This study investigates the reliability of Xsens-based lower extremity joint angles during unconstrained running on stable (asphalt) and unstable (woodchip) surfaces over five testing days. The researchers determined the within-day and between-day intraclass correlation coefficients (ICCs) and minimal detectable changes (MDCs) for discrete ankle, knee, and hip joint angles. The results show that Xsens-based joint angles generally exhibit good to excellent reliability within the same day (median ICCs > 0.9), but between-day reliability is generally lower (median ICCs < 0.9). Initial hip, knee, and ankle angles in the sagittal plane show good reliability, while ankle and hip angles in the frontal plane show poor to moderate reliability. The reliability is largely unaffected by the surface type. The study concludes that the Xsens Link system can capture within-day adaptations in lower-extremity running kinematics but suggests caution when trying to capture longitudinal adaptations, especially for ankle and hip joint angles in the frontal plane.This study investigates the reliability of Xsens-based lower extremity joint angles during unconstrained running on stable (asphalt) and unstable (woodchip) surfaces over five testing days. The researchers determined the within-day and between-day intraclass correlation coefficients (ICCs) and minimal detectable changes (MDCs) for discrete ankle, knee, and hip joint angles. The results show that Xsens-based joint angles generally exhibit good to excellent reliability within the same day (median ICCs > 0.9), but between-day reliability is generally lower (median ICCs < 0.9). Initial hip, knee, and ankle angles in the sagittal plane show good reliability, while ankle and hip angles in the frontal plane show poor to moderate reliability. The reliability is largely unaffected by the surface type. The study concludes that the Xsens Link system can capture within-day adaptations in lower-extremity running kinematics but suggests caution when trying to capture longitudinal adaptations, especially for ankle and hip joint angles in the frontal plane.