Abstract: A device and method for measuring the amount of knee extension loss, hyperextension, or extensor length associated with arthritis, trauma, surgery or other medical conditions. This device and method includes measuring the difference in heel heights of a patient while patient is lying prone on a table with their legs, at the level of their knee caps, off the table. This device and method also measures extensor length in a patient's knee joint, by assessing heel height difference while a patient is lying prone on a table with a prone knee bend. This device and its methods provide accurate distance measurements that correlate to knee straightness and flexibility; these distinguishing metrics are unattainable by a standard “disc” goniometer. In addition, this device permits convenient measuring of a knee's mobility for two unique perspectives, namely in prone with legs straight or with a knee bent.