Range of Motion Testing Flex-Tester® Flexibility is the ability to move a joint through its complete range of motion. The Flex-Tester® is specifically designed to evaluate the flexibility of selected set of joints and small muscles including the lower back vertebrae, larger hamstring muscles and hip flexor joints, all of which must work together effectively to provide optimum flexibility. The PEAK Human Performance Clinical Research Core Laboratory offers a wide variety of protocols using this instrument to assess range-of-motion and flexibility. Our protocols can accommodate individuals with limited mobility and frailty. The PEAK Human Performance Clinical Research Core Laboratory also offers a wide variety of protocols using a full line of goniometers to assess range-of-motion and flexibility at all joints.