So you've mastered the standard crunch. But have you tried the reverse crunch? When you reverse the crunch by raising your pelvis — rather than your shoulders — off the floor, you still activate the rectus abdominus, but also engage multiple other muscles to assist and stabilize the action.
Understand the Rectus Abdominus
The reverse crunch is said to work the lower abs. In reality, you don't have a lower ab muscle, you have a lower region of the rectus abdominus muscle. This muscle is one large sheath of fibers, divided in six sections by tendinous creases.
You technically can't independently activate the right or left side or the upper and lower regions. When you do a reverse crunch, the whole muscle works; however, you do experience more muscle shortening in the lower region of the muscle — essentially, putting more emphasis on this section of the muscle.
Recognize the Assisting Muscles
The obliques at the sides of the waist assist as you perform a reverse crunch. When you draw the knees in toward your torso to lift the pelvis inward, several hip and thigh muscles also assist. These include the illiopsoas, tensor fasciae latae, quadriceps and adductors.
In some versions of the reverse crunch, you lie on a bench and hold onto the sides for leverage as you rock your pelvis and legs up and in toward your trunk. This action requires stabilization from your pecs at your chest, the latissimus dorsi of your back, the rear deltoids at the shoulders and the triceps at the back of the upper arms. The teres major, just under your armpit, also activates.
Make the Reverse Crunch More Effective
The standard reverse crunch is performed on the floor or lying on a flat bench. Increase the intensity of the move by performing it on an incline of 30 degrees. Orient your body, so your head and shoulders are on the higher portion of the bench.
This adjustment makes the move more effective in working the upper and lower parts of the rectus abdominus, internal obliques and latissimus dorsi, when compared to the traditional crunch and situp, reported a 2006 issue of Physical Therapy.
The researchers noted that the incline reverse crunch does intensify activation of the rectus femoris, a muscle of the quadriceps that crosses the hip joint -- which could aggravate lower back pain in vulnerable populations.