Recent literature suggests that passive prosthetic foot-ankle devices, when optimized, may provide similar energy performance as the natural foot, but this has yet to be realized with current designs. Of concern is the inconsistency between biomechanical and structural measures of device performance. Biomechanical studies of energy performance rely on estimating power based on the deformation velocity. However, a consensus has yet to be reached regarding its definition and interpretation, due, in part, to a lack of clarity regarding the nature of deformation in these devices. Specifically, a majority of analyses use rigid body assumptions to estimate the angular velocity of these deformable systems.

An extension of current methods is developed, termed the Deformable Link Segment (DLS) model, in which the deformation velocity is estimated using a series of instantaneous equivalent linkages. The novelty of the approach is that an estimate of the angular velocity is not required and the deformation velocity can be calculated without rigid body assumptions.

Practical application of the DLS model is demonstrated for biomechanical analyses with a small cohort of prosthesis users. DLS deformation velocity and deformation power of prosthetic foot-ankle devices are calculated and results are compared to current methods in the literature. These models are distinguished by the reference points and segment angular velocity. Comparison indicates that the DLS model produces results that are distinct, yet similar in general trend to estimates in the literature.

A mechanical testing method to characterize prosthetic foot deformation is developed based on a simplification of the DLS model. Structural properties are measured from a series of force-deflection curves at specific loads and shank angles. Five prosthetic feet with differing designs and structural properties are analysed to provide the predicted vertical deflection throughout simulated stance. A method is proposed to estimate foot-ankle deformation velocity from these measures.

The framework established will further enable researchers in the design and optimization of the energy performance of prosthetic foot-ankle devices and inform clinicians in the selection of patient-specific prosthetic systems.