Natural human ankle motion includes dorsiflexion/plantarflexion (the major contributor to gait) as well as smaller contributions from inversion/eversion and abduction/adduction motions. Many of the commercially available passive prostheses for amputees are either stiff in all these motion directions or are compliant mainly in the dorsiflexion/plantarflexion direction. This can make it difficult for amputees to walk on uneven or sloped surfaces, and leads to increased risk of falling [1–4].

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