Using high-speed motion pictures, electromyography, a dynamic piezoelectric force plate, and computer analysis of the data, the gait patterns of fifteen children, four to sixteen years old, with spastic cerebral palsy and genu recurvatum were analyzed to determine the mechanisms producing genu recurvatum and the effect of fixed-ankle below-the-knee orthoses. In all children the recurvatum during stance phase began when the tibia stopped moving forward and disappeared when tibial movement resumed. In six patients (Group I), excessive activity of the calf muscles in response to the increasing dorsiflexion moment about the ankle produced by the foot-floor reaction force arrested the forward motion of the tibia. In six others (Group II), the contraction of the calf muscles was not sufficiently strong to resist the dorsiflexion moment and the tibia moved forward until maximum dorsiflexion had occurred and then stopped. In both instances recurvatum was produced when the femur continued to move forward over the stationary tibia and an extension moment was produced at the knee. In no patient did activity of the knee flexors prevent recurvatum, which was eliminated only by resumption of forward movement of the tibia. This movement of the tibia was produced either by heel-off (Group I) or by sudden unweighting of the limb due to opposite heel-strike (Group II). In Group I, when tibial motion stopped in the first half of stance phase the position of the hip rapidly changed from flexion to extension and there was forward leaning of the trunk, while in Group II the change from hip flexion to extension occurred with backward leaning of the trunk. The fixed-ankle below-the-knee orthosis, by preventing excessive dorsiflexion and plantar flexion, produced more normal moments about all joints, especially the knee. In the three children (Group III) whose recurvatum was permanently corrected by the brace, no explanation for the improvement was evident in these studies.