I have placed two sensors on subject to calculate cervical full range of motion; one positioned on T4 (vertebrea 4 of thoracic spine) an danother one on forehead. Starting position of sensor one was 20° and forehead sensor -30°, when I instruct subject to move his head forward and backward to the end poit of ROM, howevere, plotting data as curves appear close to other. Device is valid, but I am wondering if i should the full range of motion relative to zero line or relative to T4 sensor.