Achieving Dynamic Balance in Lower-Limb Exoskeletons: A Survey of Principles and Practices

Abstract The lower-limb exoskeletons (LEEs) have the capability to change the entire dynamics of the restoration of mobility in people with neurological disorders besides being able to increase the performance of non-disabled people drastically. A major problem that comes with their application, however, is the requirement to make the users do the least possible voluntary postural corrections, and that is why maintaining dynamic balance is so important. The goal of this review is to put together the progress made in different areas in balance control for LEEs from the year 2015 up to 2025, with emphasis equally on biomechanical stability criteria and adaptive control architectures. We are going to look at the five major balance strategies: (i) static equilibrium via the support polygon and center of pressure (CoP), (ii) dynamic stability through the zero-moment point (ZMP), (iii) predictive fall prevention using the extrapolated center of mass (XCoM) and its velocity-scaled extension (PXCoM), (iv) whole-body coordination via centroidal angular momentum regulation, and (v) orbital stability through gait periodicity and limit-cycle dynamics–systematically. Besides these, we will also take into account how the “smart” systems of today combine user intent detection, closed-loop adaptation, and hybrid control to get the human-exoskeleton interaction to be really seamless. Also, trends that are being seen like the following ones are mentioned: human-in-the-loop optimization, multimodal sensing, and soft wearable platforms. We end by giving the key challenges and future directions: personalized stability margins, anticipatory terrain-aware control, and real-world generalization of learning-based strategies.