The double crunch exercise involves two movements: a traditional abdominal crunch and a reverse crunch or knee-in. The reverse crunch portion of this exercise is supposed to target the lower portion of the abdomen. However, it is important to note that whenever the knees move toward the chest from a supine position (on your back), the hip flexors are the prime movers.
Begin this exercise by lying on your back. Your legs should be bent at 90 degrees or greater. Interlace the hands behind your head. Once in position, perform an abdominal crunch while simultaneously moving your knees toward your chest. Be sure to contract the abdomen, then slowly return to the starting position.
Notes: You should exhale as you crunch the abs. Never crank the neck during the crunch. Your head should remain facing upward toward the ceiling at all times.
Lie down with feet bent. Place hands behind your head.
Simultaneously crunch the abdomen and bring knees toward the chest.
Return to the starting position and repeat the exercise.