PT on the Net Research

The Core - Part 1

This article should go some way to answering some of those questions that you have, it will help you to achieve better results for yourself and your clients and to formulate your own informed opinion with regard to TVA function.


We’ve all read the figures suggesting that more than 80 percent of us will suffer debilitating back pain at some stage in our adult lives. We all know that low back pain is the single leading cause of time out of work – far out-shooting even the common cold! So what’s the relevance of all of these figures to us as fitness, conditioning and healthcare professionals? Well, there are several relevant issues:

  1. We work with people, 80 percent of whom will suffer with low back pain at some time in their lives.
  2. These people are most likely to suffer low back pain when under stress.
  3. Stress is cumulative and can therefore not only build over the months and years, but it can come from a diversity of sources (physical, emotional, hormonal, environmental and biochemical/nutritional).
  4. Many people forget that exercise is a major stress physically and hormonally – and often, they use exercise to “treat their stress.”
  5. We therefore have the potential to bring on an episode of low back pain if we mindlessly push our clients through their usual programs or if we fail to assess their individual needs at each training/rehabilitation session.
  6. On the contrary, this means we also have the ability to avoid such a situation or to help them if they DO come into our gym or clinic in pain.

So the possibilities for us are manifold:


In our increasingly litigious environment, we may be in line for blame if we fail to recognize a problem and continue to exercise a client inappropriately for their condition. Problem recognition or injury vulnerability/susceptibility/predictability are not difficult to spot - when you know what you’re looking for and understand some basic principles.


We will find ourselves in the situation where we are sought after for our expertise after a problem has already occurred. In such an instance, the ability to assess and interpret is vital (whether it is you doing the assessment or being able to interpret another expert’s assessment).


Of course this is the ideal – an ounce of prevention being better than a pound of cure. However, in some ways, this is the most difficult area for you to work in. To be effective in preventing injury, you need to have a highly comprehensive assessment system to evaluate all of the potential stressors described in #3 above:

Furthermore, you need a good enough understanding of such stressors to be able to effectively sell your service to the client. After all, how many people think far enough ahead to actually want to prevent injury such as back pain? Most people purely react to their circumstances, and those that are most likely to want preventive work are those who have already experienced a severe bout of pain or dysfunction. The information contained in this article should help you to understand more thoroughly the role of the TVA within the function of the core and therefore help to make more informed decisions in managing your clients.

The Core

One of the most exciting developments in exercise and rehabilitation over the last decade or more has been the concept of core stability. Ever since it was realized that movement emanates from the core, many methods of core training have become commonplace in gyms and on training courses. However, its popularized status has resulted in a great level of misunderstanding and misconceptions in terms of what core stability actually is, how to effectively assess it and how to interpret your results and apply them to a training/rehabilitation program.

Since the early 1990s, there has been greater information available to us regarding the core musculature and its role in the stabilization of the spine. This focus has been largely in response to the fact that low back pain is both the leading cause of time off work, and therefore, its cost to industry in lost revenue is dramatic.

Figure 1: The abdominal wall. The transversus abdominis is the deepest layer, and mainly runs transversely around the abdomen like a corset. The obliquus internus runs from the thoracolumbar fascia posteriorly and up toward the rib cage anteriorly. While the obliquus externus courses from the rib cage posteriorly down toward the pelvis anteriorly. All three layers of the abdominal wall interdigitate with the abdominal aponeurosis enveloping the rectus abdominis anteriorly. Reproduced with permission from Chek (1996).

Many therapists have been quick to take up this new information and run with it, without questioning its validity or taking a step backward to see how this might fit into other models of health care. Others have been quick to denounce this research as some kind of fad. Very few, however, have a full grasp of how core stability fits as an integral part of our function as a living organism. One objective of this article is to look at where this new and valuable research has its limitations and to consider how it can best be applied to a more encompassing health care model.

This great attention in the manual therapy and in the fitness world on the core has focused in particular on the role of the transversus abdominis (TVA) and its function in stabilizing the lumbar spine. However, as Siff and McGill have pointed out, there is much more to stability of the lumbar spine than the transversus abdominis. Caught up in the excitement of new findings, many researchers have perhaps over-focused on the “TVA,” thereby limiting their training/rehabilitative scope. Focus on any single subject area, to the exclusion of others, potentially results in methodological flaws or inefficiencies in training and rehabilitation protocols.

Siff quite correctly states that some of what has been written in this field does not accurately reflect what is known of spinal stability. A classic example of this would be the fact that many papers and subsequently many therapists have focused so intensely on transversus abdominis function on the couch that they forget the client has to stand up and move around while practicing their new skill of TVA contraction. This is one of many common oversights we will discuss, made by many course instructors, therapists and trainers, perhaps for good reason or perhaps because they don’t understand how the body moves in motor patterns.

