Oblique crunch

The oblique crunch works all your abdominal muscles with an emphasis on your obliques. Pay special attention to form if you have a history of lower back or neck discomfort.
Getting set
Lie on your back with your knees bent and your feet hip-width apart and flat on the floor. Place your left hand behind your head so your thumb is behind your left ear. Place your right arm along the floor beside you. Bring your elbow out to the side and round it slightly inward. Tilt your chin so your chin and your chest are a few inches apart. Pull your abdominals in.

The exercise
As you curl your head, neck, and shoulder blades off the floor, twist your torso to the right, bringing your left shoulder toward your right knee. (Your elbow won’t actually touch your knee.) Lower back down. Do all the repetitions on one side and then switch to the other side.

Other options
Legs-up crunch with a twist (harder): Lift your bent knees off the floor and cross one ankle over the other. Straight-arm crunch with a twist (harder): Reach for your opposite knee with your arm straight rather than your elbow bent. Reach past the outside edge of your knee.