Lie on your back with your knees bent and feet flat on the floor.
Place a ball between your knees.
Tighten your abdominal muscles and squeeze the ball, pressing the inside of your knees into it.
Keeping this pressure on the ball, place your hands by the side of your head and perform a sit-up.
Lift your feet off the ground and bring your chest and knees towards one another.
Control the movement as you lower your body and feet back down, and then repeat.
