Ab Training

July 27, 2011


Do Crunches Cause Poor Posture?

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.

Stay Fit!

Brad

References

1. Butterfield TA, Leonard TR, Herzog W. Differential serial sarcomere number adaptations in knee extensor muscles of rats is contraction type dependent. J Appl Physiol. 2005 Oct;99(4):1352-8

2. Lehman GJ, Gordon T, Langley J, Pemrose P, Tregaskis S. Replacing a Swiss ball for an exercise bench causes variable changes in trunk muscle activity during upper limb strength exercises. Dyn Med. 2005, 4:6.

3. Lynn, R and Morgan, DL. Decline running produces more sarcomeres in rat vastus intermedius muscle fibers than does incline running. J Appl Physiol 77: 1439–1444, 1994

4. Lynn R, Talbot JA, Morgan DL. Differences in rat skeletal muscles after incline and decline running. J Appl Physiol. 1998 Jul;85(1):98-104.

5. Mika A, Unnithan VB, Mika P. Differences in thoracic kyphosis and in back muscle strength in women with bone loss due to osteoporosis. Spine (Phila Pa 1976). 2005 Jan 15;30(2):241-6.

6. Morris CE, Greenman PE, Bullock MI, Basmajian JV. (2006), Vladimir Janda, MD, DSc: Tribute to a Master of Rehabilitation. Spine, 31(9), 1060-4

7. Schoenfeld, BJ (2010). Squatting kinematics and kinetics and their application to exercise performance. Journal of Strength and Conditioning Research, 24 (12), 3497–3506

8. Sinaki M, Itoi E, Rogers JW, Bergstralh EJ, Wahner HW. Correlation of back extensor strength with thoracic kyphosis and lumbar lordosis in estrogen-deficient women. Am J Phys Med Rehabil. 1996 Sep-Oct;75(5):370-4.

9. Toigo, M and Boutellier, U. New fundamental resistance exercise determinants of molecular and cellular muscle adaptations. Eur J Appl Physiol 97: 643–663, 2006


10 Comments »

  1. Boom! I think I love you Brad…er…I mean…great post. :)

    Congrats on the publication as well! I’m excited to see the stir this one causes.

    Comment by Mark Young — July 27, 2011 @ 1:48 pm

  2. Thanks Mark. Expecting some fireworks, but everything in the article is evidence-based. Hopefully people will base opinion on the research, not the hype that has been promoted by some in the field.

    Brad

    Comment by Brad — July 27, 2011 @ 5:54 pm

  3. Thanks Brad. I hope the paper reaches a large audience and the pendulum shifts rightly back to the middle. I’m looking forward to my edition of the strength and conditioning journal this month!

    Comment by Jonathan Goodman — July 28, 2011 @ 9:19 am

  4. Amazing article, and could not agree more!!

    Comment by Patrick Beriault — July 28, 2011 @ 1:48 pm

  5. Thanks Jon and Patrick :)

    Comment by Brad — July 28, 2011 @ 6:58 pm

  6. Great article and I think there’s finally a trend in the industry that is actually asking questions about the accepted paradigms of functional training and looking for research that actually backs them up or not. Been in the industry for almost 30 years now and have seen the fads come and go and it’s about time we stop throwing the baby out with the bathwater when it comes to introducing new concepts into our field and long overdue that we require scientific proof before we accept something as fact and then pass it onto our clients or the general public.

    Comment by Patrick Marsh — July 28, 2011 @ 7:08 pm

  7. [...] also consider those interested to check out Brad Schoenfeld’s latest post called “Do Crunches Cause Poor Posture?” that I think dispells yet another fitness [...]

    Pingback by Mark Young Training Systems » » Friday Quick Hits — July 29, 2011 @ 12:21 am

  8. Posture aside what about the low back? McGill seems to think crunches are a sure-fire way of hurting your back?

    Comment by Dimitrije — July 29, 2011 @ 4:15 am

  9. Dimitrije:

    My current article, “To Crunch or Not to Crunch” that I co-wrote with Bret Contreras addresses the topic of spinal flexion causing disc damage in great detail. I encourage you to check out the article.

    Brad

    Comment by Brad — July 29, 2011 @ 5:57 am

  10. I think the same way, because after so many years and many strong core bodies from crunches excercises, today is unreasonable to think that they are non-func tional and potential hazard to spinal. Only now raises two questions
    1. Where is the doctor’s explanations Stwart MacGill in all this?
    2. What is the opinion regarding abdominal exercises Hypopressive Dr. Marcel Caufriez?

    Comment by Carlos Amador — July 30, 2011 @ 6:53 pm

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