Influence of military lower extremity leg compliant element on tibia response.

OBJECTIVE: Lower extremity injuries are a major concern in military environments due to underbody blast (UBB) events. There is a need in characterizing the transfer of axial loads to the lower extremities and evaluating their influence on injury mechanisms using surrogates. The Military Lower Extremity (MiL-Lx) leg was designed to replicate the biomechanical response of the human leg under vertical loading. While previous studies have reported global force responses of the MiL-Lx, the mechanistic role of its compliant tibial element in shaping force-time characteristics has not been systematically quantified. This study addresses this gap by isolating and analyzing the effect of structural compliance on dynamic load transmission.

METHODS: Controlled vertical impact tests were conducted using the Modified Lower Limb Impactor (MLLI) at varying impact severities. Both upper and lower tibia load cells were measured to evaluate load transfer through the compliant tibia element of the MIL-Lx leg. Force-time histories were analyzed to assess peak forces, plateau regions, and time-to-peak responses.

RESULTS: Peak forces in both the upper and lower tibia increased with impact severity. A distinct force plateau was observed in the lower tibia corresponding to maximum compression of the compliant tibia element. At higher severities, a characteristic double-peak emerged, with the initial peak associated with shock loading and the secondary peak linked to recoil following tibial element compression. Time to peak for the lower tibia decreased with severity, whereas the upper tibia time to peak remained comparatively stable. The compliant tibia element significantly modulated force transmission, altering both the temporal evolution and magnitude of loads transferred to the upper tibia.

CONCLUSIONS: This study provides the first detailed mechanistic characterization of how the compliant tibial element of the MiL-Lx influences force-time response under UBB-relevant loading. The findings demonstrate that structural compliance governs load attenuation, temporal filtering, and peak force transmission to proximal segments, with direct implications for injury risk assessment and the design of blast-mitigating protective systems.