Introduction

In biomechanical terms, the cardinal process involved in lifting is the balancing of moments. Gravity acting on the trunk and any weight to be lifted exerts a forward bending moment on the trunk. To raise the trunk, and
thereby the weight, this moment must be balanced and overcome by a backward bending moment, and such a moment can be provided by the back muscles acting
posterior to the axes of sagittal rotation of the lumbar vertebrae.

In the absence of any external load, the back muscles are sufficiently strong to raise the trunk from a forward bent position. However, various calculations have shown that the back muscles alone are insufficient to raise the trunk when substantial external loads are to be lifted 1-4. Their contraction is unable to overcome the combined forward bending moment generated by the weight of the trunk and the large weight to be lifted. To lift a weight, the back muscles must be assisted by an additional mechanism that also contributes a backward
bending moment.

The earliest theory that describes such a mechanism is the abdominal balloon theory of Bartelink 5. The essence of this theory is that contraction of the abdominal muscles produces a rise in intra-abdominal pressure which effectively creates a ‘balloon’ in front of the vertebral column that resists compression and, therefore, forward bending. Forward bending moments are thereby balanced by a combination of back muscle activity and raised intra-abdominal pressure.

Although this theory has been popularized and supported by circumstantial evidence, recent studies have fielded incompatible or paradoxical data. Abdominal muscle activity does increase during lifting in proportion to the weight lifted or the flexion moment generated 6-13. However, while the intra-abdominal pressure correlates well with the load on the spine when it is
statically loaded, it correlates poorly with the dynamic loading and is influenced by postural asymmetry 14-16 Moreover, the large initial increase in intra-abdominal pressure that occurs during a maximum lift is only brief,
and lower pressures are present throughout most of the lift. 5.17.

Exercises designed to increase the capacity of the abdominal muscles to raise intra-abdominal pressure succeed in strengthening the muscles, but do not result in higher intra-abdominal pressures during lifting 18.19. Although patients with back pain have reduced abdominal muscle strength, their intra-abdominal pressure during lifting does not differ from that in controls 20, and when their abdominal muscles are retrained and strengthened, intra-abdominal pressure does not improve 21.

On theoretical grounds, it has been calculated 21 to execute a lift of 100 kg an intra-abdominal pressure of250mmHg (32kPa) is required to balance the bending moment. Were this level of pressure to be sustained for longer than a brief moment the abdominal aorta would be occluded and blood flow to the viscera and lower extremities would be compromised (which indeed, is a reservation raised by Barterlink himself 5). Furthermore, the known capacity of the abdominal muscles is 60 – 50psi (0- 4- 1- 0 MPa) 22, 23, which is insufficient to realize the level of hoop tension required to generate such pressure.

Not withstanding these quantitative limitations, intra-abdominal pressure is raised by contraction of the external and internal abdominal obliques and to a
lesser extent rectus abdominis 24, but it has been noted that these muscles also exert a flexion moment on the trunk, which tends to negate any anti flexion effect exerted by the raised intra-abdominal pressure 25.

These several limitations and inconsistencies have brought the intra-abdominal balloon theory into question and have led to the proposal of another mechanism
capable of producing an extensor force, as an alternative or in addition to the abdominal balloon mechanism. Gracovetksy, Farfan and Lamy 26 proposed that
the posterior ligamentous system is capable of storing sufficient tension to enable the vertebral column to overcome any flexion force that exceeds the capacity of the back muscles. The posterior ligamentous system consists of the capsule of the zygapophysial joints and the midline ligaments. and the posterior layer of the thoracolumbar fascia.

The tensile properties of the zygapophysial capsules and the supraspinous, interspinous and flaval ligaments have been studied previously 27, and collectively they can balance between 24% and 55% of applied flexion
moments. This action, however, depends on the amount of lumbar flexion, for it is necessary for the spine to elongate posteriorly, and remain essentially
fully flexed, for tension to be stored in the ligaments.

Gracovetsky et al. 28 and later Gracovetsky et al 4 proposed that the thoracolumbar fascia could provide an additional anti-flexion moment that is not dependent on the spine being fully flexed. It was suggested that,
because the fibres of the posterior layer of thoracolumbar fascia attached obliquely to the lumbar spinous processes, tension in the fascia could act to prevent separation of the spinous processes, thereby providing an anti-flexion moment. This tension in the thoracolumbar fascia could be generated by contraction of the abdominal muscles that attach to the thoracolumbar fascia and by the expansion of the back muscles that underlie the fascia. The latter process was described as the ‘hydraulic amplifier mechanism 28.

These proposals, however, were made in the absence of any appropriately detailed anatomical data on the thoracolumbar fascia. Having recently completed a study of the thoracolumbar fascia 29 We are now able to offer both a qualitative and quantitative assessment of the posterior layer of thoracolumbar fascia and its putative role in lifting.

Anatomy

The thoracolumbar fascia consists of three layers: an anterior layer which arises from the anterior surface of the lumbar transverse processes and covers quadratus lumborum; a middle layer, from the tips of the lumbar transverse processes, and a posterior layer which arises from the midline to cover the back muscles 29.

The posterior layer of thoracolumbar fascia covers the back muscles from the lumbosacral region through to the thoracic region as far rostrally as the splenius muscle. In the lumbar region it is attached to the midline, and lateral to the erector spinae, between the 12th rib and the iliac crest, it unites with the middle layer of thoracolumbar fascia forming a raphe, referred to as the ‘lateral raphe 29. At sacral levels, the posterior layer extends from the midline to the posterior superior iliac spine and the posterior segment of the iliac crest.

