures (ht in cm, wt in kg) and vertical jump height (cm) were also measured. Vertical jump height and the Sayers equation were used to estimate lower body muscle power (Watts). Spearman correlations were calculated for sex-specific associations between muscle power and bone strength measures. Because MSCA is so involved in the relationship between muscle power and bone strength, mediation analysis was used to examine the effect of muscle power and MSCA on bone strength through several pathways. RESULTS: Muscle power estimates were significantly associated with bending and compression measures of bone strength. Associations were greater for bending strength (PMI 38%, 66%: females r=.70, r=.69 males r=.76, r=76) than compression strength (BSI: females r=.48 males r=.59). Mediation analysis indicated that muscle power had a significant effect on MSCA (p<.0001 at all sites), as well as on bone strength independent of MSCA (p<.0005 at all sites). CONCLUSIONS: These results showed a significant association between muscle power and bone strength even when controlling for MSCA. This is consistent with the premise that quality and use of muscle contribute over-and-above muscle size. More fully elucidating physical activity and bone health dose-response may require conceptualizing muscle power movements as a dimension of physical activity. Supported by The National Institute of Dental and Craniofacial Research (R01-DE12101 and R01-DE09551) and The National Center for Research Resources (UL1-RR024979 and M01-RR00059).