An In Situ Examination of Running Specific Prostheses Stiffness Properties

Since the development of running specific prostheses (RSPs) in the 1980s, individuals with lower limb loss have been able to engage in more modes of physical activity including running competitively and recreationally. Researchers have been led to investigate different mechanical properties like stiffness and hysteresis using machine testing. However, machine testing is limited by loading and deformation rates that are well below those recorded during running. The purpose of this investigation is to examine the mechanical properties of RSPs in situ. Three-dimensional motion capture and force platforms were used to record deformation and external loading while two participants ran at their 1-mile pace using their prescribed prosthetic device. Linear and nonlinear prosthetic stiffness and hysteresis were calculated for each trial across loading and restoration phases using the vector magnitudes of three-dimensional force and deformation measured via markers affixed to the prosthesis. Linear stiffness during the loading phase was relatively similar across participants (∼26 kN/m), despite differences in prosthesis type, body mass, and running speeds. However, linear stiffness was reduced by 5-10% during the restoration phase. Overall, both prostheses force-deformation relationships were nonlinear, exhibiting variable stiffness throughout each phase, a crossover point ("pinched hysteresis curve"), and smooth transitions between loading and restoration phases. Machine tested stiffness values from the literature were approximately 5 kN/m lower compared to in situ stiffness values. These findings illustrate the importance of understanding the in situ properties for RSPs, user applications, and clinical recommendations for prostheses prescription.