Starting Position: Attach a cuff (cable or band resistance) to one ankle with the anchor or resistance point in front of your body. Stand with your feet hip-width apart, toes pointing forward or turned slightly outwards, with arms by your sides. Engage your abdominal muscles ("brace") to stabilize your spine. Pull your shoulder blades down and back. Avoid arching the back.
Slowly bend the knee of the cuffed foot and lift it slightly off of the floor (1-2"). Find your balance on your standing leg. Try not to shift your hips or lean with your torso. Hold on to a stable surface for balance as needed.
Curling Movement: Exhale. Bend your knee, bringing your heel towards your buttock. Keep your thighs with one another and your body. Maintain your balance and vertical posture.
Continue to bend your knee until your heel nears, or touches your buttock, or you reach the limits of your comfort level. If you notice your hips or torso beginning to tilt forward, you have reached the end limits of the movement.
Downward Movement: Inhale. Slowly return your leg back to your starting position without losing your balance or changing the position of your torso or standing leg.
Exercise Progression: The intensity of this exercise can be increased by (1) increasing your range of motion by moving your heel closer to your buttock, (2) increasing the resistance in the cable / band, or (3) removing any balance assistance, or standing on an unstable surface (e.g., Airex pad, BOSU, etc.).
The ability to perform this exercise unassisted offers the added benefit of balance training.