A system and method is provided for determining the orientation of a body segment using an inertial measurement unit (IMU) sensor capable of measuring its orientation relative to Earth. In general, the method mounts a primary IMU sensor on a first body segment, with an unknown first alignment orientation relationship between the primary IMU sensor and the first body segment. A primary IMU sensor orientation is measured, and an alignment orientation relationship is calculated between the primary IMU sensor orientation and a first body segment orientation. The method may also measure a primary IMU sensor initial orientation and a subsequent orientation. As a result, a subsequent orientation of the first body segment is determined based upon the primary IMU sensor initial and subsequent orientations, as well as the calculation of the alignment orientation relationship between the primary IMU sensor initial orientation and the first body segment orientation.
