A Simple Minimum-Setup Pipeline for Using Leg-Worn Inertial Sensors to Track Knee Flexion: Validation on 10 Movements

Knee motion is a key biomarker in chronic musculoskeletal diseases, yet conventional in-lab optical motion capture falls short of identifying how knee motion continuously im-pacts joint health outside the lab. Inertial measurement unit (IMU) provides a clinically at-tractive approach for continuous real-world motion tracking. Our goal was to establish a clinically practical, minimum-setup pipeline for leg-worn IMUs to estimate knee flexion and determine its concurrent validity to optical motion capture during various knee movements. We recorded thigh and shank-worn IMU data with concurrent marker-based and markerless optical motion capture on 10 healthy adults, who performed 10 common movements including walking, running, and stair navigation. We combined IMU func-tional alignment with data fusion to estimate knee flexion during each movement and compared IMU-based estimate against both motion capture systems using Pearson corre-lation (Rxy) and root-mean-square difference (RMSD). IMU-estimated knee flexion strongly correlated with motion capture (Rxy ≥ 0.9). RMSDs were smaller for slower movements like walking (RMSD = 4.4°–6.0°) while larger during faster movements like running (RMSD = 5.4°–9.4°). Wearable IMUs track knee flexion with comparable results to motion capture during daily activities typical to older adults, highlighting their potential for continuous patient monitoring. Our simple pipeline makes IMU-based knee motion tracking more practical and compatible with clinical research. Future research should seek IMU wearing best practices to secure clinically meaningful data on real-world knee mobility.