The ankle plays an important role in human walking. Biomechanical studies show that the ankle generates more power than hip and knee in walking [1]. Without the power generated by the ankle, transtibial (TT, also known as below knee) amputees fitted with the traditional passive prostheses suffer from multiple problems, e.g., asymmetric gait and greater metabolic energy expenditure.

Motivated by this reality, researchers have expended a significant amount of efforts in creating energetically active (i.e., powered) TT prostheses. Typical examples include the multiple prototypes developed by the MIT Biomechatronics Group (e.g., Ref. [2]), the SPARKy prosthesis, and the Vanderbilt transtibial prosthesis. These prostheses are all powered with electric motors, augmented with mechanical springs to simulate the elastic characteristics of the biological ankle. For example, the prosthesis design by Au et al. uses springs in series with the electric motor to form a series elastic actuator, and...

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