Perturbation Recovery Time Identifies Subtle Human Balance Impairments and Features

Falls are a leading cause of injury and growing healthcare costs, particularly in aging populations. However, early-stage balance impairments often remain undetected until severe problems arise. Quantitative assessment of human balance therefore remains a critical challenge. Here, we introduce Perturbation Recovery Time, a novel state-space-based balance metric inspired by nonlinear dynamic system theory that quantifies the duration required for gait dynamics to consistently return to a steady-state neighborhood following a perturbation. Unlike conventional approaches based on steady-state walking, this framework evaluates balance through externally induced perturbations that reveal control mechanisms not observable during unperturbed gait. Using healthy participants with experimentally induced impairments, we identified key balance-related features, including the anterior-posterior center-of-mass-center-of-pressure distance, whole-body angular momentum in the frontal and sagittal planes, and vertical center-of-mass position and acceleration. These features capture the core mechanisms of balance control, including foot placement, inverted pendulum dynamics, push-off control, and trunk regulation. Perturbation Recovery Time reliably detected within-subject changes in balance associated with controlled impairments in sensory input, motor coordination, and movement consistency. These findings demonstrate that Perturbation Recovery Time provides a quantitative and sensitive measure of balance, offering a framework for early detection of balance impairments.