Ankle prostheses are limb replacement solutions that provide functional performance for transtibial (below-knee) amputees. An estimated 623,000 major transtibial amputations were performed in the U.S. in 2005 [1]. Having a functional limb replacement that the amputee feels physically and socially comfortable wearing significantly improves his or her quality of life [2]. Most commercially available ankle prostheses are passive devices that do not have actuators or batteries to provide powered push-off at the ankle. Without push-off at the ankle, the amputee relies primarily on his or her intact limb for propulsion. The natural ankle performs net rotational work, but the passive prosthesis is characterized by energy dissipation. This contrasting behavior produces asymmetric gait (walking pattern). Active prostheses with motors and batteries can produce powered push-off, but tend to be bulky and expensive. Collins and Kuo [3] developed a passive prosthesis that demonstrates push-off by storing...

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