Force-Feedback Coordinates Stance and Balance Locomotor Subfunctions in Hopping

Simplifying legged locomotion through abstraction and splitting it to subproblems (locomotor subfunctions) pioneered by Raibert hopper is addressed in this paper. Using neuromechanical models, we focus on coordination between stance and balance locomotor subfunctions in a vertical hopping task. Stance and balance control relate to the axial and perpendicular terms of the ground reaction force (GRF), respectively. We show that using springs for knee extensor muscle (VAS) and biarticular thigh muscles (RF and HAM) with appropriate lever arm ratios, these two elements of GRF can be controlled, separately. Therefore, proper mechanical design of these muscles can generate repetitive hopping. However, for synchronization between them - especially when facing perturbations - additional sensory information is required. We depict that leg force feedback in the context of force modulated compliant hip (FMCH) improves stability and robustness of hopping against perturbations in addition to generating more rhythmic hopping patterns.