The Reaction of Skin and Soft Tissue to Shear Forces Applied Externally to the Skin Surface

The reaction of skin and soft tissue to shear force applied externally to the skin surface has been investigated by the experimental measurement of skin blood flow and model analysis of internal stresses. Skin blood flow has been measured using laser Doppler flowmetry while variable shear forces and normal force were applied to the skin surface. The experimental results show that the skin blood flow is reduced by increasing either normal force or shear force; the magnitude of the reduction being nearly proportional to the resultant of the normal and shear forces. The internal stresses were analysed by simplified models incorporating elasticity theory. The analyses indicate that shear force changes the stress distribution mainly in the superficial layer of the soft tissue and increases the stress in the area ahead of the direction of application; the maximum internal compression stress was found to be determined by the resultant of normal and shear forces. The study suggests that the resultant force is the most critical parameter in assessing the effect of load applied to the skin; and shear force has the same effect on skin and underlying tissue as normal force.