Siff also poses the question: can anyone justifiably state that any single muscle is involved more in trunk stabilization than another? Again, another valid point, but perhaps one that requires some level of informed discussion. What Siff was referring to is the concept that the TVA is seen by some therapists as the “be all and end all” in trunk stabilization. Of course, this is not the case, as the TVA is merely a component of a much larger muscular system which, in turn, is only a component of a much larger stability system. The stability system is just one system in a “system of systems” – the cybernetic organism that is the human body.

The Local and Global Stability Systems

The term “local system” is one that was coined by Bergmark in 1989 and has been adopted by researchers from Australia where the cutting edge of this research has emanated. This local stability system, also known as the inner unit, is used to describe muscles whose primary role is to stabilize a joint, whereas the term global stability system, or outer unit, describes the muscles whose primary role is to mobilize a joint and provide gross stabilization.

Inner unit muscles typically are unisegmental (span one joint) and often have a preponderance of slow twitch postural fibres, while outer unit muscles tend to be multisegmental (span a number of joints) and have a preponderance of fast twitch fibres. It is clear from discussion in the academic press that these systems are classifications that are generally agreed upon but are not incisive in their definition or parameters. Welcome to the human body!

Figure 2

Local System/Inner Unit Global System/Outer Unit
Sling systems of the body, e.g:
TVA Anterior sling
Diaphragm Posterior sling
Multifidus Deep longitudinal sling
Pelvic Diaphragm Lateral system
(Deep Fibres of Psoas Deep P-A sling
(Medial Quadratus Lumborum) Deep diagonal sling

Figure 2: The inner unit and outer unit systems of trunk stabilization. The concept of sling systems is one that is well accepted in the literature and can be very useful clinically. However, the concept of muscular, slings, chains or “trains” (as popularized by Myers) should only be viewed as a useful model; a better model to describe these functional pathways are contractile fields within which the slings are incorporated.

As an example of the way each tissue is often a component of different systems, let’s take the interaction between respiratory diaphragm and the TVA. The diaphragm is known to form an important component of the local stability system of the low back. It generates intra-abdominal pressure, and through the counter-force of the non-compressible viscera*, a decompressive lift is created via the crura of the diaphragm (attaching to L2-3) through the lumbar spine. However, the diaphragm’s primary role is in respiration. The transversus abdominis is also a muscle of respiration, though this is not its primary role; it is more important in stabilization of the spine. Therefore, the diaphragm chiefly has respiratory function and provides musculoskeletal stability as a secondary function, while the transversus abdominis is primarily musculoskeletal in nature but is respiratory as a secondary function.

Figure 3: Co-contraction of the transversus-palvic/respiratory diaphragm complex: results in compressionof the non-compressible viacera - therefore they can only "escape" through pressing upward into the respiratory diaphragm creating life and therefore decreasing compressive forces through the lumbar spine. Figures taken for CHEK (2001) "Abs in or Out" article.

Essentially, for most researchers who look at the function of this muscle, the TVA is seen as a muscle of stabilization. But, as we will show, the TVA is clearly important in the generation of power as well as being critical in the efficient function of many different bodily systems. Consequently, if the TVA is dysfunctional, it will compromise any system it is a component of while, conversely, it will support the function of all systems it is a component of if it is working well. Additionally, the TVA can be affected by any one of these systems if they are not working properly.


Recognizing the human body as a cybernetic organism is a realization many researchers have failed to make as, by convention, they have been conditioned to refine and reduce – to look down the microscope – in order to keep their research protocols academically acceptable. However, this mode of thinking has come at a cost. There is a sceptical saying about the reductionist paradigm used in most research, which states: "We gradually know more and more, about less and less, until eventually we’ll know everything about nothing.” But this is perhaps a little unfair. The reductionist paradigm most researchers use should not negate the benefits of their work. It merely highlights the importance of keeping a broad and encompassing mind when reading and applying their work. In this context, any research can be useful to the rehabilitation and/or exercise professional, so long as it is read from an open, cross-disciplinary perspective. Some of the research flaws are discussed further on in this article.


The very fact that we use the term “systems” to describe the various muscles responsible for either stabilizing or mobilizing us implies that in order to stabilize or mobilize requires several component parts. Stabilization cannot be achieved by one muscle alone. According to the Oxford English dictionary, the definition of a system is: “A complex whole: a set of connected things or parts; an organised body of material or immaterial things.”

