The use of rocker models to understand functional tasks of the human lower limb is attractive because of their simplicity. Recent studies have determined a consistent feature of able-bodied walking termed the roll-over shape (ROS), which is the effective rocker shape that the lower limb system conforms to between initial contact and contralateral initial contact during walking. However, it is unclear what effective rocker shapes are used for fore-aft swaying. A better understanding of these shapes could be used to develop improved prostheses for this task, perhaps improving balance and balance confidence, and reducing the occurrence of falling in lower limb prosthesis users. We measured effective rocker shapes used by 11 able-bodied persons during walking and fore-aft swaying. We hypothesized that the curvature of the swaying shapes would be smaller (radius larger) than that of the walking shapes, providing a more stable interface with the ground during swaying. The radius (measured as the inverse of the curvature of the shape) was found to be about 1/3 of the leg length for walking, but over two times the leg length for swaying. A model examining the effective ankle stiffness necessary to achieve these curvatures suggests that the stiffness of a biomimetic prosthetic ankle would need to be over three times higher for fore-aft swaying than for walking. These results suggest that two separate modes would be needed in an ankle-foot prosthesis to mimic the physiologic system for walking and swaying.