This paper presents a fully passive exoskeleton knee that resists knee buckling during the stance phase, encourages knee flexion to assist in toe clearance during the swing flexion phase, and allows free swing extension. This knee device utilizes a spring to store and release energy in different phases. It realizes a complex implementation with a simple architecture by using a four-bar linkage mechanism with the knowledge of the mechanism with variable topologies (MVTs). In this article, a fully passive knee prototype was designed, optimized, and built. The knee device is hypothesized to enable walking and assist toe clearance in exoskeleton walking. This article opens a paradigm of the passive knee devices that tackles the problem of low toe clearance, not only for implementation of the exoskeletons but also for orthoses and prostheses.

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