This last point is important in our line of work, as although we may often think of the body as a collection of connected material things, we often forget that the immaterial – such as thoughts, words and deeds - are just as much part of our “system” as the material muscles, organs and joints. This line of thinking has been assessed by Mayers et al, Liebenson and others from the perspective of “pain behaviour” or illness behaviour in motor control. This is a highly significant, yet immaterial, part of how our clients’ systems functions.

Indeed, the integration of the motile parts was the premise that Bobath worked upon in her highly successful approaches to neurological rehabilitation. She focused on training movements, not muscles, and maintained that activity of individual muscles and muscle groups was secondary to their co-ordination in patterns of activity. These are the fundamentals that have been adopted and expanded upon by many successful rehabilitation and exercise specialists.

Anyone who understands systems theory will know that one of the most fundamental rules governing systems is that any system is only as good as its weakest link. A Ferrari with a set of Volkswagen Beetle brakes may initially be able to go as fast as any other Ferrari, but this won’t be the case for very long. As soon as its engine starts to outperform the capacity of the braking system, it is likely to crash. Likewise, for a local stability system to work effectively, it must have each of its component parts working effectively. Equally, just as it is risky but possible to move quickly in our Ferrari with a substandard braking system, it is risky but possible for the global stability system to move the body with a substandard local stability system. As you will see later, there are many, many people – particularly in Western society - who are riding their Ferrari around with Volkswagen Beetle brakes, and are accidents waiting to happen!

To carry the Ferrari analogy slightly further, the outer unit system, being primarily a mobiliser system, is a fast twitch system. It moves you quickly, just like a Ferrari. This means it drinks a lot of “juice” – working predominantly in the fast glycolytic and fast oxidative energy systems.

Figure 4: TVA dysfunction is akin to a faulty breaking system in a car. Any muscle which exhibits inhibition or dyssynergic contraction will create faulty and/or suboptimal movement patterns. The body can compensate for this for a short while, but will eventually come unstuck, resulting in injury and degenerative change; just like a car with a suboptimal braking system.

It has been suggested that the TVA is not an essential component of trunk stability and that the body can effectively compensate for TVA dysfunction, and this is a fair point. But if the outer unit system is used to compensate, it will fatigue early and subsequent injury is more likely. It’s important to understand that a compensation is still just that; “a compensation” – it is a compromise. This not only means sub-optimal performance –in activities of daily living (ADLs) or in sports, but also increased risk of injury. In the manual therapy literature it is well documented that whenever there is compensation, if uncorrected, across time that compensation will turn into what’s termed “decompensation.” This is the point at which symptoms usually appear – often at a site distant from the original problem. This scenario is extremely common and requires a highly skilled manual therapist/practitioner to be able to track the problem back to its source. Clinically it is sent that such decompensation is a common sequela (outcome) of TVA dysfunction and TVA dysfunction in our client population is extremely common.

Further, the function of the TVA is neurologically looped with the other muscles of the inner unit, so TVA dysfunction will usually be associated with some level of dysfunction in the multifidus, pelvic floor and diaphragm (see below).

Shear, Compression and Torsion

There are three major types of force that create damage in the lumbar spine; these are shear, compression and torsion. Nevertheless, these are also necessary parts of the locomotive process, which is why synovial joints have evolved to have a friction coefficient less than that of steel over ice.

Co-contraction of the inner unit muscles of the lumbar spine, primarily “stiffens” the spine, generates hoop tension through the thoraco-lumbar fascia and increases intra-abdominal pressure to stiffen the trunk during movement. Without such co-contraction, the entire mechanism is flawed and the local stability system will break down, resulting in dependence on the outer unit, or global stability system, as well as a host of other potentially serious dysfunctions discussed below.

While the outer unit can function to stabilise the lumbar spine in the absence of inner unit function, its own efficacy drops dramatically (see the visceral fulcrum theory below), and the shear, compression and torsion through the passive structures of the lumbar spine (joints, ligaments, joint capsules and discs) increases substantially. So Siff was correct in stating that the body can compensate for transversus abdominis dysfunction and that the TVA is not the most important muscle in the body.

Is TVA Dysfunction a Problem?

What Siff may have missed in his observation was the fact that although the TVA may not be the most important muscle in the human body, it is a most important muscle. If you have a good knowledge of applied anatomy, you will know that the obliques do not attach to the same parts of the thoracolumbar fascia (in fact the EO doesn’t attach at all) as transversus abdominis does and therefore cannot truly compensate for a dysfunctional TVA. Additionally, the obliques have a higher level of fast twitch fibres and so could not compensate for the TVA for long –even if they did attach to the deep and middle layers of the thoracolumbar fascia.

