Research shows that the crunch—also called the curl-up, the abdominal curl, or the trunk curl—has a high “challenge-to-compression” ratio [1]. In other words, it challenges your abs while imposing minimal compressive load to the spine.

The usual advice is to “pull your belly button in toward your spine” or “pull your abs in” during the crunch. But there’s a far better way to make the exercise more effective.

In fact, researchers from Omaha’s University of Nebraska Medical Center have found that “pulling the abs in” actually reduces rectus abdominis activity [2].

A group of 25 subjects performed the crunch in accordance with different sets of instructions.

The first set of instructions was intended to emphasize rectus abdominis activity. The second set of instructions was intended to emphasize the obliques.

Electromyographic activity was recorded from the upper and lower rectus and the internal and external oblique abdominis muscles. A physical target was used to make sure that the trunk was raised to the same height for all conditions.

The crunch worked rectus abdominis harder when subjects were told to tighten their stomach muscles, rather than pull them in. Here are the instructions they were given:

“Push your belly button out by tightening your stomach muscles. Try to shorten the distance between the bottom of the ribs and the groin area when you curl up. Focus on drawing your ribcage down and in while you curl up.”

But the same exercise worked the external and internal obliques more than rectus addominis when these instructions were given:

“Try to suck in your stomach to move your belly button toward the floor. Keeping your stomach tight, try to flatten your back against the floor. Focus on keeping your ribcage flared out as you curl up.” Professor Stuart McGill, an expert in spine function and injury prevention at the University of Waterloo in Canada, offers the following tips for those wanting an advanced version of the crunch with minimal stress to the spine:

• One leg is bent with the knee flexed to 90° while the other leg remains relaxed on the floor.
• No cervical (upper spine) motion should occur, either chin poking or chin tucking.
• For anyone experiencing neck discomfort, place the tongue on the roof of the mouth behind the front teeth, which helps to promote stabilizing neck muscle patterns.
• Brace the abdominals. This involves tightening the abdominal muscles as if you’re about to take a punch in the gut. When the brace is performed correctly, the abdominal wall is neither hollowed in nor pushed out.
• Curl up against the brace, and then breathe deeply in the “up” curl-up position while maintaining the brace. Remain in the up position long enough to take a few deep breaths.

This technique will challenge rectus abdominis while minimizing compressive load to the lumbar spine. Try it the next time you train your abs. You’ll be surprised at how such a simple exercise can easily be made more challenging and effective.

References:

1. Axler, C.T., & McGill, S.M. (1997). Low back loads over a variety of abdominal exercises: searching for the safest abdominal challenge. Medicine and Science in Sports and Exercise, 29, 804-811
2. Karst, G.M., & Willett, G.M. (2004). Effects of specific exercise instructions on abdominal muscle activity during trunk curl exercises. Journal of Orthopaedic and Sports Physical Therapy, 34, 4-12

Christian Finn hold a masters degree in exercise science and lectures on the subject of fitness at both universities and private training organizations around the United Kingdom. You can find Christian’s articles published in numerous magazines, leading industry journals and websites worldwide, including Men’s Health and Muscle & Fitness (UK edition). His research on high-intensity intermittent training has been published in the online journal Sport Science (March 2001).

You can reach Christian by email at christian@christianfinn.com or on the web at www.christianfinn.com.