Set your bench on an incline to work your abs.
According to research commissioned by the American Council on Exercise, incline curl-ups are one of the most effective exercises for working your rectus abdominis, the so-called "six-pack" muscle. But there's more than a little confusion around the terminology for this exercise, so you might see the same movement described as a decline curl-up, a decline sit-up or an incline sit-up exercise.
Getting Onto the Bench
Although the apparatus you'll use for decline curl-ups is sometimes referred to as an incline bench (because it's placed at an incline), it's more properly known as a decline bench because your head ends up lower than your knees. This can make getting onto the bench a little awkward. Depending on how it's constructed, you first hook your feet under a support, then lower your body onto the bench with your knees bent as you lie back; alternatively, you drape your knees over a foam roll and then hook your feet under another support, and lie back on the bench.
If you're not confident in your ability to control your body position as you get onto the bench, build strength and confidence by doing sit-ups on a flat bench, or setting the incline/decline bench to a milder, flatter angle. You can also ask for a spotter until you get comfortable with the process of getting on and off the bench and the incline sit-up technique.
Doing Decline Curl-Ups
Once you're on the bench, doing decline curl-ups (which you might also see called incline/decline crunches or sit-ups) works just like doing crunches flat on the floor. Squeeze your abs as you curl up, lifting your shoulders and upper torso from the bench. Think of bringing your lower ribs toward your pelvis. This is the exercise that was evaluated by the American Council on Exercise.
What About Sit-Ups?
Although the words "decline sit-ups" or "incline sit-ups" can be used to refer to the exercise just mentioned, they also (confusingly) refer to a slightly different variation. Get onto the decline bench just as you would for decline curl-ups. But instead of flexing or curling your spine, you're going to keep your torso straight and hinge from the hips instead, in what some would call an "old-school" sit-up. It might help to think of folding your body into the shape of a taco shell, with your torso forming one side of the shell and your thighs the other, with your hips as the hinge at the midpoint.
The Difference Between Sit-Ups and Curl-Ups
As you can see, the decline sit-up and the decline curl-up are so similar that their names are often interchanged. But there is a notable difference between the two of them: The straight-torso sit-up variation places more emphasis on your hip flexors in addition to your abs. While this makes it an efficient exercise, if your abs aren't strong enough it can place a lot of stress on your lower back. With that in mind, choose the variation that best suits your fitness level and the state of your lower back.