Impact of Physical Fatigue on Lower-Limb Inter-Joint Coordination During Overground and Treadmill Walking

Background: Walking relies on synchronized joint actions guided by the central nervous system and sensorimotor circuits. Physical fatigue can alter these functions and affect walking performance, but its impact on inter-joint coordination is poorly understood. Research question: How does physical fatigue affect inter-joint coordination under different walking conditions? Methods: Twelve young adults performed a progressively inclined treadmill walking to induce physical fatigue, monitored by respiratory exchange ratio, heart rate, and perceived exertion. Sagittal plane joint kinematics of the right leg was analyzed pre- and post-fatigue during overground walking under preferred and fast speeds and treadmill walking. Inter-joint coordination was assessed using continuous relative phase metrics: mean absolute relative phase (MARP), deviation phase (DP), and cross-correlation coefficient. A two-way repeated measures ANOVA evaluated the effects of fatigue and walking conditions. Results: For hip–knee coordination, MARP and DP differed significantly across walking conditions during the stance phase ([Formula: see text]), with fatigue-related changes in MARP observed in fast walking. Cross-correlation coefficients exceeded 0.79 in both the stance and swing phases. For knee–ankle coordination, MARP varied significantly across walking conditions ([Formula: see text]) and fatigue states ([Formula: see text]), with cross-correlation coefficients also differing across conditions ([Formula: see text]). Significance: Physical fatigue reduces inter-joint coordination variability during walking, potentially increasing the risk of lower limb injuries but may be serving as an adaptive strategy for complex tasks like fast walking. Variability differs across gait phases and joints due to their functional roles. Treadmill walking may mask natural variability, warranting careful data interpretation.