Foot ulceration: hypotheses concerning shear and vertical forces acting on adjacent regions of skin.

During gait the plantar surface of the foot experiences distributed shear and compressive stresses due to tangential and vertical forces respectively. At any given point under a foot, the ratio of tangential/vertical forces gives a value (mu Rmin) for the minimum coefficient of friction required to prevent slipping. If mu Rmin is greater than the actual coefficient of friction (mu A), then localized slipping will occur. Three possible scenarios exist which could lead to skin ulceration: at a localized area the skin may tend to slip (i) towards, (ii) away from or (iii) parallel to a neighboring skin region where mu Rmin < or = mu A (i.e. a region that doesn't slip). The first of these possibilities is similar in concept to the loading conditions that lead to a carpet becoming 'wrinkled', i.e. one region of the carpet slips towards another that is stationary. The second and third possibilities would tend to cause tearing in the carpet or shearing of its fabric respectively. In terms of skin breakdown the relative importance of each of these biomechanical hypotheses needs to be determined.