Exercise, muscle, and the applied load-bone strength balance

A fracture occurs when the applied load is greater than the bone can withstand. Clinical practice guidelines for the management of osteoporosis include recommendations for exercise; one of the few therapies where the proposed anti-fracture mechanisms that include effects on both bone strength and applied loads, where applied loads can come in the form of a fall, externally applied loads, body weight, or muscle forces. The aim of this review is to provide an overview of the clinical evidence pertaining to the potential efficacy of exercise for preventing fractures in older adults, including its direct effects on outcomes along the causal pathway to fractures (e.g., falls, posture, bone strength) and the indirect effects on muscle or the muscle-bone relationship. The evidence is examined as it pertains to application in clinical practice. Considerations for future research are discussed, such as the need for trials in individuals with low bone mass or students that evaluate whether changes in muscle mediate changes in bone. Future trials should also consider adequacy of calorie or protein intake, the confounding effect of exercise-induced weight loss, or the most appropriate therapeutic goal (e.g., strength, weight bearing, or hypertrophy) and outcome measures (e.g., fracture, disability, cost-effectiveness).