Concurrent Validity and Reliability Between Noraxon Ultium™ IMU and Vicon OMC for Lower Limb Gait Assessment Across Variable Walking Speeds.

Background: Wearable inertial measurement units (IMoCAPs) are increasingly used in clinical gait analysis due to their portability and ability to capture data outside laboratory settings; however, validation across operating conditions is essential. Objective: To evaluate the concurrent validity and reliability of the Noraxon Ultium™ IMU system against a Vicon optical motion capture (OMC) system for lower-limb kinematics during walking across different speeds and time intervals. Methods: Ten healthy adults performed overground walking at slow, normal, and fast self-selected speeds. Kinematics were recorded simultaneously using both systems. Discrete variables (Max, Min, ROM) were analyzed using three-factor repeated-measures ANOVA (Device × Speed × Time, p < 0.05). Agreement was assessed using Bland-Altman analysis, RMSE, and ICC (3,1), and waveform differences were evaluated using Statistical Parametric Mapping (SPM). Results: Time effects were minimal across all planes. Sagittal-plane kinematics showed strong agreement, with small biases (<3°), low RMSE (≤2.5°), and moderate reliability (ICC = 0.65-0.74). Both systems detected increased hip and knee motion with speed, although Device × Speed interactions indicated greater IMoCAP underestimation at higher speeds. Frontal-plane agreement was poor to moderate (RMSE: 1-3°, ICC: 0.42-0.74). Transverse-plane kinematics demonstrated the largest discrepancies (RMSE up to 6-7°, ICC: 0.17-0.26), particularly for hip rotation. SPM revealed significant waveform differences across all planes. Conclusions: The Noraxon Ultium™ IMU provides valid sagittal-plane gait assessment, moderate frontal-plane agreement, and limited reliability for transverse-plane kinematics, requiring cautious interpretation at higher speeds.