A Wearable Accelerometer-Based System for Knee Angle Monitoring During Physiotherapy

Wearable inertial sensors can revolutionize the field of physiotherapy by enabling continuous remote assessment, early detection of mobility issues, and monitoring of recovery from injury or surgery. This can be achieved by integrating these sensors with onboard processors and a communication technology. In this work, we propose a wearable system that uses accelerometer data to obtain a clinical knee angle. The system comprises sensors on the thigh and shank along with embedded firmware. A simple accelerometer-inclination method was used to compute the thigh and shank angles, and the knee angle was calculated from these leg segment angles. The experimental results demonstrate that the proposed approach is sufficient for measuring the knee angle, obviating the need for additional sensors. This reduction in computational complexity was achieved without compromising the accuracy of knee angle computation. Measurements were performed in clinically relevant positions (supine, sitting, standing, and during level walking) to validate the system, and the results were compared with a video-based reference system. The measurements were closely aligned with the reference system, as indicated by the average root mean square error (RMSE) of 1.71 degrees for the exercises and 2.51 degrees for the leveled walk. This is an acceptable accuracy because the minimal clinically important difference (MCID) for the knee angle is 6 degrees. However, it was demonstrated that sensor placement can affect the accuracy. This portable, cost-effective, noninvasive, and embedded system, capable of continuous assessment in free-living environments, would be advantageous for acquiring quantitative patient data.