The Human Ankle-Foot Complex as a Multi-Configurable Mechanism during the Stance Phase of Walking

The objective of this study is to investigate the biomechanical functions of the human ankle-foot complex during the stance phase of walking. The three-dimensional (3D) gait measurement was conducted by using a 3D infrared multi-camera system and a force plate array to record the Ground Reaction Forces (GRF) and segmental motions simultaneously. The ankle-foot complex was modelled as a four-segment system, connected by three joints: talocrural joint, sub-talar joint and metatarsophalangeal joint. The subject-specific joint orientations and locations were determined using a functional joint method based on the particle swarm optimisation algorithm. The GRF moment arms and joint moments acting around the talocrural and sub-talar joints were calculated over the entire stance phase. The estimated talocrural and sub-talar joint locations show noticeable obliquity. The kinematic and kinetic results strongly suggest that the human ankle-foot complex works as a mechanical mechanism with two different configurations in stance phase of walking. These lead to a significant decrease in the GRF moment arms thereby increasing the effective mechanical advantages of the ankle plantarflexor muscles. This reconfigurable mechanism enhances muscle effectiveness during locomotion by modulating the gear ratio of the ankle plantarflexor muscles in stance. This study also reveals many factors may contribute to the locomotor function of the human ankle-foot complex, which include not only its re-configurable structure, but also its obliquely arranged joints, the characteristic heel-to-toe Centre of Pressure (CoP) motion and also the medially acting GRF pattern. Although the human ankle-foot structure is immensely complex, it seems that its configuration and each constitutive component are well tuned to maximise locomotor efficiency and also to minimise risk of injury. This result would advance our understanding of the locomotor function of the ankle-foot complex, and also the intrinsic design of the ankle-foot musculoskeletal structure. Moreover, this may also provide implications for the design of bionic prosthetic devices and the development of humanoid robots.