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Lower Back Pain (LBP) as a generic term and is the main cause of work related absence and disability in industrialised societies (Frank, Kerr, Brooker, DeMaio, Maetzel, Shannon, Sullivan, Norman and Wells, 1996).  It is suggested by Maniadakis and Gray (2000)  that musculoskeletal disorder is the most commonly reported type of work related illness and back pain is the nation's leading cause of disability, with 1.1 million people disabled by it (Labour Force Survey, 1998). An estimated 80% of people will experience back pain at some time in their lives (O'Sullivan. 2005; Back Pain Association, 2006). Conditions experienced can be varied and include prolapsed discs, disc degeneration, osteoarthritis of the apophyseal joints, osteoporosis, and spondylolisthesis (Kelsey and White, 1980) and forty nine percent of the UK adult population report back pain lasting at least 24 hours (Back Pain Association, 2006.)

It is these musculoskeletal disorders that have been linked to trunk instability of the stabilizing muscles situated in proximity of the lower back (Marras and Mirka, 1996; Gorbet, Selkow, Hart and Saliba, 2009). But what is the classification of trunk stability?

Trunk stability is defined as the ability of the spinal column to survive an applied perturbation (Grenier and McGill, 2008) or a loss of spinal stiffness (Pope and Panjabi, 1985). Instability is often a term used to describe a mechanical lack of stability between two spinal segments and is considered to be present when there is "an excessive range of abnormal movement for which there is no protective muscular control" (Maitland, 1986) from stabilizing muscles, with the result being a loss of spinal stiffness (Pope and Panjabi, 1985) and a lack of survival application to applied perturbation (Grenier and McGill, 2008). Amongst the many muscles which may contribute to such stability, recent focus has turned its intention to the performance of the abdominal muscularity and in particular one abdominal muscle, known as the transverses abdominals (TVA) (Robinson, 1992).

The TVA is the deepest of the abdominal muscles (Miller and Medeiros 1987; Jull, Richardson, Hodge and Hide, 1999) and is said to be used for trunk stabilization purposes (Reiman, Harris and Cleland, 2009). The anatomical attachment of the TVA was described by Gray (2008) graphically and verbally. The TVA  arises, as fleshy fibres from the lateral third of the inguinal ligament, from the anterior three-fourths of the inner lip of the iliac crest, from the inner surfaces of the cartilages of the lower six ribs, integrating with the diaphragm, and from the lumbodorsal fascia.

The muscle ends interiorly in a broad aponeurosis, the lower fibres of which curve infer medially (medially and downward) and are inserted, together with those of the internal oblique muscle, into the crest of the pubis and pectineal line, forming the conjoint tendon. In layperson's terminology, the muscle ends in the middle line of the linear Alba, in conjunction with the rectus abdomens and the internal and external oblique.

Functioning like a corset (within a serape effect; Konin, Beil and Werner, 2003) due to its orientation to the linear Alba, the TVA (according to Bliss, 2005) function is to stabilize the lower back before movement of the arms and/or legs occurs. This function is critical if wear and tear of the joints in your low back is to be prevented. Richardson, Sniijders, Hides, Damen, Pas and Storm (2002) also highlight that one of the most important actions of the TVA that achieves the most stabilisation is to draw-in the abdominal (hollow) or to brace in activating this corset. This in turn forms a relationship with the process of intra-abdominal pressure and its numerous correlated mechanical principles (Porter, 1986).

Hodges and Richardson (1997) undertook a study which used needle electrodes on the TVA, deltoid and upper legs in attempt to establish the activation timing sequence of the TVA prior to limb movements. They suggested that when arm movement is performed, the onset of TVA activity precedes that of deltoid by approximately 30 milliseconds. In contrast, when the leg is moved (producing reactive forces of greater magnitude due to the increased mass), activation of TVA precedes that of deltoid by more than 100 milliseconds. This supports the proposal that the contraction of the TVA muscle prior to the movement of a limb may contribute to the control of stability of the adjacent joints in addition to controlling the position of the centre of gravity within the base of support, especially when the stability of the lumbar spine is challenged by rapid motion of the upper limb. They continued and suggested that the TVA is the first trunk muscle to activate, and the onset of its activity is not significantly affected by the direction of the reactive forces.

A few years before the Hodges and Richardson (1997) findings, Creswell, Oddsson and Thorstensson  (1994) reported a similar trend with self-induced ventral perturbations trunk loading and where the time of arrival of the insult could be predicted. These authors reported activity of TVA to proceed that of the rectus abdominals and internal and external obliques by 65-120 ms, which is comparable to the range reported in the Hodges and Richardson (1997) study of 10-117 milliseconds. With such contrasting values a look at the possible reason for this is worth investigating.

