The Transverse Abdominus (TvA)
A lot of focus has been placed on the TvA as a muscle that is integral in reducing lower back pain and has been individually targeted during the training of “the core.” The TvA has become the muscular equivalent of “core stability” as an industry buzzword. It has grown in prominence that far outweighs its importance in preventing pain and contribution to functional movement.
By placing so much emphasis on one muscle, we are moving away from integrating joints and muscles as we do during movement. These relationships create reactions that allow proprioceptive information to flow. This facilitates effective eccentric muscular loading and concentric unloading all over the body.
By understanding proprioceptive mechanisms, we know that muscles do not work in isolation and motion in function happens in a subconscious manner. Nineteenth century anatomist Beevor suggests that the body does not know muscles but only movements.
Trying to consciously activate a muscle concentrically or isometrically, such as the TvA, in isolation and without dynamic proprioceptive joint input simply does not make sense in light of our modern understanding of how muscles work.
PW Hodges et al at the University of Queensland have done much of the research into the TvA and have been referenced by many when writing about or discussing this subject. His 1996 study, “Inefficient muscular stabilization of the lumbar spine associated with low back pain. A motor control evaluation of the transversus abdominis,” found that the timing of the activation of the TvA in subjects with lower back pain (LBP) was altered. The conclusion was that the later activation of the TvA was an indicator that the TvA and its timing of activation is a cause of, or has a pivotal role to play in LBP.
An alternative hypothesis could be that individuals with LBP have an altered or reduced proprioceptive flow and muscular response. Muscle response or activation in function is eccentric first as a response to joint motion. This means that the late TvA activation may actually be a reaction to reduced joint motion and subsequently altered proprioceptive information flow. If a joint below or above the lumbar spine at the ankle, hips or spine is giving reduced proprioceptive information then this vital information transmission may be disrupted and hence not allow the eccentric activation of the TvA. This would mean that to gain successful TvA activation we would have to mobilize the joint that has altered the proprioceptive flow. It could be that this joint dysfunction is also a contributor to the LBP by increasing forces on the lumbar spine. Gary Gray and Dr Dave Tiberio have made a straightforward but eloquent review on this subject. They simply state, “If it is not turning on, what turned it off?” Learman et al. found that “proprioception deficits may correlate with pain level” in cases of chronic low back pain. If the late TvA activation were a response to whatever is causing the LBP rather than the actual specific cause of LBP, this would then reduce the validity of concentrating on TvA as a remedy to LBP.
The TvA could also fire first because of its deep positioning within the body and relationship with spinal movement and joint proprioceptors, the Ruffini endings. The Ruffini endings operate with a very low mechanical threshold, this means that very little movement is required for them to send information required for muscular activation. The TvA fiber type is mainly slow twitch. This means that it probably operates most of the time during movement but has limited ballistic ability. This would be in accordance with the low threshold of its activating mechanisms. The Ruffini endings are proximally located and also respond mainly to transverse movements such as occur in the spine to which the TvA has significant interaction with through the MLF (Middle layer of lumbar fascia). This would also correlate with the anatomical orientation of the TvA. If the Ruffini endings have less input from other joints in the body, they would also reduce their output to the muscle spindles and Golgi Tendon organ of the TvA. Subsequent disrupted muscle function of the TvA could be a result.
Solomonow has led research into the neurological and mechanical properties of ligaments and their role as a major sensory organ. He suggests that as well as providing structural support they also function as a proprioceptive feedback mechanism. It is interesting to note that the Ruffini endings are also found in the proximal extrinsic ligaments that inhabit the spinal joints in the low back area.
Muscle Fiber Type and Involvement in Stabilization
Does muscle fiber type hold a clue to stabilization of the lumbar spine? The predominantly slow twitch fibers found in muscles classed as slow twitch, including the TvA and many deep core muscles, have a twice the concentration of collagen. Collagen is visoelastic, meaning it behaves elastically under rapid loading and displays an inherent stiffness. Stiffness being the value of how little an elastic material stretches under a load. This stiffness means that it will resist deformation, therefore lengthen less and keep the stability of the lumbar spine.
