Core stability from the inside out

The DNS approach to core stabilization
By Hans Lindgren, June 14, 2011

Existing theories

I would like to start off by mentioning that I am very reluctant to use the term core-stability. The core-stabilization concept has been so extremely over-used and misunderstood for the last decade that many people are really starting to question the value of it. I am only using the term “core” here because it is universally accepted. The fact is that a massive amount of time and energy have been invested into core-training. It has been regarded as a vital part of functional rehabilitation programs, and the health and fitness industry has been promoting it heavily and developing their own systems for training the core. Many athletes have taken valuable time out of their training regime to strengthen the core and yet almost every time a research study has been performed to evaluate the effect of core-training the result has not been favourable at all. There have been comments like this:

“There were no significant correlations between core stability, functional movement and performance. Moderate to weak correlations identified suggest core stability and FMS are not strong predictors of performance. In addition existent assessments do not satisfactorily confirm the importance of core stability on functional movement. Despite the emphasis fitness professionals have placed on functional movement and core training for increased performance, our results suggest otherwise. Although training for core and functional movement are important to include in a fitness program, especially for injury prevention, they should not be the primary emphasis of any training program”. (29)and:

“The clinical outcomes of core-strengthening programs have not been well researched. Studies are hampered by the lack of consensus about what constitutes a core-strengthening program. Some describe remedial neuromuscular retraining whereas others describe sports-specific training and functional education. To our knowledge, no randomized controlled trial exists on the efficacy of core-strengthening. Most studies are prospective, uncontrolled, case series”. (1)

Additionally we have got “The myth of core stability” (24) by Eyal Lederman which more or less attempted to tear the whole concept of core-stabilization apart with the backing of a long list of quality references.

Core stabilization more or less started with the theory of Transversus Abdominis and the abdominal hollowing exercises after research by Hodges et al showed that the Transversus Abdominis is activated prior to any voluntary movement of the limbs occurring and that people with back-problems had delayed onset of activation of Transversus Abdominis and Multifidus muscles. Suddenly all exercises were supposed to be performed with abdominal hollowing to activate the deep core.

Instability training was introduced, and it was said that to be able to counter-act the instability the abdominals and the deep back muscles would have to work harder, and thereby the core would get much stronger and keep the spine stabilized. One study (25) showed that only the Rectus Abdominis and Erector Spinae showed increased activity when performing exercises on a Swiss-ball. This actually indicates that the core was not activated at all, since these muscles were trying to take the load off. The use of an unstable base of support has been questioned because the work-load is dramatically reduced since a much lower resistance is possible to handle. Flanagan et al (6) stated that greater activation of the core musculature appears to occur by lifting a heavier weight overhead than by lifting a lighter weight overhead with an unstable load, or on an unstable surface.

All along there have been strong voices coming from the camp of strength athletes that specific core exercises are unnecessary if you just squat and dead-lift with decent weights. The Pilates methods have also emerged as a popular and effective alternative for core stabilization. Pilates emphasizes diaphragm breathing combined with Transversus Abdominis activation for core stabilization.

Vera-Garcia and colleagues (33) have shown that abdominal bracing is superior to abdominal hollowing in stabilizing the spine.  Abdominal bracing is the tensing of the entire abdominal wall as if to protect you from being punched in the stomach.  Is Abdominal bracing the answer to core-stabilization? Bracing certainly increased trunk stability, but at the cost of increasing spinal compression load.

Why the core concept has not been working

The first obvious reason why the core stability concept has failed is the lack of a proper definition of what core stability involves. Every author and self-claimed guru has their own definition and list of muscles involved in the core-stability phenomenon. Overall, there has been a tendency to put an anatomical description on the stability function-however Stability is a function not a structure. All the present theories are looking at the core from the outside in, with. The emphasis is on tightening and working the abdominal muscles and back muscles, which is still treating the torso like a hollow tube. The definitions of a core are: centre, nucleus, middle, interior and kernel.

