Knee and ankle joint forces during steps and jumps down from two different heights.

Two healthy subjects were filmed on TV-video when performing 14 different steps and jumps from two different heights (0·20 m and 0·43 m) onto two Kistler force platforms that recorded the reaction forces. The ankle and knee load moments and joint forces were calculated using a sagittal plane semi-dynamic biomechanical model. Steps with the ball of the foot from the lower height induced a vertical ground reaction force (Fz) close to 1 body weight (bw) and around 2 bw from the greater height. From the greater height the knee moments for the majority of the step-downs studied were about 100 Nm, which gave knee joint compressive forces of 4-7 bw. The ankle was exposed to loads of similar magnitude but not so much influenced by step height. The reasons why patients with knee pain prefer performing backward step-downs are discussed.