Performing abdominal curls while holding your legs in the air is a good way to protect your back from overarching — but you need to keep your legs very still and your knees slightly bent. This move isolates your front and side abdominal muscles without stressing your neck or overtraining your hip flexors. Mix it with other types of crunches for added variety in your routine.
1 Lie face up on a padded surface or a mat. Raise your legs in the air with your feet together and your knees bent directly over your hips. Place your fingertips behind your head to support your neck.
2 On an exhale, contract your abdominals, rotating your left shoulder toward your right leg. Reach your left hand toward your right outer ankle. Pause, with your left shoulder blade as high off the floor as possible. Make sure your knees are together and your hipbones stay level while you twist across. Slowly lower your shoulders to the floor and repeat on the other side. Continue alternating sides until you have done 15 to 20 reps on each side.