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Tibial stress fracture is a common injury in athletic and military populations. Bone strain arising from repetitive loading can lead to initiation and accumulation of microdamage. If this is not adequately repaired by the remodelling process, then a stress fracture can result. Our understanding of stress fracture development is hindered by the ethical and practical limitations associated with human studies. The authors are to be congratulated for obtaining in vivo tibial bone strain data during the activities of jumping and running. While the difficulties are acknowledged, including small sample and problems with strain gauge technology, the results of this study did not provide support for the concept that high impact loading in the form of drop jumping produces higher principal strains and strain rates than fast running. As the authors highlight, these results apply to the non-fatiguing exercise. The calf muscles in particular have a role in attenuating the load on the tibia. The authors conclude that hight impact exercise need not be avoided in training programmes. However, it may be the change in training rather than the absolute training that is the more important stimulus for stress fracture development. Thus high impact exercise may increase the risk of stress fracture if introduced suddenly into a training programme. Furthermore, the cause of stress fractures is likely to be multifactorial and training parameters will interact with other factors such as bone density and bone geometry.