Through the research of many great therapists and academics, we now know that the TVA should contract before engagement of the global system or “mobilizer” muscles in order to prevent excessive wear on the spine’s passive structures. We know that the TVA should co-contract with other deep muscles of the local system, such as the pelvic floor, diaphragm and multifidus muscles, in order to stiffen the spine (reducing potentially traumatic rotation moments*). Co-contraction is also critical so that the intra-abdominal contents do not “leak” or herniate, becoming extra-abdominal contents each time the TVA contracts. Any dysregulation in the forces produced from the TVA, pelvic diaphragm and respiratory diaphragm will result in any combination of the following:

How many of your clients suffer with one or more of these issues?

*By the term “rotation moments,” what is inferred is rotation through any axis – sagittal, frontal or transverse or any combination thereof.

We also know that low back pain will create “inhibition” or shutting down of the neural drive to the TVA and deep stability muscles of the core, and what’s more, it only necessarily takes just a history of low back pain for this to happen and for the core stabilising muscle to remain inhibited.

Can your client walk after they’ve left you?

Some trainers, and therapists in particular, are notorious for over-descending their client’s exercise program. If the client is able or likely to walk after they leave their appointment with you, you can use this as a guideline to see if you have descended their rehabilitation too far. There is little point, for example, in you prescribing that your client does prone TVA contraction to “condition” their abdominal wall in isolation, unless they are going to spend the rest of their week between appointments lying on their stomach! This is not how the body works – it operates in motor programs derived from central pattern generators.

To create new (or relearn old) motor patterns, the new pattern has to be generated cognitively. This is one of the great misgivings of manual therapy in general; the patient can not and will not change the way they move, unless they are put through a cognitive process of motor re-education. Therefore, massage, manipulation, mobilisation of joints, fascial release and every other kind of passive manual therapy has serious limitations in how it will change your client’s movement patterns and biomechanical utilisation; such therapies must be a means to an end. Your clients need to know how to integrate their new found skill into purposeful and practical activities of daily living, such as walking, bending, lifting, squatting.

There are many manual therapists who vehemently reject the idea that their client with a disc bulge should be taught how to perform a functional activity, such as squatting. However, the point such therapists may be missing is that it was the inability to squat with proper form that probably caused their client’s disc problem in the first instance. If the client is truly in a state where they should not learn to squat, then their pain should be so severe that they cannot sit and should only get into bed via a mechanical crane – they certainly should not be driving, as just getting into a car usually requires a single legged squat with a twist – a challenging movement pattern for even the most “functional” back! (Note: If your evaluation reveals TVA inhibition secondary to axial loading, inner unit training in non-compressive positions becomes justified.)

Some clients will respond better to organised rehabilitation, such as a structured exercise program three times per week. While others will respond better to an unstructured program which may include visual cues setting off specific exercises (such as every time you see a red car when driving, engage your TVA for 15 seconds), or auditory cues – such as every time you hear a phone ring check your sitting position and ergonomics. Other ways to do non-specific exercises is to place red sticker dots on specific household implements – such as on the kettle, on the tap in the bathroom, on the top of your computer screen, on your TV remote control) and whenever you see this stimulus you perform your designated exercise.

Machine Culture

So armed with this information, it is no wonder that the thoughtful therapist/conditioning specialist should follow the way the body naturally does things. They should first assess and condition the TVA and other core muscles, and educate correct movement sequencing, before working on the mobiliser system. In the gym however, it is too often the other way round – the mobiliser or prime mover muscles are usually trained in preference; as this is what the machine culture has promoted. Consequently, if the TVA and inner unit function should always be the starting point, (both in movement and in rehabilitation) it is not surprising that, to the uninitiated, it may seem that it is over-focussed upon; as it is always where any good conditioning program should start (discussed below - see Abdominal bracing versus abdominal hollowing).

Let’s qualify this last statement: To produce a conditioning or rehabilitation program for any client, it is first critical that you should be able to effectively assess that client’s needs and function. Cyriax, a famous orthopaedic surgeon and author of many text books states the treatment can only be as good as the assessment. Hence, you should be comfortable with a series of screening assessments or be able to refer out to someone who can perform such assessments so that you know the level at which you need to design their program.

Firstly, the transversus contracts before voluntary or reactive movement of the fore or hind limb (arm or leg) – therefore the old paradigm of the legs initiating movement is flawed; movement emanates from the core. This pre-contraction, or feed-forward mechanism, is essential to minimise micro-trauma to the passive structures of the spine (as described above) which houses our central nervous system, and carries our control centre, the head balanced atop of it. With a functional pre-contraction of the inner unit, the outer unit musculature (sling systems) are to some extent relieved of postural stabilisation of the osteo-articular system and can focus more of their energetic expenditure on mobilising the organism. This will result in greater force generation and better performance.