Bracing is still only tightening the outside of the core. During a study of the therapeutic effect of the “big-three” exercises (plank, side-bridge and bird-dog) by McGill (27), a surprising observation was that the stabilization actually decreased when taking a deep breath. Another interesting comment from this study was: “Further, subjects showed varying muscle activation linked to inspiration and expiration, indicating that some patients can entrain their respiratory muscles to function independently of their spine-stabilizing function.”

A new paradigm of core stability:

At this point it is a good time to look at two interesting diaphragm studies by Kolar et al

  1. Dynamic MRI demonstrated that the diaphragm has got a postural function and that individuals are capable of moving involuntarily. The study showed that the diaphragm’s postural position is lower or similar to that of tidal breathing in 81% of the subjects. The diaphragm ROM during both respiration and postural activities differ among individuals. There is a significant correlation between the respiration volume and the diaphragm ROM during tidal breathing.Results of this study indicate that the diaphragm does not participate as one single functional unit in stabilization but different segments can be activated to different degrees. (20)
  2. Dynamic MRI demonstrated the diaphragm’s dual function (ventilation and posture) and that they can be performed simultaneously. The diaphragm can achieve its respiration function from a lowered position to ensure sufficient Intra-abdominal pressure is produced when required for a postural task. (21)

Comments from the authors: “There exists an individual ability to control the postural function of the diaphragm. This supports our clinical evidence that the activity of the diaphragm during stabilization varies greatly among individuals. Individuals with limited capability to contract their diaphragm for stabilization of the body may have higher likelihood for development of back pain. The respiratory movement of the diaphragm is synchronized with its stabilization function. Dysfunction of this synchronization (Insufficient and uncoordinated diaphragm activation) in people with weak body-stabilizing function of the diaphragm leads to overloading of spinal segments”.

Chronic lower back sufferers have a different position of the diaphragm not enabling them to properly activate the core (IAP). Both chest and belly breathing are signs of dysfunctional diaphragm position and function.

How would the theory that core-stabilization comes from the inside out and is generated by the synchronized activation of the diaphragm fit in with other available research studies?

Hodges et al (10) were on track with the dual function (breathing and posture) of the diaphragm already in 1997. They showed that the diaphragm, together with the Transversus Abdominis, Multifidus and pelvic floor muscles, were activated for stabilization prior to moving the limbs. The whole world went Transversus and Multifidus crazy. Why did the diaphragm get so completely ignored?

Hodges and colleagues (11)have also measured movement in the lumbar spine when IAP was transiently increased by electrical stimulation of the diaphragm via the phrenic nerve. The conclusion of the study was that when IAP was artificially increased to 15% of maximum IAP amplitude that could be generated voluntarily with the trunk positioned in flexion, a trunk extension movement was recorded. Results provided the first in vivo data of the amplitude of extensor moment that is produced by increased IAP.

Hodges and Gandevia (13) found that continuous rapid movement of the upper limbs activated the diaphragm posturally in a manner unrelated to the breathing function.

It has been showed that weight-lifters (26) and other muscular individuals performing heavy manual labour (3) have hypertrophy of their diaphragms and their transdiaphragmatic pressure (Pdi) is greater than that of less muscular people.

Al-Bilbeisi and McCool (2) reported increased diaphragm activity, evidenced by increased transdiaphragmatic pressure (Pdi), during various weight-lifting activities. The subjects were told to breathe in during the exertion phase in order to develop a higher and more constant transdiaphragmatic pressure. They concluded that during a range of weight-lifting activities the diaphragm is recruited and the transdiaphragmatic pressure is raised to a level that may provide a significant strength-training stimulus to the diaphragm.  They also stipulated that the observation that Pdi increases with weight-lifting activities supports the idea that the diaphragm is an important postural muscle.

DePalo and colleagues (4) used sit-ups and biceps curls to train the diaphragm for 16 weeks and concluded that nonrespiratory manoeuvres can strengthen the inspiratory and expiratory muscles in healthy individuals and because diaphragm thickness increased with training the increase in maximal pressures is unlikely to be due to a learning effect.

