Changes of postural feedback gain scaling with constrained somatosensory cues

Abstract

It is widely accepted that the postural balance of human beings is achieved by integrating information from the visual, vestibular and somatosensory systems. However, the role of each individual sensory system in that task of maintaining a balanced posture is not fully understood. From these sensory information, somatosensory signals are known to have an important role in human postural responses. To clarify the role of somatosensory information from the lower limbs of humans in postural response we applied external perturbation to the subject in two different conditions; control condition and constraint condition. In control condition, subjects stood barefoot on a firm supporting surface, a servo-controlled dynamic platform, which can translate in posterior horizontal direction and in constraint condition we inserted a layer of foam between sole and hard platform surface to make a compliant surface. Magnitude of applied backward translation was varied from 3 cm to 15 cm to test the subjects’ ability to scale their postural responses to the magnitude of the translation. 5 healthy volunteers (4 men, 1 woman) aged 25-29 years were volunteered in this research. We set the scaling of feedback control gain as the criterion to estimate the changes of human postural responses induced by constrained somatosensory cues. Human body, in sagittal-plane, was modeled as 2-segment inverted pendulum and linear full-state feedback control model was used to analyze the human postural balance. Joint torque was calculated using redundant least square method and optimization method was used to find best-fit feedback control gains for each trial. The results showed that joint angle gains were scaled with perturbation magnitude both in control condition and constraint condition. Ankle gains scaled downward and hip gains scaled slightly downward as the perturbation magnitude increases. In constrained somatosensory condition, the gain slope of ankle joint was steeper than controlled...