The crunch is one of the most common abdominal exercises. It primarily works the rectus abdominis.
Start lying face up on the floor with your knees bent. The movement begins by curling your shoulders towards your pelvis. Your hands can be behind or beside your neck or crossed over the chest.
Do not push against your head or neck with your hands. This can cause injury. Your head can rest against your hands though.
Crunch exercises may be performed on exercise balls. Weights may be used, typically held under the chin, to increase resistance. Increasing the distance will also increase the load on the abdominals due to leverage.
The curl-up, as taught by spine biomechanics professor Dr. Stuart McGill. Stuart McGill considers the curl-up to be a safer alternative to the crunch, which differs from the sit-up. Dr. Stuart McGill has done extensive research on the effects of crunch exercises on the back, which can be especially harmful for those rehabilitating their backs from an injury.
If anybody is looking to develop the muscles that create a "six pack" this is one of the best known ways.
Differences between a crunch and a sit-up
Unlike the sit-up, when performing a crunch the lower back should not leave the floor. This is said to eliminate any involvement by the hip flexors, and make the crunch an effective isolation exercise for the abdominals. The difficulty of the crunch can be increased by lying on a declined bench and/or holding a weight on the chest or behind the head.
- The reverse crunch is a crunch done with the legs instead of the chest.
- The dragon flag (created by Bruce Lee or simple "flag" popularized by Sylvester Stallone in his movie Rocky IV), called "dragon" after one of Bruce Lee's nicknames, "little dragon". Begin lying horizontal on a bench. The arms are used to hold the underside of the bench to anchor the shoulders to it to prevent rolling forward. The rest of the body inferior to the scapulae is then pulled from the bench. Beginning with legs flexed, the legs can be extended slowly to increase the difficulty of the movement (and tension on the core muscles) due to moving the weight further from the levers. Throughout the movement the legs and hips are meant to remain locked while movement occurs in the abdominal region.
- The twisting crunch is performed by lifting one shoulder at a time. More emphasis is placed on the obliques.
- The Thai crunch is performed by hitting the stomach after full contraction. This variation is used by Muay Thai fighters to condition the core to take hits from punches or knees.
- The cable crunch is performed while kneeling upright by curling the body to pull down on a cable machine. The hips are kept motionless, so the bodyweight does not assist the movement.
- The bicycle crunch is performed by bringing together alternating elbows and knees, resulting in a leg motion similar to pedaling a bicycle. The bicycle crunch is the most effective in targeting the rectus abdominis and the obliques, according to a study done by the American Council on Exercise.