Conventional passive prosthetic feet cannot provide net positive mechanical energy, causing a strong reduction in push-off work. This decrease the comfortable walking speed and increases the metabolic cost of walking in amputees, as more than half of the positive work performed during able-bodied gait is done during the powered plantar flexion at the ankle joint. A possible contribution to enhance push-off is to utilize passive prostheses that can store and release energy by means of springs. This restores the push-off partially, enabling a higher self-selected walking speed and a lower metabolic cost.
Effective energy release at push-off is not just a matter of higher energy storage, but the release also needs to be well timed. Controlling the release rate of energy in current passive prosthetic feet is often constrained due to the unnatural ankle joint mechanics caused by simple spring behavior. The Variable Stiffness Prosthetic Ankle-Foot (VSPA-Foot) of Shepherd and Rouse (2017) tackles this problem by using a cam and follower transmission to decouple the leaf spring mechanics from the mechanics of the ankle joint. The cam profile determines the mechanics of the ankle joint, whereas the stiffness of the spring determines the energy stored for a particular deflection. Despite achieving this control of ankle mechanics, the VSPA still acts spring-like. This means that energy is stored and released in the exact same manner. The purpose of this study was to decouple the energy storage and release characteristics. A prototype was build that uses two cam-profiles in order to do so. These cam profiles can differ in the way they store and release energy, as long as the total energy stored or released is at most equal (thereby not violating the laws of thermodynamics). By using multiple cam profiles, energy can be stored in the initial part of the stance phase. Rather than returning this energy instantaneously, it is released during late stance to enhance the push-off. This is a continuous approach to the energy recycling concept originally proposed by Collins and Kuo (2010). Implementing the energy recycling concept in the VSPA foot redesign removed the hysteresis losses found whilst using a single cam profile and showed net positive work done during the push-off phase.