A Self-Aligned Method of IMU-Based 3-DoF Lower-Limb Joint Angle Estimation

Inertial measurement unit (IMU)-based lower-limb joint angle estimation (JAE) that enables wearable and real-time monitoring of kinematic and biomechanical states has been studied for decades. However, 3-degree-of-freedom (DoF) JAE of lower-limb joints still requires cumbersome calibration procedures. In this article, we propose a self-aligned method by formulating a universal 3-DoF kinematic constraint and solving it by dividing it into three subproblems. Particularly, we divide the issue of optimizing the 3-DoF kinematic constraint by: 1) estimating the main axis following a widely adopted hinge-joint assumption; 2) unifying the reference frames according to a comprehensive metric; and 3) estimating the rest two joint axes. By estimating the coordinates of the three rotational axes of the 3-DoF joint, we further estimate the 3-DoF joint angles by decoupling the joint's rotations into rotations around the axes. We evaluate our method by accuracy comparison with other methods, test-retest reliability, and ablation experiments. The performance demonstrates that the RMSEs of our method are below 2.9 degrees for hip and knee, below 2.6 degrees for ankle, which are comparable with other methods and able to estimate 3-DoF angles in a self-aligned manner. The agreement between test and retest is good, and the ablation experiments demonstrate the effectiveness of sequentially solving the three subproblems. The proposed method is feasible for self-aligned JAE of 3-DoF lower-limb joints. This study may aid the wider application of IMU-based JAE in field conditions and provide a powerful component for wearable human intent decoding.