This study evaluates the reliability of Xsens IMU-based lower extremity joint angles during in-field running on stable (asphalt) and unstable (woodchip) surfaces. The research involved 17 recreational runners (8 female, 9 male) who participated in five testing days. The study determined within-day and between-day intraclass correlation coefficients (ICCs) and minimal detectable changes (MDCs) for discrete ankle, knee, and hip joint angles. Results showed good to excellent within-day reliability for most joint angles, with median ICCs > 0.9. Between-day reliability was generally lower, with initial hip, knee, and ankle angles in the sagittal plane showing good reliability (median ICCs > 0.88), while ankle and hip angles in the frontal plane showed poor to moderate reliability (median ICCs 0.38–0.83). Surface type had little effect on reliability. The study found that within-day adaptations in lower-extremity running kinematics can be captured with the Xsens Link system. However, between-day reliability suggests caution when trying to capture longitudinal adaptations, particularly for ankle and hip joint angles in the frontal plane. The reliability of Xsens-based joint angle measurements during running is less well understood. While some studies have investigated the validity and reliability of Xsens-based full-body kinematics during walking, running, or other functional tasks, many open questions remain. The study aimed to determine the between-day, within-day, and calibration reliability of discrete hip, knee, and ankle joint angles in the sagittal and frontal planes during running at a self-selected speed on stable and unstable surfaces. The study also investigated potential sources of between-day variations in Xsens-based joint angles, including running speed, stride frequency, and surface type. The results indicated that between-day variations in running speed and stride frequency were significant predictors of between-day variations in joint angles. The study concluded that the Xsens Link system is well suited to capturing within-day adaptations in lower-extremity movement in response to different running surfaces. However, for repeated measurements on different days, the system showed good to excellent between-day reliability for sagittal plane hip, knee, and ankle angles just before foot contact. Hip and ankle frontal plane angles showed poor to moderate between-day reliability, likely due to higher calibration errors. The study recommends caution when using the Xsens Link system for monitoring day-to-day changes in frontal plane hip and ankle angles. Potential ways to improve between-day reliability include controlling running speed, stride frequency, and standardizing the subject-specific calibration pose (N-Pose) between testing days.This study evaluates the reliability of Xsens IMU-based lower extremity joint angles during in-field running on stable (asphalt) and unstable (woodchip) surfaces. The research involved 17 recreational runners (8 female, 9 male) who participated in five testing days. The study determined within-day and between-day intraclass correlation coefficients (ICCs) and minimal detectable changes (MDCs) for discrete ankle, knee, and hip joint angles. Results showed good to excellent within-day reliability for most joint angles, with median ICCs > 0.9. Between-day reliability was generally lower, with initial hip, knee, and ankle angles in the sagittal plane showing good reliability (median ICCs > 0.88), while ankle and hip angles in the frontal plane showed poor to moderate reliability (median ICCs 0.38–0.83). Surface type had little effect on reliability. The study found that within-day adaptations in lower-extremity running kinematics can be captured with the Xsens Link system. However, between-day reliability suggests caution when trying to capture longitudinal adaptations, particularly for ankle and hip joint angles in the frontal plane. The reliability of Xsens-based joint angle measurements during running is less well understood. While some studies have investigated the validity and reliability of Xsens-based full-body kinematics during walking, running, or other functional tasks, many open questions remain. The study aimed to determine the between-day, within-day, and calibration reliability of discrete hip, knee, and ankle joint angles in the sagittal and frontal planes during running at a self-selected speed on stable and unstable surfaces. The study also investigated potential sources of between-day variations in Xsens-based joint angles, including running speed, stride frequency, and surface type. The results indicated that between-day variations in running speed and stride frequency were significant predictors of between-day variations in joint angles. The study concluded that the Xsens Link system is well suited to capturing within-day adaptations in lower-extremity movement in response to different running surfaces. However, for repeated measurements on different days, the system showed good to excellent between-day reliability for sagittal plane hip, knee, and ankle angles just before foot contact. Hip and ankle frontal plane angles showed poor to moderate between-day reliability, likely due to higher calibration errors. The study recommends caution when using the Xsens Link system for monitoring day-to-day changes in frontal plane hip and ankle angles. Potential ways to improve between-day reliability include controlling running speed, stride frequency, and standardizing the subject-specific calibration pose (N-Pose) between testing days.