The Mechanophysiololgy of Stress Fractures in Military Recruits

Stress fractures (SFs) are of the most common and potentially serious overuse injuries. Many athletes, naïve exercisers, and military recruits who are engaged in frequent and repetitive activity may suffer a SF; the most common site for SF is the tibia. SF is regarded as fatigue fracture—when training yields bone strains in a range where the micro-damage formation in the bone exceeds the ability of a remodeling process to repair it and ultimately this cumulative tissue damage might result with a spontaneous fracture. The registry of SFs among athletes is incomplete, but in military recruits the incidence of SFs range between 5 and 12 % (female soldiers are 2–10 times more prone to SFs compared to their male counterparts). Recovery from a SF is primarily achieved by halting any load bearing activities and on rest. This might be detrimental to athletes and military recruits, as results in loss of training days and consequently a reduction in physical capacity. The ample risk factors for SFs can be categorized as internal factors depending on the individual (e.g. gender, bone geometry) and external factors (e.g. training volume). It follows that in many cases SFs are preventable. Recruits engaged in a reasonable level of physical activity, especially impact exercise in the years prior to joining the military, and also maintain adequate nutrition, may lower their risk for SFs. Yet, several fundamental issues in regard to SFs are still left unresolved. For example, how muscle forces provide a protective effect against SFs, how many cycles (i.e. steps or strides) can an individual perform before he or she will be at a risk of suffering a SF, or is it necessary to implement prophylactic interventions in order to protect those who are identified at a greater risk? New experimental tools and improved computational modeling frameworks for investigating and better addressing the above questions that are reviewed in this chapter can be used to improve the knowledge on the etiology and prevention of SFs.