Comparison of static and dynamic biomechanical measures in military recruits with and without a history of third metatarsal stress fracture.

BACKGROUND: For Royal Marine recruits in training, the third metatarsal is the most common site for stress fracture. Previous evidence regarding biomechanical factors contributing to metatarsal stress fracture development is conflicting, possibly due to the lack of differentiation between the metatarsals. The present retrospective study compares static anatomical characteristics and dynamic biomechanical variables for Royal Marine recruits with and without a history of third metatarsal stress fracture. METHODS: Ten Royal Marine recruits with a history of third metatarsal stress fracture were compared with control subjects with no previous stress fracture occurrence. Selected static anatomical variables were measured to describe the ankle and subtalar joints. Peak ankle dorsi-flexion and rearfoot eversion were measured during running. In addition, peak vertical and horizontal ground reaction force variables were compared for the two study groups. FINDINGS: No significant differences in static anatomical variables were identified between study groups. During running, peak rearfoot eversion was found to occur significantly earlier for the stress fracture group than for their matched controls, suggesting an increase in time spent loading the forefoot. The peak applied resultant horizontal force during the braking phase was directed significantly more laterally for the stress fracture group. In addition, the peak magnitude of resultant horizontal force applied during the propulsion phase was significantly lower for the stress fracture subjects. INTERPRETATION: The findings of this study highlight the importance of including dynamic biomechanical data when exploring variables associated with the development of third metatarsal stress fracture and indicate that successful interventions to reduce the incidence of this injury are likely to focus on forefoot function during braking and propulsion.