7.2 Reaction forces at the foot supports during single-leg stance

Ergoniza Intelligent models two possible point contact areas between each foot and the floor. When the model is in single-leg stance and only one contact area exists between the foot and the floor, all reactions are concentrated at that single support.

When two contact areas exist, the vertical reactions at the heel and forefoot balance the vertical components of body weight and the supported external loads. Their distribution between the supports depends on the position of the center of pressure, as illustrated in Figure 22. The distribution is inversely proportional to the distances from each support to point O, where O is the intersection between the line joining the supports and a line perpendicular to it that passes through the CoP.

The reaction forces at the contacts have horizontal components when the supported external loads also have them. The horizontal reaction forces are calculated separately for the X and Y components.

When two contact areas exist, the distribution of the horizontal components must satisfy both force and moment equilibrium. The sum of the horizontal reactions must be equal and opposite to the horizontal resultant of the external forces. In addition, the moment produced by the loads about the center of the foot must be balanced by the reactions to prevent the foot from rotating.

These conditions alone do not determine a unique solution. Ergoniza Intelligent separates the calculation into the X and Y axes and distributes the moment about the vertical axis between both directions according to the orientation of the foot. When the longitudinal axis of the foot coincides with the model's Y axis, the moment is balanced by the X components of the heel and forefoot reactions; when it coincides with the X axis, the moment is balanced by the Y components. For intermediate orientations, both contributions are combined using continuous weighting based on the square of the cosine of the angle between the foot and the Y axis.