Direction-specific response of shear traction forces generated underneath the hallux and lesser toes due to multi-directional perturbations applied in balanced standing

Existing experimental studies on human balance have primarily focused on biomechanical responses of major lower-limb joints, while the role of hallux and lesser toes of human foot in response to external perturbations has not been fully explored. Although toe grip strength may significantly influence balance performance, a more physiological-relevant toe grip evaluation, has not yet been established. This study investigates the biomechanical grip strength of the hallux and lesser toes during perturbations by quantifying the shear interactions (i.e., horizontal traction forces) at the foot–ground interface. A robotic platform with an instrumented multi-axial force platform was employed to analyze the involvement of the hallux and lesser toes in maintaining standing balance due to random ground perturbations. Our results indicate that hallux and lesser toes demonstrated significant direction-specific shear responses, and the proportion of shear traction force in the hallux and lesser toes significantly increased, particularly for perturbations in the posterior half-plane (p < 0.05). A substantial increase in shear traction force underneath the lesser toes was observed during contralateral perturbation events (p < 0.05). Importantly, the lesser toes could generate substantial shear traction forces by enhancing griping following perturbation, a capability not shown in other foot regions. This study introduced a novel approach for precisely quantifying the grip functions of individual toes at in-vivo perturbing conditions. The information provided is envisaged to have important implications on improved interventions for posture and balance rehabilitation.