Knee moments: origin in normal ambulation and their modification by double-stopped ankle-foot orthoses

While the primary function of an ankle-foot orthosis is to stabilize the ankle complex, these devices often cause an unstable knee joint. The effect of a double upright ankle-foot orthosis on knee stability was evaluated during the stance phase of the walking cycle. The external moments applied to the knee in the anteroposterior plane were determined by measuring the ground reaction forces and the position of the knee with respect to these forces. Total knee moment and individual moment contributions of the fore/aft shear and vertical ground reaction forces were analyzed. Moments generated during normal ambulation were compared with those generated by an ankle-foot orthosis with an adjustable double stop. Knee moment components due to fore/aft shear and vertical ground reaction forces tended to counterbalance each other, minimizing the magnitudes of total knee moment. Fore/aft shear moment was most affected by the magnitude of the fore/aft shear force and had the major effect on total knee moment when the fore/aft shear force was peaking during the heel-strike and push-off phases. The vertical force moment was most affected by the length of the moment arm (position of knee center with respect to force) rather than magnitude of force, and had the major effect on total knee moment during midstance phase.