“Core exercises” (sit-ups and leg lifts) have been shown to provide training stimulus for the diaphragm (32) where inspiration against a contracted abdominal wall improved the strength of the diaphragm (increased transdiaphragmatic pressure). It was also said that the study provided an insight into diaphragm recruitment during different core exercises.

“Recent studies have revealed that not only does the diaphragm receive feed-forward drive prior to some limb movements, but that it also contracts both phasically and tonically during repetitive limb movements. Thus challenges to posture can indirectly challenge ventilation, while coordinated diaphragm contraction may contribute to control of the trunk.”(7)

“Results show that coactivation of the diaphragm and abdominal muscles causes a sustained increase in intra-abdominal pressure, whereas inspiration and expiration are controlled by opposing activity of the diaphragm and abdominal muscles to vary the shape of the pressurized abdominal cavity”(14)

 “We found that that thickening and shortening were greatest during a breath taken with the abdomen. The measured diaphragm muscle lengths from the chest radiographs show almost twice as much shortening with abdominal breath. This was also evident in the ultrasound studies.” “The abdominal breathing pattern presumably caused near-maximal shortening of the diaphragm in our studies.”(34)

 “Rapid repetitive movement of the upper limb during apnoea (breath holding at end-expiration) also activated the diaphragm. There was a large variation in level of activation between the subjects.” (13)

 “The findings of this study suggest that high-intensity inspiratory muscle training results in increased contracted diaphragm thickness and increased lung volumes and exercise capacity in people who are healthy”.(5)

“The inspiratory muscles, including the diaphragm, are morphologically and functionally skeletal muscles and therefore should respond to training in the same way as would any locomotor muscle if an appropriate physiological load is applied” (23)

Analyses indicated that IAP has a substantial spinal unloading effect for all directions of generated external movements. The unloading results from an extension moment generated by the IAP that exceeds the flexion moment generated by the abdominal wall activation forces. These findings support the idea that intra-abdominal pressurization is beneficial because it unloads the spine.

“The result of this study provides evidence that the stiffness of the lumbar spine is increased when IAP is elevated” (12)

“The Inhalation-hold form of breath control produced significantly greater peak intra-abdominal pressure than all other forms of breath control. Breath control is a significant factor in the generation of IAP magnitude during lifting tasks. The effect of respiration should be controlled in studies analysing intra-abdominal pressure during lifting”. (9)

When examining the natural breath control of subjects during lifting tasks, Hagins and Lamberg (8 ) noted that amongst  the subjects there were significant increases in magnitude of inspired volume and the frequency of occurrence of inspiration immediately prior to lift-off. When examining the effect of load there was a significant increase of inspired volume and occurrence of breath-holding when lifting the heavy load compared to the medium and light loads. Their results suggest that distinct patterns of natural breath control occur during lifting tasks and breath control is responsive to the timing and the magnitude of the load lifted.

 “The abdominal muscles are often recruited during activities involving the upper extremities and trunk such as when lifting heavy objects. Contraction of these muscles increases intra-abdominal pressure. A benefit of increasing intra-abdominal pressure during weight-lifting activities is that it lessens the axially directed compressive forces on the spine that are associated with these manoeuvres. However, if the glottis is closed and the diaphragm is passive during these activities, the elevated intra-abdominal pressure is transmitted to the thorax. High intra-thoracic pressures, in turn may have adverse hemodynamic and central nervous system effects such as decreasing venous return from the extremities, increasing systemic blood pressure and increasing CNS pressure. To avert these complications the diaphragm may be recruited during strenuous truncal activities. A tensed diaphragm will minimize or avert the rise in intra-thoracic pressure that occurs when the abdominals are forcefully contracted. Thus, by recruiting the diaphragm, one can preserve the benefits of increased intra-abdominal pressure in minimizing the compressive forces on the spine while avoiding the complications related to high intra-thoracic pressure. In our subjects, intra-thoracic pressure remained subatmospheric even when gastric pressures were elevated to level greater than 100cm H2O. In addition, activating the diaphragm not only maintained intra-thoracic pressures at subatmospheric levels but also allowed the individual to breathe during the activity.” (2)

