The Difference Between Sit Ups and Crunches
Even though nowadays the crunch and sit-up are different exercises, the sit-up is the original. Over time, and with research, the crunch evolved from the sit-up. Let's start with the original. To perform a sit-up, you start by laying on your back and placing your feet flat on the ground with your knees bent. Then, you lift your head and shoulders, middle back, and low back off the ground coming to a seated position where you are mostly balanced on your tailbone. It's common, but unnecessary, to have someone hold your feet down while doing sit-ups. Ideally, you're strong enough that you don't need a buddy. In fact, if you've taken a Pilates class, you may have done the Pilates version of a sit-up, called the roll-up. There's nobody there to hold your feet - it's all about using your core!
Now on to the crunch. The crunch starts the same way by laying on your back, feet grounded, knees bent. The difference is the amount of movement. During the crunch, you lift your head and shoulder blades off the ground, then lower. Your mid and lower back stays on the floor the entire time. Your hands can be placed behind your head or crossed over your chest.
Both the sit-up and the crunch work your abdominals; specifically, the muscle - that when well-trained - gives you a 6-pack. However, the sit-up also works the muscles that flex your hip joint. The sit-up works your abs until you lift your lower back off the floor. Once your back leaves the ground, you are using your hip flexors to finish the exercise. There's nothing wrong with this...it's just something to be aware of.
So, if you want to work out your abs, the crunch is a very effective exercise. If you want to train your abs and hip flexors, the sit-up is great. Either way, your abs will thank you!