Lie on the floor/mat on your back. Keep lower back in contact with the floor, as your bend knees and bring them up to your hips (not pictured). Straighten legs in line with hips as much as possible, squeezing feet and legs together. Place hands to sides or under lower back for support.
Keeping legs straight and together and lower back flat and controlled, lower your legs toward the ground slowly and with control, stopping at about 45 degrees or the point at which you can no longer lower the legs without your back arching. Contract abs and bring straight legs back up to hips to complete one rep.
Make sure back stays flat on floor and abs are tight (pull navel in towards spine). Modify this exercise and reduce any back strain by ending the knees instead of keeping legs straight.
Muscles Worked: Abs, hips