In a prospective study, quantitative measures of the structure of the longitudinal arch of the foot were established and related to the incidence of stress fractures in the bones of the lower limbs of military recruits. In addition, the role of a semirigid orthotic device (Langer military stress orthotic) in preventing stress fractures was evaluated as a function of the structure of the longitudinal arch. Femoral and tibial stress fractures were found to be more prevalent in the presence of feet with high arches, whereas the incidence of metatarsal fractures was higher in feet with low arches. The use of an orthotic device reduced the incidence of femoral stress fractures only in the presence of feet with high arches and the incidence of metatarsal fractures only among feet with low arches. The findings suggest that the normal foot with a low arch acts as a better shock absorber than the normal foot with a high arch, and that an orthotic device may improve the shock absorbing capacity of the arch.