Crunches are unquestionably one of the most popular exercises. We do them on the floor, exercise balls, BOSU’s, exercise machines and with cables. Sometimes we vary the exercise by adding rotation. But lately, crunches have become a controversial topic among personal trainers and fitness enthusiasts. The concern is that repeated spine flexion (bending) can damage spinal discs, the shock absorbers between each of the spinal vertebrae. Physicians and physical therapists generally recommend that patients with disc problems avoid excessive amounts of spine flexion and flexion with rotation, especially under loaded conditions. Given that activities of daily living like lacing your shoes require certain amounts of spine flexion, the concern is that too much spine flexion could speed deterioration of spinal discs. During spine flexion, pressure is increased in the posterior part of the disc increasing the risk of injury.

A recent study published in the Strength and Conditioning Journal² reviewed the scientific research to determine if there is evidence to support the limitation of exercises involving spine flexion like crunches. Much of the data involving spine flexion comes from in vitro studies which are performed in a lab where the body part, in this case the spine, is removed from the body. These types of studies subject the spine to repeated cycles of flexion and extension with loads similar to those experienced during crunches. Studies² on pig spines report that a majority of discs showed complete or partial protrusion of disc material when subjected to 4,000-85,000 cycles of flexion and extension.

This certainly is cause for concern. However, in these studies the muscles were removed from the spine so that the spine does not function exactly as in normal movements. Another potential difference between in vitro studies and living organisms (in vivo studies) is that tissue adapts to progressive exercise by becoming stronger and more resistant to injury. Also, the number of repetitions performed in these studies without a rest period is not representative of a typical exercise program. In most exercise programs, fewer consecutive cycles are performed, usually not more than 20-25 repetitions per set followed by a recovery period and additional rest between workouts. These rest periods allow for tissue recovery and adaptation. 

Some studies^5,6 have reported better nutrient flow to the posterior region of the disc, the area most vulnerable to injury, during flexion. Also, crunch type exercises have been shown to work the Rectus Abdominis more than other core muscles.⁷ If your goal is muscle hypertrophy, the crunch may provide more activation of the Rectus Abdominis than other exercises. Also, dynamic exercises like the crunch include an eccentric or negative (lowering phase of the crunch) contraction which is not present during the static types of contractions used during a plank exercise.  Eccentric exercise preferentially recruits fast twitch muscle fibers which has greater potential for hypertrophy.³  

Data from human studies suggest that family history¹ and a sedentary lifestyle⁴ increase the likelihood of disc injury. Thus, caution should be used if you have a history of disc problems or experience pain or discomfort when performing trunk flexion exercises. In these cases spine flexion exercises should be used only after a therapist or physician determines whether the benefits outweigh the risk. 

If you have a healthy back, then how much spine flexion is too much? No one knows for sure how much is too much. Contreras and Schoenfeld² recommend starting with 2 sets of 15 repetitions at a speed of 2 seconds per repetition not to exceed 60 total repetitions per workout for nonathletic populations. Allow at least 48 hours or more between workouts for the core muscles. Recovery is important as the discs have a poor blood supply and are slow to adapt and recover. For this reason, it is not recommended to train core muscles every day. To improve muscular endurance, it is probably safer to do static types of abdominal exercises like planks rather than many repetitions of crunches. A recommendation for static abdominal exercises is 3-4 sets of 10-15 second holds in multiple planes progressing to 60 seconds. Remember that many of the exercises performed during a workout will also activate core muscles. Worth noting, spine disc pressures are highest in the morning just after rising so it is best to wait 1-2 hours after waking to perform core exercises involving spine flexion. 

When deciding whether crunches are right for you, it is also helpful to examine your posture. It is estimated that as many as 90% of Americans have rounded shoulders and a forward head position. This means we are constantly walking and sitting in a position of thoracic (mid) spine flexion which increases stress on the discs and tightens structures on the front of the spine such as ligaments and muscles. To improve posture, static core exercises like planks may be more appropriate. Planks performed in different positions can be substituted for crunches. However, proper form should be emphasized to ensure that the shoulders are not rounded. Remember that many of our activities of daily living require our core muscles to contract statically as in the plank rather than dynamically as in a crunch. Also by including exercises for the low back, you can build strength in core muscles on both sides of the spine. Finally, exercises to stretch the chest and abdominal muscles will help restore posture and muscle balance.  

Your health history, exercise goals, posture, and individual response to exercise should determine whether crunches, planks or a combination are right for you. So don’t overdo it and give yourself plenty of rest. Also, if you have to do a lot of repeated spine flexion as part of your job, it may be a good idea to choose static exercises for your workouts to limit the number of cycles of flexion and extension. Weigh in with your own personal fitness experiences with crunches vs. static core exercises.

¹Battie`, M.C. & Videman, T. (2006). Lumbar disc degeneration: Epidemiology and genetics. Journal of Bone and Joint Surgery,  88(Suppl 2), 3-9.
²Contreras, B. & Schoenfeld, B. (2011). To crunch or not to crunch: an evidence-based examination of spinal flexion exercises, their potential risk, and their applicability to program design. Strength and Conditioning Journal, 33(4), p. 8-18. doi: 10.1519/SSC.obo13e3182259d05
³Farthing, J.P. & Chilibeck, P.D. (2003). The effects of eccentric and concentric training at different velocities on muscle hypertrophy. European Journal of Applied Physiology, 89, 578-586.
⁴Frymoyer, J.W. (1989). Epidemiology. In: New Perspectives in Low Back Pain. Frymoyer, J.W., Gordon, S.L., eds. Park Ridge, IL: American Academy of Orthopaedic Surgeons, 19-34.
⁵Holm, S. & Nachemson, A. (1982). Nutritional changes in the canine intervertebral disc after spinal fusion. Clinical Orthopedics and Related Research, 169, 243-258.
⁶Holm, S. & Nachemson, A. (1983). Variations in the nutrition of the canine intervertebral disc induced by motion. Spine,  8, 866-874.
⁷McGill, S.M. (2002). Low Back Disorders. Champagne, IL: Human Kinetics.