Set up: Upper back and head resting on a fitball, feet in front of knees your knees, open up through hips so body is one long line
Set your core, raise one heel off the ground. Avoid arching through the lower back, slightly tuck under with the pelvis to counteract this. Keep hips level and avoid rotation. Alternate heel rises.
To progress this exercise, extend one leg to “kick out”. Make sure the opposite hip (of the non-moving leg) doesn’t drop with each kick. Alternate with one leg at a time.