No exercise has been more maligned in recent years than the abdominal crunch. That’s right, the good old crunch, a staple exercise in almost every athletic program, is constantly dissed by certain fitness professionals as being non-functional and, worse, a potential hazard to spinal health. I have co-authored a peer-reviewed article with my good friend and colleague Bret Contreras that evaluates the current research on spinal flexion exercises such as the crunch and draws relevant conclusions based on the evidence. The article appears in the current issue of the Strength and Conditioning Journal. You can view the article at the following link: To Crunch or Not to Crunch.
One of the more attention-grabbing criticisms on the topic is that crunches have a negative effect on posture. The theory goes something like this: Crunches shorten the rectus abdominis (i.e. the “six pack” muscle). Since the rectus abdominis spans from the sternum/rib cage to the pelvis, continually shortening the muscle will pull down your ribcage, ultimately resulting in kyphosis (i.e. a roundback posture). It’s an interesting theory. It’s also completely unfounded.
As with many theories, its basis has a kernel of truth. Specifically, placing a muscle in a shortened position for a prolonged period of time does in fact cause it to assume a shorter resting length. For example, if you immobilize your arm in a cast at a flexed position for several weeks, your arm will tend to remain flexed once the cast is removed. This is due to an adaptive response whereby the elbow flexors (i.e. biceps, brachialis, etc) lose sarcomeres in series (Toigo & Boutellier, 2006). This has been coined “adaptive shortening.”
There also is some evidence that certain types of exercise can affect the number of sarcomeres in series. Lynn and Morgan (1994) showed that when rats climbed at an incline on treadmill (i.e. repeated shortening contractions), they had a lower sarcomere count in series than those who walked at a decline (i.e. lengthening contractions). This suggests that repeated concentric-only actions lead to a decrease in the number of sarcomeres in series while exercise consisting solely of eccentric contractions results in a serial increase in sarcomere length. Similar results have been noted by other researchers (Butterfield et al. 2005; Lynn et al. 1998).
Perhaps you can see the flaw in hypothesizing that performing a crunch will shorten the rectus abdominis: namely, crunches are not solely a shortening exercise! Rather, the movement also involves eccentric actions where the rectus abdominis is returned to its resting length. Thus, any potential negative effects of shortening contractions on sarcomere number are counterbalanced by the lengthening effect of the eccentric actions (Butterfield et al. 2005; Lynn et al. 1998). The net result is no change in resting length and thus no negative effect on posture.
Some anti-crunch proponents take an alternate hypothesis, claiming that performing spinal flexion exercises (i.e. crunches) overly strengthens the rectus abdominis to the the point that it overpowers its antagonists and thereby pulls down on the ribcage. Again, this is a straw man argument. Certainly it’s true that an imbalance between muscles can cause postural disturbances. I’m sure you’re familiar with guys who perform the “nightclub workout” (chest and arms every time they hit the gym) and end up so internally rotated that they have difficulty scratching the back of their head. Does this mean you shouldn’t perform chest presses and arm curls? Certainly not! The issue here is one of poor program design, not an indictment of presses and curls. Simply train the antagonistic muscles and there’s no postural problem.
With respect to crunches, the same principle holds true. Sure, if you perform a gazillion crunches every day and don’t train other muscle groups then you’re setting yourself up for a postural deviation. But this is a non-issue as long as you adhere to a balanced routine that emphasizes strengthening of the back extensor muscles (Sinaki et al. 1996; Mika et al. 2005). And performance of virtually any standing, non-machine based exercise will heavily involve the posterior core muscles that antagonize the rectus abdominis (Schoenfeld, 2010; Lehman, 2005). This will negate any potential “over-strengthening” of the rectus abdominis associated with the crunch. In fact, the average person tends to have weak abdominals (Morris et al. 2006), so they could very well benefit by directly strengthening the anterior core musculature.
To sum things up, there is no convincing evidence that performing crunches as part of a total body resistance training routine will have any negative effects on posture (assuming there is no inherent postural issues to begin with). This is yet another instance where a theory can seem logical on the surface but have little basis of support once you look deeper into the facts.
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