The synergy between the diaphragm and the abdominal wall has been addressed earlier. From (10) “contraction of the abdominal muscles contributes to trunk stability prior to and during movement of the limbs (Hodges and Richardson 1997 a,b) and this action is increased when respiratory demands increase (Hodges, Gandevia & Richardson 1997)”

 “The result of this study indicates that contraction of the diaphragm contributes to increased intra-abdominal pressure prior to the initiation of movement of large (but not small) segments of the upper limb. The contraction is independent of the phase of respiration. This provides the first direct evidence that the diaphragm may contribute to the postural control of the human trunk in addition to its role in respiration. Furthermore, the findings show that this preparatory contraction of the diaphragm is associated with initial shortening of its muscle fibres and occurs simultaneously with activation of transversus abdominis. “From (10) again

One more from (10) “Two previous human studies have provided indirect evidence of a contribution of the diaphragm to postural control. These studies documented contraction of the diaphragm prior to contraction of rectus abdominis in preparation for rising onto the toes (Skladal, Skarvan, Ruth & Mikulenka 1969) and a close relationship between trans-diaphragmatic pressure and intra-abdominal pressure with lifting (Hemborg et al 1985).”

Okay just one more from (10). “The contraction of the diaphragm associated with limb movement cannot be mediated by a spinal or supra-spinal reflex response to the arm movement since it precedes the motion of the arm and it is too early to be a response to contraction of other posturally activated muscles such as those in the leg. Thus, the response in the diaphragm must be pre-programmed by the CNS and may be initiated as part of the motor command for movement (Cordo & Nashner 1982; Horak, Esselman, Anderson & Lynch 1984; Bouisset & Zattara 1987)”

“For large loads in the hands most subjects appeared to stabilize the trunk with large muscle forces relegating the responsibility of creating lung air-flow to the diaphragm. When reasonable small low-back demands were coupled with a breathing challenge and higher ventilation rates two out of eight subjects demonstrated entrainment of abdominal activity to breathing that resulted in additional cyclic low-back compression loading of the order of 1000N. Ergonomists should consider the additional tissue loading from the physiological demanding tasks and the related ventilation challenge, together with the tissue loads required to support external objects and maintain body posture.” McGill (28)

Core stabilization from the inside out

Synchronization of the diaphragm’s dual functions is the key for activating the core from the inside out. DNS (Dynamic Neuromuscular stabilization) teaches methods for testing and activating the proper diaphragm function and the resulting IAP. For information about DNS see: www.rehabps.com

Testing and activation of synchronized diaphragm contraction will be covered in another article.

Once an individual has learnt to activate the diaphragm functions properly almost all activities would pass as a core exercises. Add proper diaphragm activity synchronized with activation of the entire abdominal wall and the pelvic floor to all existing “core exercises” and the effect will be dramatically improved. Exercises which previously only worked the surrounding layers of the abdominal wall through bracing activities will take on a completely new dimension of stabilization once the inside of the core is properly filled and pressurized. The previous theories of muscles supporting the spine like guy-wires are good, but why not add an internal air-bag to the equation as well? Proper core activation from the inside out validates most other previously existing core theories. Instability exercises would work, the stability would increase when taking a deep breath in when performing the “big 3” or similar exercises and heavy squats and dead-lifts would really work the core.

The only method that still would be counter-productive is the abdominal hollowing. The Transversus Abdominis together with the rest of the abdominal wall are supposed to perform an isometric or eccentric contraction when the core is properly activated. Too early or too strong activation of the abdominals would interfere with the diaphragms ability to contract properly and thereby reduce stabilization. Synchronization of the diaphragm’s dual functions will play a vital role in rehabilitation as well as sports-performance programs.

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