I generally do full sit ups, I just prefer them to crunches.
But are there any benefits or drawbacks of full sit ups over crunches?
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Consider for a moment that a major job for your abs is not to pull your chest to your knees (crunch and sit up style), but to remain erect and not collapse like when doing a front squat:
Crunches and sit ups don't have a large isometric component, as where something like front squats, deadlifts, and planks do. Most of what we want our backs to do in daily life is related to posture, where that lengthened and upright position is really helpful. Also, a lot of lower back injuries can be traced to weak abs, and not being able to maintain a tight (and erect) mid section.
Personally I'd vote for ab wheel rollouts (using an ab wheel) over crunches and situps as it engages far more muscles. Those combined with deadlifts, front squats, and planks make for terrific abdominal strength.
More muscles are used in a sit up but crunches seem to focus on the abdominal muscles more.
For myself I can do 2-3 times as many crunches, and always do some crunches after sit ups to continue working some of the muscles. The main group of muscles neglected in crunches seem to be the back and legs. In the military you get 2 minutes to do as many as possible, and I found that to do this I also had to push myself down and go up without relying on people holding your feet. So in my experience I usually used alot more than just the abs. Hope this helps
The main differences I've seen between the two:
Crunches: Shorter range of motion, abs are more isolated, little to no lower back movement.
Sit-ups: Longer range of motion (more time under tension), involves hip flexors, may involve rounding of the lumbar spine, easier to load with resistance.
Of the two, I feel crunches are a better option as there may be less risk involved in terms of back injury, and targeting only the abs.