Quiet standing: a simple motor task but a hard modeling challenge

Maintaining quiet balance is a simple yet indispensable motor task that is crucial for human health and well-being, as it is essential in everyday activities. The loss of balance ability-due to aging and traumatic or degenerative disorders-is associated with an increased risk of falls and injuries. Therefore, it is critical to identify the underlying neural control mechanisms of healthy and impaired posture and to develop better and more targeted ways to predict, prevent, and treat postural disorders. After providing context on relevant biomechanical and neurophysiological aspects of quiet stance, this review offers a detailed analysis of the different computational models used in existing literature to explain unimpaired and impaired quiet balance. We identify six key aspects of modeling upright stance which are currently discussed in the literature, and we delve into the different neuromotor control hypotheses that have been proposed. We critically discuss the reviewed findings and provide our perspective on how upright and unperturbed posture may be an independent neuromotor control primitive for human motion, or in this case, non-motion. Furthermore, we highlight the need for continued investigation into impaired balance models to advance rehabilitation, enhance assistive robotics, and improve the prediction and mitigation of balance impairments. In conclusion, our review uniquely synthesizes the modeling challenges of quiet stance and provides a critical overview of the most debated controversies in this field. By analyzing such controversies, the review establishes a reference point for future research efforts aimed to clarify and further comprehend upright unperturbed stance.