Design and validation of a foot-ankle dynamic simulator with a 6-degree-of-freedom parallel mechanism

An in vitro simulation test using a designed well-targeted test rig has been regarded as an effective way to understand the kinematics and dynamics of the foot and ankle complex in the dynamic stance phase, and it also allows alterations in both internal and external control compared to in vivo tests. However, current simulators are limited by some assumptions. In this study, a novel foot and ankle bionic dynamic simulator was developed and validated. A movable 6-degree-of-freedom parallel mechanism, known as Steward platform, was used as the core structure to drive the tibia, with a tibial force actuator applied with different loads. Four major muscle groups were actuated by four sensored pulling cables connected to muscle tendons. Simulation processes were controlled using a software developed based on a proportional-integral-derivative control loop, with tension-compression sensors mounted on tendon pulling cables and used as real-time monitor signals. An iterative learning module for tibial force control was integrated into the control software. Six specimens of the cadaveric foot-ankle were used to validate the simulator. The stance phase was successfully simulated within 5 s, and the tibia loads were applied based on the body weight of the cadaveric specimen donors. Typical three-dimensional ground reaction forces were successfully reproduced. The coefficient of multiple correlation analysis demonstrated good repeatability of the dynamic simulator for the ground reaction force (coefficient of multiple correlation > 0.89) and the range of ankle motion (coefficient of multiple correlation > 0.87 with only one exception). The simulated ranges of the foot-ankle joint rotation in stance were consistent with in vivo measurements, indicating the success of the dynamic simulation process. The proposed dynamic simulator can enhance the understanding of the mechanism of the foot-ankle movement, related injury prevention, and surgical intervention.