Standing up using one leg is a challenging task for those with a transfemoral amputation, particularly for elderly users with a low activity level. Active prostheses are generally not accessible to this group and available passive prostheses do not support standing up. This article presents the design and evaluation of the “Energy Restoring Intelligent Knee” (ERiK), which stores energy during sit-down in a pneumatic cylinder and returns it during stand-up. We hypothesized that the system would reduce the time needed to perform transitions and also enable higher load sharing by the prosthetic leg. However, the results of an experimental study with seven participants with transfemoral amputation contradict these hypotheses: the participants could neither move faster nor make more use of the prosthetic leg to share their body weight during transitions. We observed that a major obstacle to the useful functionality of the leg was the absence of ankle dorsiflexion – the foot tended to slip during stand-up initiation, such that only low pre-pressures and therefore support levels could be set. The rather binary action of the pneumatics also complicated movement initiation. The lessons learned from this study may be helpful to those seeking to create better designs in the future.@en

The Sit-to-Stand (SitTS) and Stand-to-Sit (StandTS) transitions are among the most demanding activities of daily living in terms of required muscle strength. Many elderly struggle with the transitions, which become even harder after a transfemoral amputation. The current state of the art in knee prostheses does not supply any support to reduce the required muscle strength. A new, energy-recovering ankle-knee prosthesis was designed to provide support during SitTS and StandTS, with a natural torque profile. The prosthesis uses pneumatic cylinders to store energy during StandTS, and recover it during SitTS. To make the use of the prosthesis as intuitive as possible, the design was optimized to mimic natural torque profiles during the transition. The addition of an ankle joint enables a more posterior placement of the prosthetic limb, which should reduce the required knee torque. The prototype provides a peak extending knee torque of 21.7 Nm and a peak plantarflexing ankle torque of 10.8 Nm during SitTS, which can be further increased by increasing the pre-pressure in the system. It is concluded that the current prototype has the potential to provide support during the SitTS and StandTS transitions, but that alterations are required to improve the available joint torques. The next phase will be user tests with transfemoral amputees.