Development of a passive knee mechanism that realizes level walk and stair ascent functions for transfemoral prosthesis

Passive mechanisms have been well developed to realize the safe level walk for transfemoral prosthesis users, preventing unintended knee flexion during prosthetic stance phase. However, for stair ascent, a mechanism had not existed for a long time. We previously developed the knee joint unit whose mechanism realized prosthetic knee extension using load or external force on the knee joint unit itself. Although the level walk and stair ascent functions have been accomplished with different mechanisms, these functions should be combined into one mechanism for the prosthetic knee to contribute to regaining locomotion capacities. Therefore, the purpose of the present study was to develop a passive knee mechanism that realized both functions. We proposed a passive mechanism combining the pre-existing mechanisms. The proposed knee was designed to function as a four-link mechanism that prevented unintended knee flexion during level walking. For stair ascending, it was designed to extend the knee, using load on the prosthetic leg when the prosthetic leg contacted a step of the staircase with the knee flexed. The gait experiment with the proposed knee on a transfemoral prosthesis showed successful level walking, stair ascending, and their transition without any assistive device such as the use of a handrail. According to the experimental results, the functions worked and switched appropriately during the transition from level walk to stair ascent, depending on gait conditions without manual handling.