A fast twitch muscle has more elastic properties and therefore will deform more and increase range of movement of the associated bones decreasing stability. Bosco et al. suggest the cross bridges of ST (slow twitch) fibers may stay attached for longer periods than FT (fast twitch) fibers. This also points to less deformation and would contribute to the stiffness and stability of the spine.
Fascia also displays high collagen content. The deep trunk muscles also have multiple fascial connections that would also provide stiffness through resistance to deformation, especially collectively such as at the thoracolumbar fascia.This would mean that core muscles would not contract concentrically or isometrically, shortening and pulling together or staying the same length but instead eccentrically lengthening while resisting deformation and compliant behavior through stiffness along with their fascial connections.
This would reduce the validity of conscious isometric contraction training, although it is understandable with the small ranges of eccentric motion occurring why some could have come to that conclusion regarding contraction type without understanding the inherent fiber and connective tissue properties. Resistance of force and deformation through stiffness during movement would in this model recreate the authentic reaction of the TvA and other deep core muscles during function. Smaller ranges of muscular lengthening and shortening are also thermodynamically more economical. This fits with the low mechanical threshold of proprioceptive and muscular operation and therefore the high work level and potential energy consumption in this area.
In the Hodges study, the participants were asked to perform differing arm movements to evaluate the TvA. The movement that caused the most TvA reaction was abduction at the shoulder. This abduction would rotate the spine to the ipsilateral side causing a proprioceptive reaction and TvA activation as a response to this spinal rotation. This indicates the rotational nature of TvA. In the frontal plane the spine would laterally flex to the contralateral side also indicating frontal plane nature of TvA. These reactions would actually also create muscular tension that could help stabilize the spine. Spinal rotation and lateral flexion would logically therefore have to be incorporated into the training of the TvA, if specific training of the TvA was so desired. Cresswell et al. recorded “Reciprocal patterns of activity between the left and right sides of transverse abdominis” during trunk twisting movements. From a purely visual perspective the fiber orientation of the TvA also would indicate its role in the transverse and frontal planes. This evidence however does not seem to have been used when adopting a TvA training strategy within the fitness industry. Instead it has been approached using isometric contractions and predominantly static positions. Hodges study did not define the nature of LBP of the participants and as we know the functional reasons for the cause of LBP are many. This also points to the generic nature of an approach to understanding and treating LBP. This means we have developed ineffective responses to LBP such as a focus on the TvA.
Looking at the anatomy of the core does not shed any light on the reasoning behind singling out the TvA for special attention. The TvA attaches to the middle layer of lumbar fascia (MLF) along with internal oblique’s (IO) and external oblique’s (EO), which then attach to the lumbar transverse processes. Also attached to the L2-4 transverse processes are the psoas and QL. Why then do we individually accredit the TvA with such an important role in lumbar spine stability. The collective concentric tension of these muscles combined with the thoracolumbar fascia would cooperatively provide more stiffness and hence stability during functional movement. The TvA individually could not resist the combined pull force of the other muscles of the trunk on lumbar spine segments.
The lumbar spine also displays an intrinsic structural strength of its own. If we look at the lumbar spine versus the more mobile thoracic spine we can see the larger and thicker vertebrae. An element of structural stability must also be present to hang such large muscles off. A symbiotic balance between bone, muscles and connective tissue exists to provide the optimum balance between motion and stability. This is key to understanding the core and should guide us to train the area as a ‘whole’ to be successful.
Grieve et al. suggest that Bergmark’s (1989) work that divided the trunk into ‘local’ and ‘global’ muscles is often misinterpreted. Bergmark classified the TvA, portions of the IO and lumbar multifidus as "local" and the rectus abdominis, EO and lumbar erector spinae as ‘global’. Others influenced by Bergmark’s research have renamed them as the inner (local) and outer (global) units. Greive et al. say “Bergmark’s work are often misinterpreted as identifying the muscles that are spine stabilizers (local) and the muscles that are moment generators (global)” they add “The overall stability of the spine depends on the individual forces, and hence stiffness, of all trunk muscles.”
Cholewicki and Vanviet (2002) found that “No single muscle group could be identified as the most important spine stabilizer and no clear distinction was found between the local and global muscles as related to stability” in fact they further state “no single muscle group contributed more than 30% to the overall stability of the lumbar spine."
