Imagine standing on a tightrope, delicately teetering to maintain your balance. Just as a tightrope walker uses a combination of small ankle and hip movements to stay steady, our bodies employ similar strategies even when we’re standing on solid ground. This study delves into the intricate dance between our ankle and hip joints during upright standing. By developing a hybrid control model, researchers reveal that our bodies rely on an intermittent control system to stabilize ourselves. This system consists of two components: an active component applied to the ankle joint, which uses sensory feedback to adjust our position, and a passive component applied to the hip joint that helps maintain stiffness. Surprisingly, it seems that a single active component at the ankle joint is enough to explain the coordinated movement of both joints. The simulations conducted in this study suggest that sticking with a pure ankle strategy is more robust and energy efficient than using a mixed strategy that combines both ankle and hip movements. However, in certain situations where pure strategies become unstable, employing a stable mixed strategy can be advantageous. To grasp the intricacies of this elegant stabilization dance, dive into the full research paper!

Even in unperturbed upright standing of healthy young adults, body sway involves concurrent oscillations of ankle and hip joints, thus suggesting to using biomechanical models with at least two degrees of freedom, namely, a double inverted pendulum (DIP) framework. However, in a previous study, it was demonstrated that the observed coordinated ankle–hip patterns do not necessarily require the independent active control of the two joints but can be explained by a simpler hybrid control system, with a single active component (intermittent, delayed sensory feedback of the ongoing sway) applied to the ankle joint and a passive component (stiffness control) applied to the hip joint. In particular, the proposed active component was based on the internal representation of a virtual inverted pendulum (VIP) that links the ankle to the current position of the global center of mass (CoM). This hybrid control system, which can also be described as an ankle strategy, is consistent with the known kinematics of the DIP and, in particular, with the anti-phase correlation of the acceleration profiles of the two joints. The purpose of this study is to extend the hybrid control model in order to apply to both the ankle and hip strategy, clarifying as well the rationale of mixed strategies. The extension consists of applying the hybrid control scheme to both joints: a passive stiffness component and an active intermittent component, based on the same feedback signals derived from the common VIP but with independent parameter gains for the two joints. Thus, the hip gains are null in the pure ankle strategy, the ankle gains are null in the pure hip strategy, and both ankle and hip gains are specifically tuned in mixed strategies. The simulation of such an extended model shows that it can reproduce both strategies; moreover, the pure ankle strategy is more robust than the hip strategy, because the range of variation (RoV) of the intermittent control gains is larger in the former case than in the latter, and the pure ankle strategy is also more energy efficient. Generally, the simulations suggest that there is no advantage to employ mixed strategies, except in borderline situations in which the control gains are just outside the RoV that provides stable control for either pure strategy: in this case, a stable mixed strategy can emerge from the combination of two marginally unstable pure strategies.

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