These plank booty leg lifts work out your tush and the back of your thighs at the same time.
Lie on your belly on an exercise ball, and walk your hands out so the ball is underneath your shins. Your hands should be underneath your shoulders.
Draw your navel toward your spine to engage your abs, which will help keep your spine straight and your body stabilized.
Lift your left leg up into the air, and then slowly lower it back toward the ball, but don't let it touch. This counts as one repetition. Keep your pelvis level throughout your reps.