The crunch or curl-up is an abdominal exercise that works the rectus abdominis muscle.[1] It enables both building and defining "six-pack" abs and tightening the belly. Crunches use the exerciser's own body weight to tone muscle and are recommended by some experts[like whom?], despite negative research results[citation needed], as a low-cost exercise that can be performed at home.[2] Crunches are less effective than other exercises such as planks and carry risk of back injury.[3]
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