Exercise-induced Bone Adaptation Significantly Increases Skeletal Fatigue Resistance

Abstract

PURPOSE Stress fractures are particularly common among athletes and military recruits, and have ramifications in terms of morbidity and time lost from participation. Despite their significance, there are currently limited preventative strategies for stress fractures. As stress fracture risk is directly influenced by skeletal properties (such as bone mass and size), it has been hypothesized that modification of these properties using an exercise program may positively influence risk. Bone is inherently mechanosensitive, and responds and adapts to its mechanical environment. The aim of this study was to investigate whether the bone changes associated with a mechanical loading program can enhance skeletal fatigue resistance. METHODS Site-specific mechanical loading was performed on one forearm of adult female Sprague-Dawley rats using the axial compression loading model. Loading was performed three days per week for five consecutive weeks to induce adaptation. The loaded and non-loaded ulnas in each animal were removed following the loading program, and their material and structural properties determined. The ulna pairs were subsequently loaded until fatigue failure at the same constant peak axial load. RESULTS Loading induced consistent and predictable changes in the structural properties of loaded ulnas, with the largest change being a nearly two-fold increase in midshaft minimum second moment of area (IMIN). The mechanical-loading induced bone changes resulted in a more than 100-fold increase in fatigue resistance in loaded ulnas, with resistance being exponentially related to the structural properties of the ulna. CONCLUSIONS This study found that by enhancing the structural properties of a bone via a mechanical loading program its fatigue resistance could be significantly improved. This indicates that an exercise program aimed at modifying bone structure may be used as a possible prevention strategy for stress fractures. As large increases in fatigue resistance were generated with only small changes in its structural properties, such a program would not need to generate large bone changes to have a significant impact on stress fracture risk. Based on our data, an increase of 5% in bone mineral content would equate to a 6-fold increase in fatigue resistance. Similarly, as fatigue resistance was exponentially negatively related to strain any technique that reduces bone strain during loading will have a large impact on stress fracture risk. Running on a treadmill has been shown to reduce tibial axial compressive strain by 66% compared to overground running. Based on our data, this reduction in strain would increase fatigue resistance by 70-fold.