Cresswell et al. found the TvA linked to changes in IAP (intra-abdominal pressure); “it appears that transversus abdominis is the abdominal muscle who’s activity is most closely related to changes in intra-abdominal pressure”. Changes in IAP are also related to inspiration and expiration and the TvA may have a role to play in or be affected by breathing. Olympic lifters have employed the use of methods such as the Valsalva maneuver. This results in increased IAP, which is argued to have a stabilizing impact on the trunk. During this maneuver and other breathing manipulations we may see more TvA recruitment. It is probable that the increased volume of air has a possible stabilizing impact on the body at least as great as any TvA activation. IAP was once thought to reduce compressive forces on the lumbar spine through tensile load to the spine (Bartelink 1957). This has been shown to have less importance by modern research. Lumbar spine compression forces actually increase during IAP as found by Nachemson (1986).
Balance is Needed
The main issue to understand here is that function related compressive forces on one side of the lumbar facet joints would be coupled with decompression on the contralateral side. This would be true in all three planes. To increase one will decrease the other and vice versa. The balance between compression and distraction, symmetry and timing of spinal motion is vital for pain free optimal function.
A case for preferential recruitment of the TvA and Oblique’s has been made to reduce compressive forces at the spine (Hodges et al., 2001). However can we individually and preferentially activate muscles in isolation? Does this happen during function and how would we go about doing this during function? TvA contraction type and levels, as well as its role within a specific movement will change with different functions. It has also not been explored sufficiently in a variety of functional positions. Trunk muscle co-contraction has much more similarity to what would occur during normal everyday client activity, although usually in an ecconcentric manner. This means a healthy and stable core will allow the correct 3D motions and forces where they are needed.
The TvA could have a role to play in helping create the hydraulic amplifier affect theorized by Gracovetsky. This could cause an extension moment on the spine although a ligamentous model has now superseded the muscular model originally based on just two muscles that provided insufficient concentric force.
From a functional perspective the creation of a multi-muscular and fascial extension moment through increased spinal stiffness could help both the anterior and posterior trunk. As the pelvis translates forward and anteriorly rotates as in gait, a possible extension moment is created by muscular and fascial stiffness on the lumbar spine. This would cause the vertebrae above to flex slower causing relative extension at the joint. This increased spinal extension force moment in conjunction with the anterior rotation of the pelvis will stimulate the anterior musculature. These muscles are oriented to eccentrically decelerate the trunk and have an impact on the kyphotic or spinally flexed posture type that sees the superior spinal bone flexing faster than the inferior creating relative spinal flexion. This could also happen in both a squatting or lunging situation where an anterior pelvic tilt occurs and due to the TvA’s attachment would create proprioceptive stimulation and force resistance through stiffness. It would however need the proper motions to occur elsewhere in the function related chain to create this proprioceptive input to activate and create this muscular tension. The argument of the proliferation of trainers and therapists, who swear by the activation in isolation of individually unimportant muscles of the trunk, may be misguided and ultimately provide ineffective treatment of LBP.
Our approach to gaining “core stability” and reduction in LBP seems to go against the evidence of the available scientific research. Dynamic, functionally three-dimensional and progressively increasing muscular load movements that relate to client function have been shunned in favor of single plane movements, attempted isolation and permanently low muscular load exercises. This reduces the proprioceptive input that creates effective neuromuscularskeletal relationships within the body and also the environment around us. This input is vital to functional success that also includes the core or trunk area.
Reduction of LBP may not come from working the core or trunk area. Instead by understanding other areas of dysfunction in the body and how they may relate to LBP during function could be the key. This will enable us to treat the right areas and become specific rather than generic in our treatment of LBP.
The body gives us clues to how it wants to be worked through the understanding of its integrated anatomy, although this needs to be correctly interpreted and used in our exercise prescription. By understanding the inherent structural and mechanical properties of the area and that the body knows only movements and not muscles. We can shift our focus away from trying to consciously activate in isolation and isometrically to our muscles to moving, integrating and making our core happy. This then means our trunks can become truly stable and pain free along with the rest of the body and be ready to perform at our optimum.
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