There are very few truly bad exercises … just those that are bad in certain situations.
The classic sit-up is an exercise that definitely falls into that category.
“But I thought sit-ups were bad for your back!” I can hear the desperate cries already. While it’s true that the hip flexors do act as the prime mover during a sit-up, and that stress on the lumbar spine is probably greater than when you’re doing, say, a crunch, you can’t tell me that your abs aren’t sore the day after doing a hundred or so strict sit-ups!
I would contend that those with a ‘weak core’ or with back problems probably shouldn’t be doing sit-ups. But if you can hold a plank for a solid minute or so and you’re otherwise healthy and injury free, the sit-up could be a valuable addition to your workout routine.
I’m a fan of sit-ups because they work the abs through a full range of motion. The crunch is a partial range movement; we don’t squat partial range or do pull-ups partial range or do push-ups partial range; why do we train the abs through a partial range?
However, we can modify the sit-up slightly to get the maximum benefit from it.
Put a towel under the lower back or use an ab mat to start the abs in a stretched position and to support the lower back. The abs are worked through a full range of motion, the exercise is more effective, and everyone is happy!
Take a look at this article: Sit-Ups Redefined.
It expands on what I’m saying here and has some great digrams showing the biomechanics of a proper sit-up.