The Dynamic characterisation of ESR prosthetic running feet: an investigation of the key parameters affecting their performance.

Abstract

Prosthetic running feet (referred to as Energy Storing & Returning (ESR) or Running Specific Prostheses (RSP) but better known as ‘blades’) take the form of a carbon fibre leaf spring with a deflecting keel component. Available literature on the subject of their dynamic response is limited but suggests that the amputee with running foot can be considered to act in accordance with Simple Harmonic Motion, but this does not appear to be reflected by the prescription processes currently employed by manufacturers. The research question is asked: ‘Is the current method of prescribing prosthetic ESR running feet appropriate and are there additional factors that should be taken into consideration?’ This thesis aims to first understand the static mechanical characteristics of a single model of prosthetic foot; the Flex Run from Ossur (Reykjavik, Iceland). Previous works carried out (that aim to define the energy return efficiency of the devices but results vary from 63% - 100%) are examined and replicated using a series of fabricated jig fixtures, and the disparity in efficiency results is explained. The running action of an amputee is measured using a wearable measurement system that is developed as well as high-speed video capture. The measured action is then replicated in the laboratory using a rig capable of reproducing the dynamic response of the foot. This rig is subsequently used to manipulate the variables of Simple Harmonic Motion and evaluate the suitability of this assumption to model the running action of an amputee. The research concludes by using the gathered learning to create a tool capable of mathematically replicating the response of a prosthetic foot, and the application of such a tool is discussed. It is found during the course of the research that the available Flex Run feet possess an energy return efficiency of >99% and a variable stiffness along the length of the deflecting keel. As a contribution to knowledge, it was also found that during amputee running the ground contact point (and therefore effective stiffness) of the prosthetic changes significantly from foot strike through to toe-off and the profile of this change is defined. As such the principle of a spring-mass system cannot be applied in such a simplistic manner as previously suggested. Furthermore the relationship between amputee mass, stance length, foot deflection and response timing is defined for the first time. It was also discovered that the passive nature of the prosthetic device (and therefore fixed response) has the potential of limiting the top speed of running of an amputee and as such the current prescription method falls some way short of expectations. Methods of improving the prescription process are discussed and further work is suggested to improve the function of these prosthetic devices and therefore the user experience of the amputee athlete.