Research Update: Getting to the Core

Studies have challenged common conceptions about core training.

While you have to be careful about the methodology, number of subjects and even who sponsored a study, your core training program should be based on current research - not opinions, assumptions or traditions. These studies will make you rethink what you thought you knew about core strength training.

Do exercise balls really pose a greater challenge?

Drake, J.D., S.L. Fischer, S.H. Brown and J.P. Callaghan. Do exercise balls provide a training advantage for trunk extensor exercises? A biomechanical evaluation. Journal of Manipulative and Physiological Therapeutics 29(5): 354-62, June 2006.

This study examined back extension exercises performed on a mat versus on a Swiss ball. Although several studies have shown abdominal exercises on a ball to cause greater muscle activation, activation of spinal erectors in back extension exercises on a ball decreased in some subjects. The researchers commented, "It's important to note, however, that spinal compression and anterior/posterior shear decreased when the extension exercises were performed on the ball." In other words, performing back extensions over a Swiss ball may be helpful in post-rehab, since spinal loads are lower.

Researchers concluded, "The assumption that the use of an exercise ball will always create a greater challenge for the musculoskeletal system was not supported by the findings of this study." They found no training advantage to performing back extension exercises on a ball versus a mat in the young, healthy subjects who participated in the study.

Surprise! Quadriceps help stabilize the spine

Danneels, L.A., G.G. Vanderstraeten, D.C. Cambier, E.E. Witvrouw, V.K. Stevens and H.J. De Cuyper. A functional subdivision of hip, abdominal and back muscles during asymmetric lifting. Spine 26(6): E114-21. Mar. 15, 2001.

The normal function of the local muscle system is to provide sufficient segmental stability to the spine. The global muscle system provides general trunk stabilization, and enables the static and dynamic work necessary for daily living and sports activities. Despite asymmetrical loads, the left and right internal obliques, rectus femoris (quadriceps) and multifidus (spine) contracted symmetrically in all the exercises examined.

As you might expect, the study showed significant left-to-right differences measured in the external obliques, gluteus maximus, lats and iliocostalis. The researchers concluded that the "local muscle system," also sometimes called the "Inner Unit," stabilizes the spine during dynamic movements. Although the study tends to validate the inner unit/local muscle system theory, the inclusion the rectus femoris as part of this group was a surprise.

Does instability always increase muscle activity?

Lehman, G.J. An unstable support surface is not a sufficient condition for increases in muscle activity during rehabilitation exercise. JCCA: Journal of the Canadian Chiropractic Association 51(3): 139-143, July-September 2007.

Well, the title takes away all the suspense. This study indicates that performing an exercise on an unstable surface does not necessarily increase muscle activity for all muscles for all people. The responses of different people in the study varied remarkably. The researchers go on to mention that adding an exercise ball to certain exercises, such as wall squats and back extensions (more on that in a moment), decreases muscle activation.

Can core training help to decrease lower-body injuries?

Zazulak, B.T., T.E. Hewett, N.P. Reeves, B. Goldberg and J. Cholewicki. The effects of core proprioception on knee injury: A prospective biomechanical-epidemiological study. American Journal of Sports Medicine 35(3): 368-73, March 2007, Epub Jan. 31, 2007.

Study subjects were 277 collegiate athletes who were tested to see if deficits in core proprioception affect dynamic stability of the knee. The female subjects who later suffered knee injuries indeed showed deficits in active proprioceptive repositioning when compared to the uninjured women.

Moreover, a technique known as "active proprioceptive repositioning" predicted knee injury with 90 percent sensitivity (the percentage of people who had deficits out of the group who had injuries) in female athletes. Bottom line: Core training can help a lot more than just your clients' core - at least for female athletes.

Are core stability and running economy linked?

Stanton, R., P.R. Reaburn and B. Humphries. The effect of short-term Swiss ball training on core stability and running economy. The Journal of Strength and Conditioning Research 18(3): 522-528, August 2004.

This study was designed to look at the effect of exercise ball training on core stability and running economy. Although exercise ball training increased core stability, there was no improvement in electromyography (EMG) activity (which detects the electrical potential generated by muscle cells when the cells contract, and also when the cells are at rest) of the abdominal and back muscles, running economy or running posture. Running performance did not improve. The researchers warned, "Specificity of exercise selection should be considered."

Is this the last word on abdominal hollowing?

Grenier, S.G., and S.M. McGill. Quantification of lumbar stability by using two different abdominal activation strategies. Archives of Physical Medicine and Rehabilitation 88(1): 54-62, 2007.

