Exercise Analysis - Abdominal Reverse Crunches

There is little doubt that eyebrows were raised last month when I dropped a bombshell regarding an exercise typically performed to target the abdominal musculature. The take-home message was that the major muscle of the abdomen (the rectus abdominis) crosses and causes motion at the joints formed by adjacent spinal vertebrae. Consequently, an exercise designed to target this muscle must involve a change in spinal configuration. Specifically, the spine must be drawn from its neutral shape into flexion (curled forward to form a “C”) against opposition. Raising the legs at the hips so that they approach the neutral spine involves no such movement. Therefore, the muscles that flex the thigh at the hip are the prime movers during leg raises and the rectus abdominis is simply along for the ride.    

Muscles cross joints and are attached to the bones that form the involved articulation. When we intend to move, our nervous system commands the appropriate muscles to develop sufficient tension in their contractile elements to overcome any resistance to shortening that is present. If this is accomplished, the muscle will shorten, thereby pulling the attached bones toward one another. Ideally, when we perform resistance training, one bone will be fixed to provide an anchor against which the shortening muscle can fully contract. Stabilizing muscles hold the anchoring bone in place. When we contract our hip flexors to raise our legs at the hips, our hips are not drawn down because of the stabilizing action of the rectus abdominis. This isometric work explains why some exercisers feel this maneuver in their abs. However, a static muscle action like this does not represent the optimal way to train a muscle.

Crunches are the only exercise that are characterized by tension development and shortening of the rectus abdominis against opposition to cause the requisite change in spinal configuration. Last month, I outlined the conventional version, describing spinal flexion that causes the ribcage to move toward the fixed hip. There is, however, another way to perform the movement. If the hips move against opposition toward the fixed rib cage, the rectus abdominis will contract in a similar manner. This exercise is called the reverse crunch and is the perfect complement to the exercise described last month.

Gym lore suggests that reverse crunches work the lower section of the abdominals, but this is not the case. The two forms of crunches are identical with respect to the actual change in alignment that the spine undergoes. The only difference is the end of the muscle that is free to move. Reverse crunches are an example of a reverse origin-insertion maneuver. To understand this concept, consider the difference between curling a dumbbell at the elbow and pulling yourself up to an overhead bar. In both cases, your elbow flexors have developed tension and shortened to overcome resistance, thereby causing your elbow to bend. The difference is that in one case, your hand moves toward your shoulder and in the other, your shoulder moves toward your hand. These movements might seem different, but it’s safe to say your muscles and nervous system would react in a similar manner.

To perform reverse crunches, lie supine on a flat bench and grasp the bench with your hands positioned beside your head. This is necessary because it stabilizes the end of the muscle that will not be moving (the end attached to the rib cage). Similar stabilization is achieved at the muscle’s opposite end during conventional crunches through the lower legs by virtue of immovable pads or an assistant’s hands across the ankles. From this position, raise your straightened legs so that they form a right angle at the hip, then curl your hips up so that your feet move directly toward the ceiling. Once full spinal flexion is achieved, the contraction should be held momentarily before tension development is reduced just enough to lower the hips slowly back to the bench. When the hips touch the bench, you should initiate the next repetition immediately so that the cumulative fatigue associated with a multiple-repetition set will be maximized.

Compared to conventional crunches, reverse crunches are more challenging because the load you are working against (the legs, as opposed to the upper torso and head) is greater. Consequently, this movement might be very difficult for beginners. A remedy is to alter the leverage conditions associated with the motion taking place. Muscles work in a lever system where opposition is a function of both the resistive force and the moment arm through which that force is applied. A lever’s moment arm is the perpendicular distance between the line of pull of the motive force (in this case, the muscle creating the movement) and the pivot point(s) where the movement occurs. Armed with this knowledge, a savvy exerciser can decrease the load associated with reverse crunches by flexing the hips to an angle less than 90 degrees prior to initiating the movement. This modification places more of the opposing load directly over the joints at which the movement is taking place. Similarly, the resistive load can be increased by maintaining a thigh/hip angle greater than 90 degrees as you curl the spine forward.

This article was originally published in New Living Magazine, which can be accessed on-line at www.newliving.com.





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