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What Is Core Stability Exactly? Part 1.

The purpose of this article is to provide insight into what core stability actually is and how it can impact your training. A lot of progress has been made in recent years on the concepts of stability in musculoskeletal linkages and joints...

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Over the course of the last two weeks I have tried to provide a simplistic and workable explanation of what core stability truly is. From the first article the reader should understand that the spine and muscles around it are comparable to the rigging on a ship's mast.


Muscle balance and proper motor control must be attained as well. This must be attained to prevent injury and to reach maximal performance. This week I want to begin to look at several exercises commonly prescribed for improving core stability.

When one is selecting the ideal exercise several factors must be considered. The best exercise requires evidence on the loads the tissues are subjected to and a knowledge of how injury occurs in those specific tissues.[1,2,3] The best exercise for each individual is the one that will maximally load the specified tissues and apply the least amount of load to the joint (the spine in this case). I state for each individual because certain exercises will be of benefit to some and cause injury to others. Also, an ideal exercise will help to improve spine stability in as neutral spine position as possible and require additional elements of whole body stabilization as well.[4]

I am going to begin by looking at one of the most prescribed exercises for the back extensors (erector spinae). In my practice I see a lot of patients who have seen a number of physical therapists and trainers before they reach my office.

It's A Bird, It's A Plane, It's ...

One exercise that shows up again and again in their prescription is called "the superman." The exercise has other names but this is the most common. It is performed by laying flat on your stomach and raising the upper body and legs off the floor. In this exercise the lumbar spine pays a very high compression penalty to a hyperextended spine (approximately 4000 to 6000 N) which transfers load to the facet joints and crushes the interspinous ligament.[5] This exercise is certainly contraindicated for anyone at risk of low back injury or re-injury due to the high spine loads and the extended posture.[6] In my opinion it should not be prescribed at all.

Moving to the abdominals two commonly performed exercises are sit ups with bent knees and the "press the heels sit up" (commonly referred to as the Janda sit up). People have been told that they should perform sit ups and other flexion exercises with the knees bent to disable the hip flexors (psoas). Many have hypothesized that this reduces compressive load on the lower back by disabling psoas or by changing the line of action of the psoas. MRI based data demonstrated that the psoas line of action does not change due to lumbar or hip posture (except at L5/S1) as the psoas laminae attach to each vertebrae and "follow" the changing orientation of spine, not of the hips or knees.[7]

There is no debate as to whether psoas is shortened with the hip flexed, but is there a reduction in load to spine with the legs bent? McGill examined 12 young men and observed no major difference in lumbar load as the result of bending the knees.[8] He measured the compressive loads to be in excess of 3000 N. This definitely raises the issue of safety. Certainly anyone with low back injury or risk of re-injury would be wise to avoid the bent knee sit up.

Be Careful

Also it brings up the question of whether anyone should perform the sit up. Next sit up is the "press the heels" made famous by Czech neurologist Vladimir Janda. The exercise is performed like a typical bent knee sit up except the feet are dorsiflexed and the heels are then pressed into the floor. It has been hypothesized that by pressing the heels into the floor and activating the hamstrings this would neurally inhibit the psoas. Thus there would be increased load on rectus abdominis and decreased low back loading.

Actually EMG data showed that by pressing the heels into the floor the psoas activation actually increased![6] Athletes who are intentionally trying to train psoas will find this information informative. Those with low back injury or risk of re-injury would be wise to avoid the "press the heels" sit up.

I hope this helps some of you in your exercise selection process. The achievement of maximal performance is a worthy quest, but not at the expense of one's health. Keep this in mind the next time you are trying to decide which exercises are right for you. Also make sure you are evaluated by a competent physician or therapist who can help you select the proper exercises.

Vivo in Vitro Out

It is important for the reader to understand that there are limitations in the scientific approaches for investigating tissue loading in vivo and in vitro. The low back system is an extremely complex mechanical structure and direct measurement of tissue forces in vivo is not feasible.

According to Dr. McGill, the only tenable option for tissue load prediction is to use sophisticated modeling approaches. If interested in a description of the scientific methods used, please consult Juker and colleagues,[1] McGill,[2] and Cholewicki and McGill.[3]

One exercise that has been around for quite some time is the straight leg raise. I can vividly recall performing endless reps of these in junior high track practices. I can also remember how much I hated doing them! It always seemed that one of my teammates would let their heels fall below six inches from the floor, which led to more straight leg raises until everyone got it right.

Performance of the exercise is quite simple for those of you who have not been subjected to the torture of it. Lay down flat on your back with your legs straight and raise the heels off the floor and hold. The spinal loads are so high that sit ups are contraindicated for anyone with low back injury or risk of re-injury. The high spinal loads associated with this exercise make it a questionable exercise for those with a healthy back as well.

