An iterative method for viscoelastic modeling of prosthetic feet

Prosthetic foot designs are growing in complexity, but a few material and structural properties, including stiffness and viscoelasticity, remain critical to foot function. Consistent identification of these critical properties would aid prosthesis prescription. This investigation evaluates a new technique to model prosthetic feet as a combination of springs and dampers, and therefore characterize a foot's stiffness and viscoelasticity by means of spring and damper coefficients. A quasi-Newton iterative algorithm was developed to determine model coefficients for 9 prosthetic feet based on compressive creep, stress-relaxation, and constant strain rate tests. A broad range of current energy-storing feet including designs from Otto Bock, Seattle, Kingsley, and Ohio Willow Wood were very accurately modeled with the iterative technique. Feet without a solid ankle from Flex and College Park were the least accurately modeled. The Flex foot, tested without a cover, had a considerably lower damping coefficient. Damper coefficients were similar for most all other feet, suggesting similar material properties of the foam cover. Stiffness varied and generally agreed with published data. The ability of the model to produce two separate parallel spring stiffness constants might provide insight into foot structure. The model represents a means to objectively quantify material properties for a range of solid ankle dynamic elastic response prosthetic feet, but may be limited in its characterization of other foot varieties.