Design of a Two-Degree-of-Freedom Ankle-Foot Prosthesis Mechanism with Reduced Transmission Complexity and Unloaded Kinematic Decoupling Validation

Powered two-degree-of-freedom (2-DOF) ankle-foot prostheses have distinct requirements for dorsiflexion/plantarflexion (DP) and inversion/eversion (IE) actuation. However, current designs rely on mechanically coupled DP and IE actuation with complex transmission stages. Thus, we present a proof-of-concept 2-DOF ankle-foot mechanism with unloaded kinematic validation using two simple and decoupled transmission stages. A ball-screw mechanism drives DP and a gear-and-chain mechanism drives IE, each constituting a single transmission stage. A weighted mechanical complexity metric benchmarks this proof-of-concept architecture against five existing 2-DOF designs. Kinematic tracking is validated in Simscape Multibody simulation against gait data, and axis decoupling is experimentally confirmed using a 10-camera infrared motion capture system. The proposed design uses only 2 transmission stages instead of the 4 to 6 stages found in other alternatives. Experimental validation yields a DP-IE Pearson correlation coefficient of −0.018, confirming near-complete mechanical decoupling. The design achieves a DP range of −28.9° to 12.0° and an IE range of −22.5° to 22.5°, meeting daily-living motion requirements for level-ground walking. The unloaded kinematic decoupling validations provide a basis for further development of 2-DOF powered ankle-foot prostheses implementing our design with reduced transmission complexity.