June 2007 | Volume 4 | Issue 6 | e222 From an evolutionary perspective, we were born to run . Indeed, the human musculoskeletal system is a fi ne locomotive apparatus that can adapt to varying demands. Our bones and muscles have the inborn ability to modify their structural and material designs to accommodate additional loads. Present-day athletes, for example, show higher bone mass and more robust structure than nonathletes at loaded bone sites . Studies of skeletal remains show skeletal strength has fallen relative to body size over the course of human evolution, which is thought to be due to the rise in sedentary lifestyles . The loss of bone strength associated with the typical sedentary lifestyles of people in the developed world may result in a higher risk of fractures. One recent study, for example, comparing the proximal femur of medieval and contemporary adults, found that over about 1,000 years the femoral neck axis has become longer and its cross-section has become proportionally smaller . This change in the phenotype of the proximal femur has led to a 50% increase in fall-induced stress upon the bone compared with medieval times . Obviously, a weak bone is more likely to fail than a strong one, but without the excess stress caused by falls, even a fragile bone may cope well with normal living . The root cause of most fragility fractures is falling, which leads to a large load on the bone from a direction it is not particularly adapted to handling. Fractures due to falls among aging people have become a serious public health problem for our modern societies [6,7], so there is great interest in fi nding preventive strategies. Regular exercise is undoubtedly one of the most promising preventive options, particularly because of its proven benefi ts for both bone rigidity and neuromuscular performance .