7.4 Reaction moments at the foot supports

Reaction moments may arise when the concentrated forces calculated at the supports are not sufficient by themselves to satisfy moment equilibrium. Ergoniza Intelligent can calculate them during both bipedal and single-leg stance. The simplest case to interpret occurs during single-leg stance when the center of pressure does not lie on the line joining the heel and the front of the foot.

Figure 24 shows a situation of this type in which the model is in a stable posture. Body weight and the vertical components of the external forces and loads are modeled by the resultant force F, applied at the center of pressure. This force is balanced by reactions Fₚ and Fₜ at the front of the foot and the heel. Force F also generates a moment M about the axis joining both supports, which must be balanced by reaction moments Mₚ and Mₜ. Ergoniza Intelligent distributes M between both supports in proportion to the corresponding vertical reaction force, with Mₚ + Mₜ = M.

It is important to remember that Ergoniza Intelligent considers the model to be in single-leg stance when only one foot is in contact with the floor or when, despite both feet being in contact, the entire load is supported by one of them.

The latter situation is illustrated in Figure 25. The Stabilogram shows that the center of pressure—represented by the blue circle—is not located between the feet, but to the left of the left foot, so the entire load is supported by that foot. Although the right foot remains in contact with the floor, its vertical reaction is zero and it does not effectively participate in supporting the body; the program therefore considers this a single-leg stance.