The Richardson and Hodges (1997) study involved the identification of the sequence of contraction of the trunk and limb muscles during hip flexion, abduction, and extension, with no external resistance, and with no initial motion of the spine. However, the Creswell et al (1999) study looked at ventral or dorsal (spine) load when sudden loading was applied to the trunk which had attached to it a 5 kg resistance.  It is possible that activity of the TVA prior to that of the rectus abdominals, internal and external oblique's may have been in perturbation to the expected ventral loading condition which may indicates a feed-forward postural strategy designed to increase the stability of the TVA for the ensuing sudden load (Cresswell, 1993) and which may be partly contributed by the effect of hollowing or bracing the TVA.

A study by Richardson et al (2002), the same group who identified the above timing sequencing of the TVA, compared the effect of abdominal hollowing against abdominal bracing in lower back stabilization efficacy. They found hollowing increased the stability significantly more than bracing, suggesting that hollowing is both more functionally effective and more energy efficient. Abdominal hollowing using predominantly TVA contraction leaves the prime movers of trunk rotation; the internal and external oblique free to mobilise the trunk in the rotary fashion, (Gracovetsky, 1988). This something which may contribute to the differences in the timing of the TVA in the Richardson and Hodges (1997) and Creswell et al ( 1999) studies.

A few years later Hides, Wilson, Stanton; McMahon,  Keto, McMahon, Bryant, and Richardson (2006) investigated this corset effect using magnetic resonance imaging (MRI),  of the bilateral function of the TVA during a drawing-in of the abdominal wall. They validated the study with the use of real-time ultrasound imaging as a measure of the deep abdominal muscle during a drawing-in of the abdominal wall. They demonstrated that the MRI results indicated that during a drawing-in action, the TVA contracts bilaterally to form a musculofascial band that appears to tighten (a corset) and most likely improves the stabilization of the lower back region.

However, with all theories and beliefs, counter theories and beliefs occur. Siff (2003) discerned that the combined force directions of the external and internal obliques can produce the force direction of the TVA, thereby carrying out roles like those of TVA. In other words, if active internal oblique muscles pull along one diagonal across the trunk and active external obliques pulling along the inverse diagonal, the resultant force will exerted roughly parallel to the floor (if you are standing), in other words in the same direction as tension in the TVA.

This vector resultant alone may be more than adequate to compensate for any apparently undesirable TVA malingering. Moreover, activation of a muscle still does not prove its function. McGill (2001) also found similar findings regarding Siff's (2003) work by stating that some confusion exists in the interpretation of the literature regarding the issue of abdominal hollowing and abdominal bracing. McGill (2001) said that Richardson et al (2009) observed that the hollowing of the abdominal wall recruits the transverse abdominals. On the other hand, an isometric abdominal brace co-activates the transverse abdominals with the external and internal oblique's to ensure stability in virtually all modes of possible instability (Juker, McGill, Kropf and Steffen, 1998). McGill (2001) then neither finished by stating that in bracing the wall is neither hollowed in nor pushed out. In this way, abdominal bracing is superior to abdominal hollowing in ensuring stability.

This said, the former studies by Richardson et al (1991) and  Hides et al (2006) support the findings by Teyhen, Miltenberger, Deiters, Del Toro, Pulliam, Childs, Boyes and Flynn (2005) who looked at patients with LBP to determine the characteristics and the extent to which the abdominal drawing-in manoeuvre (hollowing) results in preferential activation of the TVA. On average, LBP patients demonstrated a two-fold increase in the thickness of the TVA during the hollowing manoeuvre. This provided validity for the notion that hollowing results in preferential activation of the TVA in patients with LBP.

Supporting the hollowing hypothesis is the concept that the TVA has a higher level of slow twitch muscle fibres (Fredericson and Moore, 2005) with a  suggestion that the TVA pre-contracts, supporting the visceral fulcrum theory and the ethos that the TVA contracts prior to any limb movement (Hodges, 1999). The visceral fulcrum theory is a counter force generated by the visceral when the TVA contracts and provides a functional cylinder around the spine (Hodges, 1999). Hodges (1999) also mentions a further contribution of the TVA to spinal stability regarding intra abdominal pressure. In the original Creswell et al (1994) study which identified TVA activation prior to other abdominal muscularity in loading, they also identified IAP in relation to spinal stability.

