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Leaving aside the health risks (As per Wikipedia, "Some argue that situps can be dangerous due to high compressive lumbar load"), is there any benefit to doing crunches as opposed to sit-ups?

From Wikipedia description sounds like sit-ups excercise more muscles than sit-ups.

If specifics matter: Person is a beginner, with no history of extensive fitness or training. Age 35, with slight amount of abdominal fat and 22.8 BMI. No history of back pain but slight scoliosis.

Excercise goal: overall fitness, increase abdominal muscle to look better[1], anecdotal advice that improved abdominal muscle tone helps improve posture.

[1] - Yes I'm aware that fat burning through calorie deficit is ALSO required as far as getting anything approximating a six-pack. But obviously developing abdominal muscles is needed just as much as fat burning

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No benefit other than doctors' CYA principle. If a doc tells you to do sit-ups and you damage yourself, you could sue them. So, they say do crunches. –  jp2code Jun 24 at 14:42
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@jp2code - What if they say do crunches and you injure yourself? Aren't they then just as "liable"? –  JohnP Jun 24 at 21:22
    
@JohnP - why are you such a hater? –  jp2code Jun 24 at 21:25
    
@jp2code - I'm not. But if it's just as easy for a doc to get sued for saying "do situps", isn't it just as easy to get sued for saying "do crunches"? –  JohnP Jun 24 at 21:26
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3 Answers 3

The rectus abdominus works to curl the torso, and bring the lower part of the ribcage forward and down. It is one muscle, and as such, any exercise that curls the body works the entire muscle. The "upper and lower" abdominal belief is pretty much a myth. Whether you curl down towards your legs or up towards your head, the entire muscle is contracting.

The difference between a situp and a crunch, is that the situp does very little active abdominal work unless the torso is curling at the same time. Situps put much more emphasis on the leg flexors. Anything where the body bends and the torso doesn't works the flexors, and the abdominal muscle is a stabilizer more than anything.

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Crunches overdevelop the top 4 ( or crown ) of your 8 pack abdominal muscles, unless you are compensating by doing a lot of lower abdominal exercises.

A sit up is also not the best abdominal exercise for core strength.

Try V-Ups, Jack knifes, Leg Raises, Weighted Planks, Front Levers, etc.

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Among the alternatives you mention, sit-ups are the only exercise which allow easily hold additional weight as your strength develops. How is that not the best? –  hamstergene Jun 24 at 20:09
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Crunches won't destroy your spine, situps will.

This isn't hearsay or 'broscience', this is based on the extensive research of Dr. Stuart McGill who has made his living studying abdominal movements from the frail to elite sports people.

Situps require an unnatural flexing of the spine under tension that dramatically increases the risk of spinal damage.

The whole abdominal girdle is there primarily for support, and situps are a biomechanically unsafe and unnatural movement.

If you would like some replacement movements that safely strengthen the abdomen (which with a proper diet will improve your chances of a six-pack), he recommends the Bird Dog, Side Bridge, & Stir the Pot

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