A taut midsection might be physically attractive, but its advantages are more than cosmetic. Strong abdominals sustain good posture, protect against injury and prevent pain in the lower back. But when push comes to shove, there are two exercises you can’t do without if you want well-conditioned abs: the sit-up and the crunch, also known as the curl-up.
While there are similarities between the two exercises, they do work different muscles. Both are done lying on the floor, usually feet on the floor, knees up and hands behind head. With crunches, the lower back doesn’t lift off the ground, whereas in a sit-up, the whole body rises.
For Abs, Crunches Rule
A study conducted by the University of Wisconsin reviewed a large selection of ab workout equipment, including the Ab Circle Pro, Ab Roller, Ab Lounge and others. It found that none of them surpassed the bsic crunch for across-the-board muscle activation. It was also shown to be better than a number of other exercises, including side planks and front planks.
“Quite a lot of definition can be had by just doing crunches,” says personal trainer and yoga instructor David Knox, author of "Body School: A New Guide to Improved Movement in Daily Life." "But there’s a limit. Since the hips and legs remain stationary during crunches, you don’t fully activate the lower abs. So crunches aren’t likely to flatten your stomach below the navel.”
Sit-ups for Posture and Trunk Stability
That brings us to the sit-up. While crunches mainly work the abdominal muscles, the full sit-up engages muscles that stabilize posture, such as hip flexors and lower-leg muscles, along with a matrix of muscles running through the chest and neck. Sit-ups should be done carefully because there is a risk to injuring the back. It’s usually necessary to anchor the feet beneath a suitably heavy piece of equipment.
Because full sit-ups involve the hip flexors, which may cause the lower back to arch, causing pain. That’s especially true if your abs are weak. The hip flexors are generally stronger than the abdominals, sometimes causing an imbalance. The goal engage the abs as much as possible while preventing too much involvement of the hip flexors.
“The position of the knees and feet in relation to the hips and belly are crucial as to whether the lower belly works hard and wants to press out or in as it works,” says Knox.
Let the Reps Rip
For abdominal exercises, the American Council on Exercise recommends 10 to 25 reps for one to three sets. The last few repetitions should find you at the end of your endurance. You can increase the challenge and intensity of abdominal exercises by using added resistance, moving more slowly or performing the exercises on a slant board or exercise ball so that your head is at a lower elevation than your legs.