Personnel onboard high-speed marine craft are exposed to eccentric slam impacts of up to 20 g due to hull separation from water during operation. To maintain postural stability, occupants adopt a semi-squatted position to attenuate the high-acceleration loading, which causes severe acute and chronic musculoskeletal injuries, and hinders post-transit performance. The harsh environment warrants a thorough understanding of human-body behaviour to predict responses and quantify energy expenditure in maintaining postural stability. A comprehensive, three degree-of-freedom sagittal-plane musculoskeletal dynamic model was developed to estimate musculotendon forces from neuromuscular stimuli and joint kinematics to provide estimates of joint torques and muscle energetics. The model was validated through experimental trials with seven participants and indicates good agreement with torque profiles obtained through inverse dynamics. The framework provides general applicability to postural stability in a wide range of motion environments and supports future investigation of injury criteria and occupant-seat interaction on high-speed craft.