Set-up: Begin by lying on your back on a lightly cushioned surface, placing a towel or other sturdy fabric evenly across your feet . Grasp the ends of the towel with your hands.
Movement: Contract your biceps, pulling the ends of the towel towards your shoulders and your knees closer to your torso. Return your legs to the start position while maintaining tension and control in your biceps. To make the exercise easier you can use your legs to assist the movement, while to make it harder you can use your legs to resist.
Emphasis: Brace through your core for the duration of the exercise.