Secure a resistance-band loop around the arches of your feet and lie faceup with your knees bent 90 degrees and your legs lifted over your hips. Place your hands lightly behind your head with your elbows flared. Lift your head, neck and shoulders off the ground and twist to the right as you bring your left knee in and press your right leg out. Return to center, then repeat on the other side. Continue, alternating sides.