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A critical examination of three approaches for the design of passive ankle walking assist devices

Current ankle assist devices aim to improve the user's walking ability and stability. Despite the success of classical ankle orthoses and powered exoskeletons, these devices are still facing challenges that drive their continued development. This paper will first introduce the distinct types of ankle devices for walking assist. Then, three common approaches used in the design and development of these devices will be discussed and analyzed. These are inverse dynamics, the inverted pendulum model, and the design based on joint quasi-stiffness. The inverse dynamics approach simplifies all soft tissue contributions to a single force moment couple regardless of the source of the forces, thus making the prediction of metabolic cost very difficult. The inverted pendulum model is focused on the conservation of energy as the center of mass advances. It assumes rigid limb support and does not consider the ankle joint or the advancement of the center of pressure along the foot. The quasi-stiffness design approach attempts to translate the ankle stiffness into a combination of linear zones in order to be easily replicated with mechanical spring elements. The stiffness of the biological ankle is highly variable and dependent upon the speed of walking. Therefore devices using mechanical springs with non variable stiffness characteristics will only be effective within a narrow range of walking speeds. The current challenges faced by passive walking assist devices may be due to the methods used to assess the requirements of the body. If the parameters advised by these analytical approaches do not match the needs of the human body, the resulting devices will be inherently flawed.