The relational motion detection system measures relative motion of a head of a subject relative to a torso of a subject. The system includes a first motion detection sensor on a first apparatus configured to be secured to a head of a subject to detect a first motion. It also includes a second motion detection sensor on a second apparatus configured to be secured to the torso of a subject to detect a second motion. The system further includes a control unit configured to obtain information regarding the first motion from the first motion detection sensor and second motion from the second motion detection sensor, wherein the control unit contains instructions for calculating motion of the head relative to the torso of the subject based on the obtained first motion and second motion.
