This exercise is a classic that has been done in Phy Ed class since middle school, the Presidential Physical Fitness competition, and Military training to this day. So, what exercise is it?
So, why is a sit-up so bad that no one should do sit-ups? There's a couple reasons. First of all, it's bad for your back. Once you get past the crunch stage of the sit up, all you're doing is putting unneeded stress on your body. Your lumbar spine already has enough stress on it from holding the majority of your body's weight and from all the sitting that we do these days. When you do a sit up, you are creating an unneeded amount of pressure on the discs in the lumbar spine and strain on the overworked muscles that surround the spine.
Another reason no one should do a full sit-up is the strain it puts on a muscle called the psoas muscle. The psoas is a muscle that helps your abdominal muscles when they are weakened by bending your body at the waist or by raising your legs up. In most people, the psoas muscle is already overworked and tightened due to weakened abdominal muscles. By doing a sit up, you are strengthening and tightening an already tightened muscle.
Finally, the last point I want to make is regarding hand position. And this doesn't just pertain to sit-ups, but also crunches. DO NOT place your hands behind your head. Whether you feel it or not, you will more than likely be pulling your head forward and straining the back of your neck. When doing a crunch (not a sit-up!), cross your arms over your chest.
So, what should you do instead of a sit-up?
Crunches. Crunches and sit-ups are very similar. A crunch is basically half of a sit up. What you want to do is pay close attention to your abs contracting. When you feel like they can't tighten any further or if it feels like a natural place to stop, that's where you stop. Sitting all of the way up is unnecessary and detrimental to your spine.
Do These (crunches): NOT these (sit-ups):