A Bionic Ankle-Foot Prosthesis with Active Metatarsophalangeal Joint Mimicking the Toe-Gripping Function of Biological Limb

This study proposes a novel ankle-foot prosthesis design with an active metatarsophalangeal joint (MTPJ), which effectively mimicking the toe-gripping function of a biological foot. The design employs an underactuated mechanism that allows for the coupled translational and rotational motion of the toe, driven by a single linear motor. We developed a kinematic model of the mechanism and used finite element analysis to simulate the ground gripping force (i.e., horizontal shear force). To validate the effectiveness of the design, we conducted experimental studies on the motion patterns and plantar shear force underneath human toes during perturbating conditions, and compared the experimental results with those of the finite element analysis. The results indicate that our bionic ankle-foot prosthesis can effectively reproduce the ground-gripping function of the biological toes, generating a ground-gripping shear force comparable to that of biological toes. Moreover, we conducted a detailed analysis of the kinematic model of the prosthesis, setting the dimensions of the linkage mechanism according to the proportions and range of motion of biological toes. Under perturbation conditions, the maximum shear force generated by biological toes is $2.1 \%$ of body weight (BW). The finite element analysis results show that our bionic ankle-foot prosthesis can maintain ground grip during static standing, generating a shear force up to 45.47 N, outperforming that of existing ankle-foot prostheses (R-MTPJ: 6.06 N). These findings further validate the effectiveness of prosthesis with toe-gripping function, offering a new approach for improved ankle-foot prosthesis design.