Tighten your abdominal muscles and lift your left leg up straight behind you until it is at hip height. Bend the left foot so the sole faces the ceiling.
Tighten your buttock muscles and lift your left foot straight up, keeping the knee bent. Only lift high enough that you feel the thigh and buttock muscles contract. Do not lift so high that you create stress in the low back.
Lower yourself until the left knee is again at hip height. Do not come all the way to the floor. This is a small and concentrated movement.
Do eight to 12 repetitions. Repeat with the other leg.