An inertial sensor system for measurements of tibia angle with applications to knee valgus/varus detection

Accurate measurement of knee motion during dynamic movements is the key to detect and highlight deficiencies in peripheral muscles and ligaments of the knee and hence to predict the risk of injury. Miniature inertial sensors are increasingly becoming a viable option for human movement measurement, given their small size, low cost and relatively good accuracy compared with traditional optical measurements. A system capable of measuring tibia angle using a shank mounted wireless inertial sensor is proposed. The system employs a simple setup with only one skin-mounted triaxial accelerometer and gyroscope module attached to the tibia segment, and an algorithm to estimate the tibia angle. The accuracy of the system was assessed by an optical tracking system (Optotrak Certus) during dynamic movements performed by three subjects by evaluating Root-Mean-Square Error (RMSE) of tibia-flexion and tibia-adduction angles over the period of motion. We achieve an RMSE of 1.6±1.1 and2.5±1.6 degrees in tibia-flexion and tibia-adduction angles, respectively. It is argued that tibia angle can be reliably used to detect valgus or varus movement of the knee and hence the proposed system provides a simple and useful assessment tool for performance enhancement and rehabilitation.