Swiss balls are great for abs, but they take up a lot of space, so use a sturdy chair instead for these knee lifts.
Sit on the edge of your seat with your feet on the floor and your knees together. Lean back against the back of the chair; hold on to the edges of the seat if you wish.
Keeping your knees together, lift them toward your chest while slightly tilting your shoulders and head toward your knees. Return to the beginning position and repeat as necessary.