Human movement variability, nonlinear dynamics, and pathology: Is there a connection? Nicholas Stergiou and Leslie M. Decker review the role of variability in human movement and its implications for health and disease. Variability is not merely error but a key feature of healthy and functional movement, characterized by a chaotic structure. Deviations from this state can lead to overly rigid or unstable systems. The review discusses three main theories: the Generalized Motor Program Theory (GMPT), the Uncontrolled Manifold (UCM) hypothesis, and Dynamical Systems Theory (DST). These theories suggest that variability is essential for adaptability and that reduced variability may indicate rigidity. However, some behaviors appear stable yet variable, indicating that variability is not always linked to instability. The study highlights that variability in movement is not simply noise but has a deterministic structure. Nonlinear measures, such as approximate entropy and largest Lyapunov exponent, are used to analyze variability and provide insights into movement patterns. The review also discusses the importance of variability in motor learning and health, suggesting that optimal variability is associated with healthy motor performance. The findings indicate that variability is crucial for adaptability and that deviations from optimal variability may indicate pathology. The study emphasizes the need for a more nuanced understanding of variability in human movement, moving beyond traditional linear measures to incorporate nonlinear dynamics. The research also highlights the importance of variability in motor development and rehabilitation, suggesting that therapeutic interventions should aim to enhance variability to improve functional mobility and health. The review concludes that variability is a fundamental aspect of human movement and that understanding its role is essential for the development of effective therapeutic strategies.Human movement variability, nonlinear dynamics, and pathology: Is there a connection? Nicholas Stergiou and Leslie M. Decker review the role of variability in human movement and its implications for health and disease. Variability is not merely error but a key feature of healthy and functional movement, characterized by a chaotic structure. Deviations from this state can lead to overly rigid or unstable systems. The review discusses three main theories: the Generalized Motor Program Theory (GMPT), the Uncontrolled Manifold (UCM) hypothesis, and Dynamical Systems Theory (DST). These theories suggest that variability is essential for adaptability and that reduced variability may indicate rigidity. However, some behaviors appear stable yet variable, indicating that variability is not always linked to instability. The study highlights that variability in movement is not simply noise but has a deterministic structure. Nonlinear measures, such as approximate entropy and largest Lyapunov exponent, are used to analyze variability and provide insights into movement patterns. The review also discusses the importance of variability in motor learning and health, suggesting that optimal variability is associated with healthy motor performance. The findings indicate that variability is crucial for adaptability and that deviations from optimal variability may indicate pathology. The study emphasizes the need for a more nuanced understanding of variability in human movement, moving beyond traditional linear measures to incorporate nonlinear dynamics. The research also highlights the importance of variability in motor development and rehabilitation, suggesting that therapeutic interventions should aim to enhance variability to improve functional mobility and health. The review concludes that variability is a fundamental aspect of human movement and that understanding its role is essential for the development of effective therapeutic strategies.