The mechanics of the heel pad have been studied extensively due to the important role of the heel in absorbing shock and reducing local tissue stresses during locomotion. Factors affecting the mechanical behaviour of the heel include tissue thickness, tissue properties, and boundary conditions such as shoe and surface properties. However, the quantitative relationship between these variables is not clear, and is difficult to determine experimentally. Therefore, the purpose of this research was to develop a finite element (FE) model of the foot to determine the effect of these variables on overall heel pad mechanics and internal tissue stresses during heel strike.

First, the properties of the relevant materials, which were not available in the literature, were determined. Using cylindrical samples in a materials testing machine, the hyperelastic, compressible properties of a typical running surface were determined. Also, the hyperelastic, viscoelastic properties of the heel fat pad were measured. Appropriate constitutive equations were determined for each material.

Second, five subject-specific, two-dimensional (2D) FE models were developed and evaluated using in vitro experimental data. In an FE model of heel indentation experiments, the previously measured material properties of the isolated fat pad samples were able to predict the behaviour of the in situ fat pad within 10% at high deformations and impact speeds of 0.01 and 350 mm/s. Validations at higher speeds could not be performed due to the limitations of the testing apparatus. Also, a hyperelastic, linear viscoelastic material model was determined for the plantar skin, such that the FE model predicted heel pad behaviour at high deformations within 10.5% of experimental results, at an . impact speed corresponding to vertical impact during walking.

Finally, the validated FE model was used to study the effect of tissue thickness and tissue properties, and shoe and surface properties on heel pad behaviour. An increase in heel pad thickness and a decrease in heel pad hardness resulted in a decrease in vertical impact force, increase in contact area, decrease in peak contact pressure, and a decrease in absolute value of peak minimum in-plane principal stress. Shoe and surface cushioning had a substantial effect on plantar pressures and stresses. However, for constant heel geometry and tissue properties, the differences between effects of shoe and surface material properties were very small (< 4%). A softer shoe and softer surface resulted in a decrease in heel pad stiffness compared to the harder shoe and surface, decrease in peak plantar pressure and a decrease in the absolute value of peak minimum in-plane principal stress.

This study was the first to report the tensile and compressive properties, as well as compressibility of typical running track materials, and the first to measure and validate the incompressible, viscoelastic properties of the heel fat pad and skin. These material properties will be useful for other FE models of the foot. 1± results of this study indicate that modeling an individual's heel pad thickness is more critical than determining subject-specific material properties for a normal, healthy adult. Also, results show that athletic shoes and surfaces have a large effect on overall heel pad mechanics compared to barefoot.

Future improvements to the model include validation for impacts speeds corresponding to running and other sport activities. Also, a substantial lack of shoe material properties in the literature must be addressed. Further work in these areas may help to improve the design of shoes and surfaces for comfort and performance, and eventually the design and prescription of footwear and orthotics for subjects susceptible to high heel loading and stresses.