Prospective study of biomechanical risk factors for second and third metatarsal stress fractures in military recruits

Objectives This prospective study investigated anatomical and biomechanical risk factors for second and third metatarsal stress fractures in military recruits during training. Design Prospective cohort study. Methods Anatomical and biomechanical measures were taken for 1065 Royal Marines recruits at the start of training when injury-free. Data included passive range of ankle dorsi-flexion, dynamic peak ankle dorsi-flexion and plantar pressures during barefoot running. Separate univariate regression models were developed to identify differences between recruits who developed second (n = 7) or third (n = 14) metatarsal stress fracture and a cohort of recruits completing training with no injury (n = 150) (p < 0.05). A multinomial logistic regression model was developed to predict the risk of injury for the two sites compared with the no-injury group. Multinomial logistic regression results were back transformed from log scale and presented in Relative Risk Ratios (RRR) with 95% confidence intervals (CI). Results Lower dynamic arch index (high arch) (RRR: 0.75, CI: 0.63–0.89, p < 0.01) and lower foot abduction (RRR: 0.87, CI: 0.80–0.96, p < 0.01) were identified as increasing risk for second metatarsal stress fracture, while younger age (RRR: 0.78, CI: 0.61–0.99, p < 0.05) and later peak pressure at the second metatarsal head area (RRR: 1.19, CI: 1.04–1.35, p < 0.01) were identified as risk factors for third metatarsal stress fracture. Conclusions For second metatarsal stress fracture, aspects of foot type have been identified as influencing injury risk. For third metatarsal stress fracture, a delayed forefoot loading increases injury risk. Identification of these different injury mechanisms can inform development of interventions for treatment and prevention.