Injury to the anterior cruciate ligament (ACL) is prevalent and detrimental in the physically active population. Previous research has identified lesser knee flexion displacement, greater knee valgus displacement, and greater vertical and posterior ground reaction force (GRF) as biomechanical factors that are associated with ACL injury. Triceps surae muscle stiffness may have an influence on landing biomechanics based on existing literature suggesting greater lower extremity joint displacements and lesser vertical GRF with soft landings. Similarly, existing literature has suggested lesser ankle dorsiflexion (DF) range of motion (ROM) may influence greater knee valgus displacement. Nonetheless, the influence of these two variables on lower extremity biomechanics during a jump landing has not been investigated. The purpose of this study was to determine the influence of triceps surae muscle stiffness and ankle DF ROM on lower extremity biomechanics during a jump landing task. Thirty-five physically active subjects volunteered for this study. Triceps surae muscle stiffness was assessed using the damped frequency oscillation method, ankle dorsiflexion range of motion was assessed using a goniometer, and knee biomechanics of the jump landing were assessed using an infrared high-speed camera system. Individuals who displayed lesser triceps surae muscle stiffness demonstrated lesser vertical ground reaction forces. Individuals that displayed greater passive straight-knee ankle dorsiflexion range of motion demonstrated greater knee flexion displacement, and lesser vertical and posterior ground reaction forces. As adaptations in muscle stiffness and ROM may be induced over time, the influence of stiffness and ROM on biomechanical factors associated with greater ACL injury-risk suggest these variables should be considered with ACL injury prevention
