Abstract: A sensor apparatus, system, and computer-usable non-transitory storage device storing program instructions for the measurement of the individual joint angles in dimensional space are disclosed. The joint angle measurements are measured in electrical quantities to represent the amplitude of joint movement in respect to a neutral position. Angular rate inertial sensors are attached to the body segments of interest. The output of the inertial sensors are fed into a precision differential amplifier and integrated. Two sequential filters are used for processing the output signals. Embodiments measure three-dimensional angles in real-time.