The influence of weight-bearing impact exercise and muscle strength on bone properties in healthy humans: sex- and age-specific effects

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Exercise and muscle strength play a critical role in developing bone strength, providing mechanical protection, and preserving skeletal tissue. Bone remodeling is influenced by mechanical forces applied to bone, including ground reaction forces and internal muscle forces. Weight-bearing impact exercise, involving moderate-to-high magnitude loads (=2-4 times body weight) and multidirectional movement patterns, is considered the most effective strategy for inducing changes in bone material and structural properties. Muscle forces can influence bone strength by applying strains to stimulate bone formation, and therefore contribute to the regulation of bone strength (known as the functional muscle-bone unit theory). Muscle-derived growth factors may also stimulate bone formation independent of mechanical loading, further demonstrating evidence of a muscle-bone relationship. Despite substantial evidence in support of a positive association between muscle mass and strength and surrogates of bone strength, we have limited understanding of which mechanical and biochemical muscle-related factors explain the most variability in true estimates of bone strength. As well, the relationship between muscle and bone size and strength may differ between sexes and change significantly with age, yet we do not know how these sex- and age-related differences impact the capacity of bone to respond to anabolic stimuli (e.g., exercise). Therefore, the purpose of this proposal is to examine the influence of weight-bearing impact exercise and muscle strength on bone properties in healthy individuals and to determine whether their effects differ between sexes and with age. The goals of the proposed research program are to examine the sex differences in the muscle-bone regulatory mechanism as well as compare its determinants in younger and older adults. We will also conduct a pilot randomized study comparing a 12-month resistance training plus high-impact loading intervention versus resistance training alone and an attentional control group. These studies will provide novel insight into the independent and interacting roles of weight-bearing impact exercise and muscle-generating loading on bone strength, geometry, and structure in humans. Combining specialized measures to quantify bone material and structural properties with assessments of mechanical and biochemical muscle factors, we aim to provide new evidence on the role of muscle in governing the regulation of bone metabolism and the downstream effects on bone strength. The results from these studies will enhance our fundamental understanding of bone responses to different stressors of daily life and exercise in men and women across the lifespan.