This paper proposes a redesign of a four-bar mechanism for an active transtibial prosthesis created by Bergelin 2010 and modified by Klein 2009. Bergelin utilized a four-bar mechanism, motor, and spring to match the prosthesis ankle moments to the ankle moments of a healthy ankle. Bergelin’s prosthesis did succeed in matching ankle moments closely, but with excessive motor energy expenditure when the prosthesis was in a neutral position. Klein proposed a redesign of the mechanism to change the motor-spring connection from parallel to series to eliminate the energy requirement when the device is in neutral position, which allowed for the application of impedance control of mechanism. This paper proposes a reoptimization of the series motor-spring mechanism configuration proposed by Klein, which further reduces the energy input configuration of the active prosthesis.