The elbow-to-knee crunch is a core strengthening exercise that targets the lower abdominals and, more specifically, the obliques.
Lie flat on your back and place your hands behind your head. Bend your knees and bring them up so that your thighs and hips form a 90 degree angle, calves parallel to the floor. With elbows flared lift your shoulder blades off the floor and hold the position. This is your starting position.
Twist your upper body in one direction bringing the elbow to the opposite knee while fully extending your other leg. Hold and then return back to the starting position to repeat in the opposite direction.
Avoid neck strain by touching your head very lightly.