Age has a detrimental effect on balance control. While several contribute to the balancing tasks, the largest displacements usually occur at the ankle and hip. Thus, the estimation of ankle and hip parameters in quiet standing can be a useful tool when analyzing compensatory actions aimed at maintaining postural stability. This work presents an experimental study based on previous theoretical results where human quiet standing is modeled with a set of the physiological parameters controlling a second-order Kelvin-Voigt System. Human quiet standing was modeled as a double inverted pendulum subject to a step-like perturbation for the ankle joint. Estimates for the ankle and hip time-varying stiffness are obtained from RGB camera measurements and using a Kalman Filter. The measured apparent stiffness at the ankle switches from positive to negative as a function of the segments angular velocity. This possibly denotes a change from a passive to an active phase during stabilization. Our simulation results support previous works where switching strategies can stabilize the double inverted pendulum. Furthermore, this paper shows that the double pendulum model may be stabilized by changing only the ankle’s stiffness.

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