Starting Position: Lie prone (on your stomach) on a mat and attach a cuff (cable or band resistance) to one ankle with the anchor or resistance point away from your body. Lie with feet hip-width apart. Gently contract your abdominal / core muscles ("bracing") to stiffen your torso and stabilize your spine. Keep your ankle lightly dorsi-flexed (toes pointed towards your shinbone) throughout the exercise.
Curling Movement: Exhale and curl (bend) your knee, bringing your heel towards your buttock without moving your thigh (the top of your thigh remains on the mat), lifting your hips off the mat or arching your low back.
Continue bending your knee until your heels near, or touch your buttock, or you reach the limits of your comfort level. The point where you notice your hips or low-back beginning to move indicates that you have reached the limits of movement in your joint.
Downward Movement: Inhale and slowly return your leg back to your starting position.
Exercise Variation: The intensity of this exercise can be increased by (1) increasing your range of motion by moving your heel closer to your buttock or (2) increasing the resistance in the cable / band.
There is often a tendency to hike the hips upwards or extend the low back (increasing the low back arch) during this exercise. By engaging (lightly contracting) your abdominal muscles, this will help prevent hip hiking and low back hyper-extension.