The effect of shear forces externally applied to skin surface on underlying tissues.

The effects of shear forces externally applied to the skin surface on the underlying tissues have been investigated. An analysis of the internal stresses and strains was conducted using a simplified model incorporating elasticity theory. Skin blood flow was measured using laser Doppler flowmetry while variable shear forces over a range of 0-250 g were applied to the skin surface. The theoretical model predicts that the application of surface shear forces alters the internal stress distribution and makes the shear and compressive components of stresses increase ahead of the surface force application point. The force resulting from concomitant application of shear and normal force determines the internal maximum stress and strain. Theoretically, the shear force should have the same effects on the underlying tissues as normal force. The experimental investigations revealed that the skin blood flow decreased roughly linearly with the increase of shear forces. When a shear force equal to the normal force was applied, the flux decreased by 45%, nearly equal to the increasing magnitude (41%) of resultant of normal and shear forces.