I was chatting with someone yesterday and they mentioned to me that they do 100 sit-ups every night before they go to bed. I'm certain they were expecting me to respond with a resounding WOW, but the best I could do was muster a shrug and ask them 'why'?
There are several inherent problems with sit-ups and crunches:
The movement actually strains the back at it's weakest point and can cause disk damage
The movement is limited to targeting the rectus abdominus muscles which can ultimately lead to a distended stomach appearance
The movement is contrary to how we typically use our abdominal muscles and is not at all functional in nature
If you're going to take the time to work your core, work it in a way that is going to benefit your overall health and give your mid-section the overall toned look you're striving for.
To achieve this, work your core muscles - back extenders, rectus abdominus, transversus abdominus, obliques, hip flexors - as a group.
Some of my favorite core exercises include:
The Plank - I have found the plank to be one of the most effective core exercises around. You can modify this exercise to make it more difficult by using a stability ball, but this is definitely a no-equipment-required exercise. Remember not to let your hips or back sag!
The Overhead Squat - I bet you were not expecting this one! The overhead squat engages all of your core muscles as once that bar is over your head, it seems to have a mind of it's own. Caution - this exercise should only be used by those of you who are more 'advanced' lifters with good squatting form.
Turkish Get-Up - Another over-head exercise that engages all of your core muscles and requires precision balancing from start to finish. I love this video and how it breaks the movement down in to several drills. Keep in mind that you do not need a Kettlebell to perform a Turkish Get-Up. A dumbbell works just fine!