Design and validation of an ambulatory inertial system for 3-D measurements of low back movements

In studies of human movement, inertial sensors (accelerometers and gyroscopes) are gaining attention as a promising alternative to laboratory-constrained video capture systems. Kinematics of various body parts and joints can be quantified by attaching inertial sensors at points of interest and integrating the observed acceleration and angular velocity signals. It is broadly accepted that this measurement procedure is significantly influenced by cumulative errors arising from sensor noise, non-linearities, asymmetries, sensitivity variations and bias drifts. In addition, it is also known that linear acceleration superimposed to the gravity acceleration introduces errors when calculating tilt angles. Recently, newer techniques using sensor fusion methods have shown error reduction in orientation measurements, but require additional hardware and consume more energy. In this paper, we assess the accuracy of a low-power wireless inertial system (ViMove) that measures Low Back (lumbar spine) orientation in three dimensions. The system consists of two inertial units (sensor), with each sensor containing one tri-axis accelerometer and one single-axis gyroscope. We investigate the accuracy of 1D, 2D and 3D simultaneous movements by means of root mean square error (RMSE) computed in comparison with NDI Optotrak, an optical tracking system. The RMSE achieved for one dimensional movements in the Flexion, Lateral Flexion and Twist planes were 1.0°, 0.5° and 2.4° respectively, and 2.1°, 2.4° and 4.6° for three dimensional movements.