Creswell et al (1994) positioned intra-abdominal pressure (IAP) transducer in the gastric ventricle with intra-muscular electromyography electrode placed the TVA, what they summarised was IAP and TVA muscle contractions may result in an increased stiffening of the inter-vertebral joints within the lumbar spine and thereby simplify the control of subsequent extensor torque development by dorsal musculature. Additionally, a trunk extensor moment, although low in magnitude, may be developed through the increased IAP.

The development of techniques such as fine wire electromyography (EMG) activity along with the guidance of ultrasound imaging has allowed the direct investigation of the recruitment of the TVA muscle (Goldman, Lehr, Millar and Silver, 1987; De Troyer, Estenne, Ninane, VanGansbeke and Gorini, 1990; Creswell, Grundstrom and Thorstensson, 1992). According to Hodges (1999) the first investigations of the TVA as a possible contributor to spinal control were performed by Creswell et al (1992). These studies were stimulated by the observation that high intra-abdominal pressure was present during isometric trunk extension, measured with surface EMG electrodes. However, little activity of Rectus Abdominals, along with External and Internal Obliques could be detected with surface EMG electrodes (Creswell and Thorstensson, 1989). Thus the TVA was postulated to be responsible for this pressure increase since it can generate pressure without opposing the trunk extensor moment (Creswell et al, 1992).  Creswell (1993) stated this as being  unexpected with the continuous (but varying) activity of TVA and its close relationship to intra-abdominal pressure leading the authors to conclude that TVA may contribute to a general mechanism for trunk stabilization rather than the production of torque or control of orientation of the spine.

This close relationship of intra abdominal pressure and TVA activation is seen to be vital as intra-abdominal pressure produces what is defined as  'hoop tension'; a force applied circumferentially to the spine that creates hoop stress, which serves to stabilize the lumbar spine, by the generating of lateral tension on the thoracolumbar fascia. The superficial lamina of the posterior layer of thoracolumbar fascia generates tension downward via its attachments at the lumbar vertebrae two (L2) and three (L3), while the deep lamina generates tension upward through its attachments at lumber vertebrae four (L3) and five (L5). These mutually opposing vectors tend to approximate or oppose separation of the L2 and L4 vertebra and the L3 and L5 vertebra, creating what is referred to as "Thoracolumbar Fascia Gain" (Gracovetsky,  1988;  White & Panjabi, 1990 and Bogduk  & Towmey, 1991).

This occurs due to the lateral raphe of the thoracolumbar fascia passing medially to the linear
Alba (Bogduk and MacIntosh, 1984). Due to this horizontal Fibre orientation, contraction of TVA results in a reduction of abdominal circumference with  a resultant increase in tension in the thoracolumbar fascia and an increase in intra-abdominal pressure. This according to (McGill, 2001) provides evidence of the TVA being a strong spinal stabilizer as it has only a limited ability to produce trunk motion.

One final point regarding the workings of the TVA is the mathematical model originally described by Gracovetsky (1988). This model is induced by the process of intra-abdominal pressure and is known as "The mechanical Amplifier Affect". Gracovetsky (1988) demonstrated the extension force produced by expansion of the erector spine muscles within the compartment created by the thoracolumbar fascia and lamina groove of the spine to be a significant contributor to one's ability to lift a load. The expansion of the muscles within the thoracolumbar fascia produces intra-compartmental pressure (ICP). The cylinder is stabilized by synergistic activation of the transverses abdominals (TVA) and posterior fibres of the internal oblique (IO). The results indicated that intra-abdominal pressure increased following the activation of TVA and was early enough to precede the onset of limb movement and could contribute mechanically to the preparatory process occurring prior to limb movement as previously mentioned (Creswell, et al 1994; Hodges and Richardson. 1999).

Throughout this essay, cases for and against the role regarding the TVA activation in the stabilisation of the lower back have been documented. This essay accepts that there was no focus on breathing mechanics and the functioning of the diaphragm, which may contribute to spinal stability. This said, future endeavours regarding breathing and the diaphragm need investigating. Regarding future studies, this essay only focused on those individuals who are able bodied and not with any form of disability which brings up the following questions: Firstly, does the activation of the TVA become more important if a disability has occurred? Secondly, would some of the above findings contribute to making the rehabilitation process quicker and smoother when recovering from an accident? Finally, does  activation of the TVA enhance  ability to use prosthesis or a wheelchair easier with less lower back discomfort which may occur due to the change in skeletal, muscular and neurone motion?  In conclusions these are all valid questions that would need further investigation and are beyond the limitations of this essay.

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