The data indicates that not only is the psoas activity higher in the straight leg raise than the bent knee sit up, but spinal compression is higher as well.[4] Although the psoas is often claimed by many to be a key stabilizer of the lumbar spine, the data does not support these claims. The activation profile of the psoas according to EMG data indicates that the role of the psoas is purely as a hip flexor.[1]

So why would anyone select the straight leg raise to improve core stability when this exercise mostly challenges psoas which appears to play no role in the stability process and at the same time applies extremely high loads to the spine? My opinion is to stay away from the straight leg raise. As always, I encourage you to review the studies and decide for yourself.

Crunch This

The last exercise I will discuss is probably the most commonly performed exercise, the proverbial crunch. Everyone has performed the crunch at some time, so I assume no explanation of technique to be necessary. Many have not only performed the crunch, but have performed hundreds of thousands or more of them. If this applies to you, then you may want to leave the room for this part of the discussion!

First of all, it has been proven by calibrated intramuscular and surface EMG evidence that the crunch is very effective at challenging the rectus abdominus.[1,5] Important to our discussion is that rectus abdominus does play a part in core stabilization.[6]

So, what is the problem with the crunch then? I thought you would never ask! After a personal conversation I recently had with Dr. Stuart McGill, I believe there is a high risk for disk injury in performing crunches. Dr. McGill told me that in lab studies, the most reliable way to injure disks was to expose them to repetitive end range flexion in a cyclic manner. Now, think about how most people perform a crunch.

Everyone that comes into my office and shows me how they do crunches go into full spinal flexion. Not only do they go into full spinal flexion, but they perform hundreds or thousands of reps a week. Thus, we have a full spinal flexion (end range) exercise performed in a cyclic manner.

Study Up

A review of the literature reveals that it is likely that the disk must be bent to full end range of motion to be herniated,[7] and that the risk is higher with repeated loading.[8,9] Maybe the crunch is not as good as most would have you believe.

Many of you are probably beginning to wonder what exercises are left for the abdominals if bent knee sit ups, "Janda" sit ups", straight leg raises, and crunches are not safe for the back. Remember that health concerns should always take precedence over performance. Stay tuned as I will introduce some exercises that not only challenge your abdominals, but will improve your core stability as well.

Check Back Soon For Part Three!


1. Juker D, McGill SM, Kropf P, Steffen T. Quantitative intramuscular myoelectric activity of lumbar portions of psoas and the abdominal wall during a wide variety of tasks. (In press) Med Sci Sports Ex.

2. Callaghan JP, Gunning JL, McGill SM. Relationship between lumbar spine load and muscle activity during extensor exercises. Physical Therapy 1998, 78:8-18.

3. Axler CT, McGill SM. Low back loads over a variety of abdominal exercises: Searching for the safest abdominal challenge. Med Sci Sports and Exerc 1997; 29:804-811.

4. McGill Sm. Stability: from biomechanical concept to chiropractic practice. J Can Chiro Assoc 1999; 43 (2) 80.

5. Callaghan J, Gunning J, McGill SM: Relationship between lumbar spine load and muscle activity during extensor exercises. Phys Ther 78 (1):8-18, 1998.

6. McGill SM. Biomechanics of the Thoracolumbar Spine. Clinical Biomechanics. Churchill-Livingston, 2000.

7. Santaguida L, McGill Sm. The psoas major muscle: a three dimensional mechanical modeling study with respect to the spine based on MRI measurement. J Biomech 1995; 128(3):339-345.

8. McGill SM. The mechanics of torso flexion: situps and standing dynamic flexion manoeuvres. Clinical Biomechanics 1995; 10(4):184-192.

9. McGill SM: A myoelectrically based dynamic 3-D model to predict loads on lumbar spine tissues during lateral bending. J Biomech.

10. Cholewicki J, McGill SM: Mechanical stability of the in vivo lumbar spine: Implications for injury and chronic low back pain. Clin Biomech 11(1):1, 1996.

11. McGill SM: A revised anatomical model of the abdominal musculature for torso flexion efforts. J Biomech 29(7):973. 1996.

12. Adams MA, Hutton WC: Prolapsed intervertebral disc: A hyperflexion injury. Spine 7:184, 1982.

13. King AI: Injury to the thoraco-lumbar spine and pelvis. In Nahum AM, Melvin JW (eds): Accidental Injury, Biomechanics and Presentation. New York. Springer-Verlag, 1993.

14. Gordon SJ, Yang KH, Mayer PJ, et al: Mechanism of disc rupture: A preliminary report. Spine 16:450, 1991.