Figure 1: Illustrates how the ankle-foot (AF) roll-over shape is mapped out using a shank-based coordinate system.  The ankle-foot roll-over shape is a measurement of the effective rocker that the ankle-foot complex conforms to during the stance phase of walking.  This shape is found, in general, by finding the center of pressure (COP) of force applied to the foot in an ANKLE-KNEE coordinate system (shank-based coordinate system) (5).  As seen in the figure, the COP map, which would lie on the floor in a room-based coordinate system, now draws out a curve which gives the effective shape on which the body rolls over.  (A) shows the movement of the knee and ankle during a step, as well as the COP on the ground below.  All three of these points are mapped in a laboratory based coordinate system.  (B) shows the same points mapped in a shank-based coordinate system.  Now the COP traces the effective rocker (i.e. roll-over shape) that the foot and ankle conform to between heel contact (HC) and opposite heel contact (OHC).  Data for both (A) and (B) are shown in black from HC to OHC and in white following OHC.  Similar roll-over shapes can be found for the knee-ankle-foot (KAF) and hip-knee-ankle-foot (HKAF) systems by altering the coordinate system.  Adapted from (5).