Drift-free 3D orientation and displacement estimation for the lower extremities using independent inertial measurement units at various walking and running speeds

This study presents a modular adaptation of the Drift-Free 3D Orientation and Displacement (DFOD) estimation method for estimating lower-extremity kinematics using independent inertial measurement units (IMU) during steady-state walking and running without calibration procedures or biomechanical models. The adapted DFOD was evaluated in ten healthy recreational runners during walking (2.5 and 5 km/h) and running (9, 11, and 13 km/h), using optical motion capture as reference. Good accuracy was achieved for the feet and lower legs in the sagittal plane (orientation) and forward direction (displacement), with mean errors below 7.2 and 4.2 cm, respectively, and Pearson correlations above 0.97. Accuracy was lower in other movement directions and for the upper legs, with mean errors up to 12.8 and 6.5 cm. However, Pearson correlations for the upper legs in the sagittal plane (orientation) and forward direction (displacement) exceeded 0.87, suggesting that waveform characteristics can be captured. Statistical analysis confirmed that movement axis and body segment were the dominant factors explaining estimation accuracy, with speed having a significant but smaller effect. Overall, larger errors were observed for movements with smaller RoM, for the upper legs, and for the slowest walking speed (2.5 km/h). These findings indicate that the adapted DFOD provides promising single-IMU–based estimates of lower-extremity orientation and displacement during walking and running for distal segments and primary movement directions.