Evaluating the Relationship between Muscle and Bone Modeling Response in Older Adults

Abstract

Background: Bone modeling, the process that continually adjusts bone strength in response to prevalent muscle-loading forces throughout an individual's lifespan, may play an important role in bone fragility with age. Femoral stress, an index of bone modeling response can be estimated using measurements of DXA derived bone geometry and loading information incorporated into an engineering model. Assuming that individuals have adapted to habitual muscle loading forces, greater stresses indicate a diminished response and a weaker bone Aims/Methods: The aims of this dissertation were to 1) evaluate the association of femoral stress with measures of lean mass and muscle strength among healthy older adults participating in the Health ABC study using linear regression; 2) determine whether femoral stress predicts incident fracture among the same cohort of older adults using cox proportional hazards models; and 3) evaluate the association of femoral stress with measures of lean mass and muscle strength in women after hip fracture participating in the 3rd and 4th cohort of the Baltimore Hip Studies using linear regression and to determine whether femoral stress changes the year following fracture using longitudinal data analysis. Results: Lean mass explained more of the variation in femoral stress than measures of muscle strength among healthy older men and women as well as in women with hip fracture. Remaining variability in femoral stress may reflect individual variation in modeling response. After adjusting for measures of lean mass and strength, women in the highest tertile of femoral stress had 77% higher hazard of fracture and men in the highest tertile of femoral stress had 84% higher hazard of fracture relative to women and men in the lowest tertile, respectively. This suggests that deficiencies in bone modeling response may be an important predictor of fracture. Femoral stress did not appear to change the year following fracture in older women. Conclusion: Future studies should focus on refining measures of bone modeling response by incorporating better measures of muscle force. While femoral stress does not have clinical applications per se, it allows us to investigate a potentially important mechanism underlying bone fragility and provides a framework for thinking about treatments that could improve the interaction between muscle and bone.