A crunch begins with lying face up on the floor with knees bent. The movement begins by curling the shoulders towards the pelvis. The hands can be behind or beside the neck or crossed over the chest. Injury can be caused by pushing against the head or neck with hands.
The difficulty of the crunch can be increased by lying on a declined bench or holding a weight under the chin, on the chest or behind the head. Crunch exercises may be performed on exercise balls. Increasing the distance will also increase the load on the abdominals due to leverage.
The curl-up[clarification needed] is taught by spine biomechanics professor Dr. Stuart McGill, and he considers it to be a safer alternative to the crunch, which differs from the sit-up. 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.
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.
Strength exercises such as sit-ups and crunches do not cause the spot reduction of fat. Achieving "six pack abs" requires both abdominal muscle hypertrophy training and fat loss over the abdomen—which can only be done by losing fat from the body as a whole.
Differences between a crunch and a sit-up
Unlike the sit-up, in a proper crunch, the lower back stays on the floor. This is said by scientific literature[specify] to eliminate any involvement by the hip flexors, and make the crunch an effective isolation exercise for the abdominals. 
- The reverse crunch is a crunch done with the upper back on the floor and lifting the hips up instead.
- 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.
- Reynolds, Gretchen (2009-06-17). "Is Your Ab Workout Hurting Your Back?". Well. The New York Times. Retrieved 2011. Check date values in:
- McGill, Stuart. Low Back Disorders: Evidence-based Prevention and Rehabilitation. Human Kinetics Publishers. ISBN 978-0-7360-6692-1.
- 3 worst core exercises for people with low back pain
- "5 Core Exercises to Do If You Hate Planks". Fitness Republic. Retrieved 2015-10-08.
- "Are Sit Ups Bad for You? The U.S. Military Seems to Think So…". www.ISSAonline.edu. Retrieved 2018-10-17.
- Reynolds, Gretchen (August 17, 2011). "Are Crunches Worth the Effort?". Well. The New York Times. Retrieved 2012. Check date values in:
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- Preparation Guide for the US Marine Corps Physical Fitness Test
- To Crunch or Not to Crunch: An Evidence-Based Examination Strength & Conditioning Journal: August 2011, Volume 33, Issue 4, pp 8–18