Crunch (exercise)
The crunch is one of the most popular abdominal exercises. It works the rectus abdominis muscle. It allows both building "six-pack" abs, and tightening the belly. Crunches use the exerciser's own body weight to tone muscle, and are recommended as a low-cost exercise that can be performed at home. Form. The biomechanics professor Stuart McGill was quoted in "The New York Times Health" blog as stating: An approved crunch begins with you lying down, one knee bent, and hands positioned beneath your lower back for support. "Do not hollow your stomach or press your back  against the floor", McGill says. Gently lift your head and shoulders, hold briefly and relax back down. In a crunch, unlike a sit-up, the lower back stays on the floor. This is said to eliminate any involvement by the hip flexors, and make the crunch an effective isolation exercise for the abdominals.