This study was designed to quantify postural reaction to a sudden toe-up rotational perturbation by twenty-two healthy young adults (eleven males, eleven females, average age of 26 years, height of 1.72 m, and weight of 649.62 N) with no physical or physiological conditions that would compromise their ability to react to the perturbation. The goal was to achieve an understanding of the mechanical requirements necessary for maintenance of balance and stance.

The human body was modeled as a system of five rigid segments. The act of reacting to a movement of the ground that suddenly dorsiflexed the ankle was monitored by a video analysis system operating at 60 Hz. Special care was taken to maximize marker resolution by covering the action with several cameras each zoomed in on part of the body. Segmental kinematics and kinetics were calculated after numerically filtering at 6 Hz and differentiating the digitized location of retroreflective markers attached to the subjects’ skin. Because no ground reaction data was available, kinetic calculations were performed in a head to toe direction.

Analysis of the segmental reactions to the postural perturbation revealed that:​ (1) All segments and joints studied were utilized in a wave like motion and the reaction was not limited to a hip or an ankle strategy. (2) The horizontal components of the ankle joint reaction force and knee joint reaction force were of approximately the same magnitude and approximately twice the magnitude of the hip joint reaction force. The vertical component of the ankle joint reaction force was larger than the knee joint reaction force, which was greater than the hip joint reaction force. The ankle joint produced the largest moment in response to the platform perturbation while the knee moment was larger then the hip moment. (3) With some reservations the larger the magnitude of the sudden angular ground perturbation distance the greater was the joint angular displacement at all joints. (4) The faster the angular velocity of the rotational perturbation the larger the joint angular displacement and peak force. Even though the peak knee torque did not increase in reaction to faster angular velocities of perturbation, the peak torque of the ankle and hip did. (5) The toques required to successfully maintain stance following a toe-up perturbation were well within the capabilities of the elderly. If any joint was to be considered the "weak link" it would be the ankle joint.