Ambulatory measurement of elbow kinematics using inertial measurement units

Using inertial measurement unit (IMU) to measure human body kinematics has gained popularity because of its low-cost and ease of handling, compared to optoelectronic and electromagnetic systems. However, its usage has to be supported by post-processing protocols that integrate the data with reliable kinematic model to improve the accuracy. In this paper, a method to calculate the rotation axes and angles of the elbow joint from IMU data is presented. The method makes use of product of exponential (POE) representation and an optimization process to decompose the rotation matrix into angles along the two rotation axes without introducing the carrying angle and assuming orthogonality of the two axes. Using the method, the estimated errors of the calculated axes of rotation are comparable to published results, and the rotation angles yield an orientational deviation of less than 1.5 degrees.