A system and method are provided for measuring body joint range of motion using inertial measurement unit (IMU) sensors capable of measuring their orientations relative to Earth. The method mounts a primary IMU sensor on a proximal body segment, and a secondary IMU sensor on a distal body segment connected to the proximal body segment via a joint. The sensor orientation relationship is measured between the primary IMU sensor and the secondary IMU sensor. In one aspect, the sensor orientation relationship is expressed as a quaternion. The sensor orientation relationship is converted into at least one physiologically modeled constituent joint rotation relationship between the proximal body segment and the distal body segment. More explicitly, the sensor orientation relationship is converted to a constituent axial axis, composite radial, or primary and secondary radial rotations of the distal body segment measured with respect to the proximal body segment.