Secondly, for the primary mobilisers of the core to start to generate force effectively, the TVA first has to contract – this is described in Figure 5.

Figure 5: The visceral fulcrum theory. The counterforce generated by the viscera when the diaphragm, TVA and pelvic floor contract together results in a functional cylinder, as described by Hodges (1999) – diagram 3. If there is no “cylinder”, when the fibres of the more superficial internal oblique (diagram 1) contract, they will “cut into” the lateral body wall; exerting more of a lateral / sagittal flexion moment (diagram 2). It is proposed that one function of the pre-contraction of the transversus abdominis is to create a fulcrum about which the obliques slings can generate an effective rotary torque (Wallden 2000).

Thirdly, a classic study by Rizk shows that the three layers of the human abdominal wall are all intermeshed in the anterior midline (at or around the linea alba). It emerges that the TVA fibres of one side (e.g., the right) blend with the TVA fibres of the opposite side but at a lower (more caudal) level – therefore potentially exerting both a compressive and a rotary force. In fact, Rizk suggests that the TVA should be called the “profundus obliquus,” as many of its fibres are not technically transverse!

Even more significant is the fact that all three layers of abdominal wall interdigitate with another – like the warp and weft of a textile. The TVA interdigitates with the internal oblique of the opposite side. The internal oblique interdigitates with the TVA of the opposite side and with the external oblique of the opposite side. The external oblique, as well as connecting to the opposite internal oblique, connects to the external oblique on the opposite side. See Figure 6 below.

Figure 6: The abdominal wall. This is a diagrammatic representation of Rizk (1980s) findings; that the internal oblique fibres of one side (in this case the internal oblique is coloured red on the left side) run into and attach to the external oblique fibres of the opposite (right) side. This image is adapted from Gray (1993), where it has actually been pasted into the book upside down (but must have missed the editors; attentions)!

The end result of all this is that there are many triangular fibre arrangements – like a kind of triangular chain-mail of our muscular armour, designed both to protect our organs and to stabilize/move our skeleton. Only triangulated structures are inherently stable, and therefore the triangulated abdominal wall is designed for strength and stability.

Figure 7: Schematic representation of the triangulated nature of the abdominal wall.One TVA fibre (black), one IO fibre (red) and one EO fibre (green) form one triangulated structure, whereas two to four fibres from each muscle forms a multiplicity of triangles. How many you can you count? Did you include the upside-down triangles?

Hence any kind of movement, muscle contraction or co-contraction is going to pre-tension and therefore stimulate the spindle cells in all other parts of the abdominal wall.

Figure 8: Abdominal Bracing and Force Interactions within the abdominal wall. Either the body consciously decides to move, sending a neural impulse to the TVA, or it is perturbed and sends an impulse to the TVA – the message being “to stabilise.” When the TVA contracts on one side (usually the neural impulse would be bilateral), it stimulates the contralateral TVA through stretching of the muscle spindles. Additionally the contralateral IO will also receive the same muscle spindle stimulus and will contract as a secondary effect of TVA contraction. Finally, the ipsilateral EO will contract through spindle stimulation as a tertiary effect. Although this discussion is hypothetical based on the applied anatomy, it is supported by the work of Richardson et al (1999) who show that the timing of onset of TVA-IO-EO contraction is as one would expect based on the applied anatomy. This further makes sense as the TVA is composed of more slow twitch fibres (less fatiguable) IO of more fast twitch fibres, and the EO of even more fast twitch fibres. This is a general principle in the body where deeper tissues have a higher preponderance of slow twitch and more superficial tissues the opposite. If there is gut inflammation, uterine inflammation, current low back pain or a history of low back pain, the TVA is likely to be inhibited and the process gets cut off at the top of the cascade. In such an instance, the body must introduce a compensatory strategy which is dysfunctional but better than no compensation at all.

This then would seem to confirm McGill’s contention that abdominal bracing is more effective at stabilizing the spine than abdominal hollowing – which is designed to isolate the TVA. However, despite there being great debate distinguishing between bracing and hollowing, if abdominal hollowing truly tensions the TVA bilaterally, the internal oblique and sequentially the external oblique will be contracted as a result of spindle stimulation, resulting in the same abdominal bracing McGill refers to!

This also leaves us with a problem: if the entire abdominal wall is used for stabilization of the lumbar spine (one of the more vulnerable parts of the spine due to its lack of costal support), which muscles mobilize the lumbar spine?

Technical Editing by Paul Chek


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