Fig. 2

MACCEPA Ankle Schematic. Schematic of a MACCEPA using rigid linkages. The main motor drives the moment arm (b) around the ankle joint (a). The moment arm is displaced by an angle α with respect to the spring axis, which is defined as the neutral axis of the actuator. This displacement compresses the main MACCEPA spring (k) along the foot shaft (C), creating an ankle torque. The parallel spring (K _parallel ) is engaged during dorsiflexion, but is not in contact during plantarflexion. The pretension of the ankle (P) was constant throughout the competition. Note that α is a combination of the output angle and the moment arm angle ϕ, which is not influenced by the ankle output