Begin sitting, and slide an optional resistance band around your legs, just above your knees. (The resistance band will make this exercise more challenging, but you can also do it without.)
Lie on one side with one leg stacked on top of the other and your feet touching. Bend your knees at a 45-degree angle.
Rest your head on your lower arm.
Brace your abs by drawing your belly button in toward your spine.
Keeping your feet touching, raise the knee of your top leg as high as you can. Be sure not to shift your hips or pelvis. Your lower leg should remain in contact with the floor.
Hold for one second, then lower your top leg back down.
This counts as one rep. Repeat for two sets of 10 reps on each leg.