Comparison of mechanical test methods for determining prosthetic foot quasi-stiffness

Prosthetic foot quasi-stiffness (QS) and ankle joint QS (AJQS) are factors impacting user biomechanics and comfort. While QS is typically determined by machine-based test methods, AJQS is usually derived from user testing. Despite the use of various methods to assess QS, the relationships and comparability remain unclear. In this study, measurements were performed using five commonly described test methods on three different prosthetic feet (SACH, K2, energy storage and return). A uniaxial test machine was used to record heel and forefoot QS, while a 2D motion tracking system allowed calculation of AJQS. The results revealed significant (p< 0.001) inter-method QS differences of up to 24% (equivalent to approximately two foot categories), as well as differences in AJQS of up to 19% for the heel and 55% for the forefoot. Anterior-posterior forces (APF) varied by as much as 161%, likely due to ambiguities in certain test methods. Overall, each method demonstrated a foot sample-specific non-linear increase in QS, AJQS, and APF. Notably, two of the five methods were unable to differentiate between foot stiffnesses of the prosthetic foot samples.