Crunch

The crunch is an abdominal exercise that works the rectus abdominis muscle. It enables both building "six-pack" abs and tightening the belly. Crunches use the exerciser's own body weight to tone muscle and are recommended by some experts, despite negative research results, as a low-cost exercise that can be performed at home. Crunches are less effective than other exercises such as planks and carry risk of back injury.
The biomechanics professor Stuart McGill was quoted in The New York... meer