This study examined whether abdominal hollowing (pulling in the navel) is more effective for lumbar spine stabilization than co-contraction of all the abdominal muscles. Abdominal bracing - that is, co-contraction of the internal obliques, external obliques, rectus abdominis and transverse abdominis - improved stability by 32 percent. Grenier and McGill wrote, "There seems to be no mechanical rationale for using an abdominal hollow, or the transversus abdominis, to enhance stability. Bracing creates patterns that better enhance stability."

Can altering stability and adding upper-body exercise strengthen the core?

Behm, D.G., A.M. Leonard, W.B. Young, W.A. Bonsey and S.N. MacKinnon. Trunk muscle electromyographic activity with unstable and unilateral exercises. Journal of Strength and Conditioning Research 19(1): 193-201, February 2005.

This study compared the effect of exercises performed on unstable surfaces, and also unilateral exercises, on trunk muscle activation. The scientists chose six trunk exercises, along with unilateral and bilateral shoulder and chest presses performed on a bench and on an exercise ball.

In the trunk exercises, instability led to a 27.9-percent greater activation of the lower-abdominal muscles. The exercise ball chest press increased trunk stabilizer activation by as much as 54 percent. One-arm shoulder presses increased the use of back stabilizers, and the one-arm chest press increased recruitment of all trunk stabilizers. Surprisingly, instability did not affect trunk stabilizers in the shoulder press.

Regardless of stability, the "Superman" exercise was the most effective trunk-stabilizer exercise for back-stabilizer activation;* the side bridge was best for lower-abdominal muscle activation. The study concluded that unstable surfaces were effective for abdominal exercises, and that unilateral "upper-body" exercises were also effective core strengthening exercises.

*Further studies reported by Stuart McGill in his book Low Back Disorders (Human Kinetics: Champaign, Ill., 2007): Evidence-based prevention and rehabilitation indicate that the "Superman" exercise yields excessively high compressive forces in the spine. The moral? When looking at studies concerning core training, you also need to consider compressive and shear forces on the spine. Don't sacrifice joints and bones to focus on muscles.

Why add spinal stabilization exercises to a routine?

Koumantakis, G.A., P.J. Watson and J.A. Oldham. Supplementation of general endurance exercise with stabilization training versus general exercise only: Physiological and functional outcomes of a randomized controlled trial of patients with recurrent low-back pain. Clinical Biomechanics (Bristol, Avon) 20(5): 474-82, June 2005.

When a stabilization exercise group was compared to a general endurance-based exercise group, the stabilization group received equal benefits for low-back pain. Both groups improved paraspinal muscle strength equally. The research team concluded that exercise, regardless of the type, was the prime determinant for improvement. The addition of spinal stabilization exercises did not appear to help prevent further episodes of low-back pain.

Core training and back pain

Nadler, S.F., G.A. Malanga, L.A. Bartoli, J.H. Feinberg, M. Prybicien and M. Deprince. Hip muscle imbalance and low-back pain in athletes: Influence of core strengthening. Medicine & Science in Sports & Exercise 34(1): 9-16, January 2002.

Despite the inclusion of a core strengthening program focusing on abdominal, paraspinal and hip extensor strengthening, the athletes in this study showed no statistically significant change in the occurrence of low-back pain. Don't give up on your clients' core training program yet, though. The researchers mentioned, "this may be more a reflection of the small numbers of subjects who actually required treatment."

How useful is the latissimus dorsi?

Bogduk, N., G. Johnson and D. Spalding. The morphology and biomechanics of latissimus dorsi. Clinical Biomechanics 13(6): 377-385(9), September 1998.

Prior to this study, some hypothesized that the latissimus dorsi (lats) could extend the lumbar spine and help stabilized the sacroiliac joint. After examining the morphology of the latissimus dorsi, these researchers concluded, "Its possible contribution to extension of the lumbar spine is trivial, as is its capacity to brace the sacroiliac joint."

Identifying core function

Vleeming, A., A.L. Pool-Goudzwaard, R. Stoeckart, J.P. van Wingerden and C.J. Snijders. The posterior layer of the thoracolumbar fascia: Its function in load transfer from spine to legs. Spine 20(7): 753-758, April 1, 1995.

These scientists studied the effect of traction to various muscles - such as the lats, gluteus maximus, spinal erectors and biceps femoris - attached to the thoracolumbar fascia. Below the level of the L4 vertebra, tension in the posterior layer was transmitted to the opposite side. In some of the cadavers, this phenomenon occurred below the level of L3 and even L2. The researchers concluded, "Anatomic structures normally described as hip, pelvic and leg muscles interact with so-called arm and spinal muscles via the thoracolumbar fascia. This allows for effective load transfer between spine, pelvis, legs and arms - an integrated system."

This study reinforces the notion that one of the functions of the core is to transfer forces not only from the lower body to the upper body (and vice versa), but - more specifically - from one side of the lower body to the opposite side of the upper body (and vice versa).