The Ankle Mimicking Prosthetic (AMP-) Foot 2.0

In general, today's prosthetic feet can be classified into three categories: Conventional feet (CF), "Energy-storing-and-returning" (ESR) feet and bionic feet.

The most common conventional feet are probably the SACH-foot, or Solid Ankle Cushion Heel, and the uni-axial foot.

ESR feet, compared to CF feet, are capable of storing energy in elastic elements and returning the major part of it to assist in forward propulsion. Hereby the push-off is improved and thus moving forward is made easier for the amputee. Examples of the first ESR feet are the Seatle foot and the Jaipur foot. Thanks to better knowledge and understanding of the human gait and biomechanics, new types of ESR prostheses were developed as the Flex-foot, the Springlite foot and the VariFlex to name a few.

To increase the amount of energy returned for propulsion, researchers have developed the Ankle Mimicking Prosthetic Foot (AMP-Foot 1.0), an articulated ESR-type foot using locking mechanisms to store harvested energy during the dorsiflexion (DF) phase of stance, and to release it at push-off. To improve even more the push-off properties of passive prostheses, Collins et al. have developed the so-called Controlled Energy Storing and Returning foot (CESR foot). While storing energy during stance, the CESR foot uses also the weight of the body at initial contact to store energy and releases it when needed.

Still on research level are the so-called bionic feet. By using an actuator (pneumatic or electric) to inject energy in the system for forward propulsion, gait is improved and the metabolic cost of the amputee is decreased. Examples are the Sparky project and the MIT powered ankle-foot prosthesis.

One can conclude that passive energy storing devices (ESR feet) are energetically efficient but do not provide the extra power needed for propulsion during walking. On the other hand, actuated devices are able to provide the necessary energy, but need heavy and bulky actuators capable of producing high torques in small periods of time.

With the AMP-Foot 2.0, the authors propose a new design of an energy efficient, powered transtibial prosthesis. Its design enables the actuator to work at low power during a much longer time period while energy is stored in springs and released when needed. Thanks to this, the size and weight of the actuator can be decreased considerably while still providing the full power necessary for forward propulsion.