At lumbosacral levels, the posterior layer consists of two laminae: a superficial lamina with fibres orientated caudo medially and a deep lamina with fibres oriented caudol aterally (Figure 1). The superficial lamina is formed by the aponeurosis of latissimus dorsi, but the disposition and attachments of its constituent fibres vary. The most lateral fibres from latissimus dorsi attach via short tendons directly to the iliac crest and do not contribute to the posterior layer of the thoracolumbar fascia. More medial fibres become aponeurotic just lateral to the lateral raphe. As they pass through the raphe the tendons are deflected medially so that they eventually gain attachment to the lower lumbar and sacral spinous processes, forming the sacral portion of the superficial lamina of the posterior layer. The most medial fibres pass through the lateral raphe and gain attachment to L3, L4 and L5 spinous processes, and form the lumbar portion of the superficial lamina of the posterior layer.

The deep lamina consists of bands of collagen fibres emanating from the midline, principally from the lumbar spinous processes (Figure 1). The bands from the L4, L5 and SI spinous processes pass caudolaterally to the posterior superior iliac spine. Those from the L3 spinous process and L3-4 interspinous ligament wrap around the lateral margin of the erector spinae to fuse with the middle layer of thoracolumbar fascia in the lateral raphe. Above L3 the deep lamina progressively becomes thinner, consisting of sparse bands of collagen that dissipate laterally over the erector spinae. A deep lamina is not formed at thoracic levels.

<img src=’https://novo.pedroprado.com.br/imgs/1990/339-1.jpg’>
Figure 1: A posterior view of the posterior layer of thoracolumbar fascia. On the right, the deep lamina alone is shown, with bundles of collagen fibres passing from the L4, L5 and S1 spinous processes to the posterior superior iliac spine (PSIS), and from L2 and L3 to the lateral raphe (lr) where they fuse wiyh the aponeurosis (apon) to transversus abdominis (TA) which forms the middle layer of thoracolumbar fascia. On the left, the deep lamina is covered by the superficial lamina which is formed by aponeurosis of latissimus dorsi (LD) and which fuses with the deep lamina and the middle layer of thoracolumbar fascia in the lateral raphe. The two laminae endow the intact posterior layer of thoracolumbar fascia with a cross-hatched appearance. (see image below)

Collectively, the superficial and deep laminae of the posterior layer of thoracolumbar fascia form a retinaculum over the back muscles. Attached to the midline medially and the posterior superior iliac spine and lateral raphe laterally, the fascia covers or ensheaths the back muscles, preventing their displacement dorsally. However, recognition of the disposition of its component fibres allows the mechanical properties of the posterior layer to be discerned.

The obliquely crossing fibres of the two laminae of the posterior layer are arranged such that at any given point on the lateral raphe a fibre from the superficial lamina extends rostro medially. The posterior layer therefore consists of a series of overlapping triangles of collagen fibres whose apices lies in the lateral raphe and whose bases lie in the midline (Figure 2). The upper border of each triangle is formed by a fibre in the deep lamina, and the lower border is formed by a fibre in the superficial lamina. The divergence of the two borders is such that each triangle subtends two vertebral levels. When the lumbar spine is in the neutral position, the fibres of the posterior layer are orientated at 30 degrees to the horizontal, but in the flexed position this angle increases to about 40 degrees.

<img src=’https://novo.pedroprado.com.br/imgs/1990/339-2.jpg’>
Figure 2: From any point in the lateral raphe (Ir) two collagen fibres diverge towards the midline through the posterior layer of thoracolumbar fascia. One passes upward in the deep lamina, the other downwards in the superficial lamina. The divergence between the two fibres spans two vertebral levels. (see image below)

Because of the triangular arrangement of its fibres, lateral tension forces applied to the posterior layer will be transmitted in a triangular fashion. Lateral tension applied to the apex of any triangle in the lateral raphe would be transmitted to the midline along the borders of the triangle (Figure 3). Because of the obliquity of the borders, the force exerted at each basal angle would consist of two components: a horizontal and a verticical vector (Figure 3). When lateral tension is applied to the thoracolumbar fascia bilaterally the sum of the horizontal vector forces at the midline will be zero, but the remaining vertical vectors in any triangle will be exerted in mutually opposite directions. This arrangement allows lateral tension in the posterior layer exert a force that tends to approximate the spinous processes, and is equivalent to an extending, or anti flexion force.

<img src=’https://novo.pedroprado.com.br/imgs/1990/339-3.jpg’>
Figure 3. Lateral tension applied to the posterior layer of thoracolumbar fascia will be distributed in a triangular fashion towards the midline. The divergence of the fibres of the posterior layer permits this lateral tension to exert an extending force on the lumbar spinous processes (see figure 4)

The middle fibres of the transversus abdominis attach to the lateral raphe of thoracolumbar fascia 29, and therefore their contraction is capable of generating lateral tension in the posterior layer. Apart from a few fibres of internal oblique that may attach to the raphe 29, transversus abdominis is the only abdominal muscle that attaches to the posterior layer, and whatever influence the abdominal muscles may have on the thoracolumbar fascia will be expressed only by the transversus abdominis. Knowledge of the detailed anatomy of the thoracolumbar fascia makes it possible to calculate the magnitude of the tension generated by the transversus abdominis and the size of any anti-flexion moment generated by it.

Calculations

Contraction of transversus abdominis would apply lateral tension (Tl) to the lateral raphe, which would be transmitted to the midline through fibres of the posterior layer of thoracolumbar fascia. If the fibres of the posterior layer are orientated at 6 degrees to the horizontal, the tension that develops in each fibre (Tf) can be resolved into a horizontal tension (Th) and a vertical tension (Tv) (Figure 4):

Tl must equal the sums of the horizontal tensions in the triangles ABC and ADC, i.e. Tl = 2Th.

The extension force (EF) approximating B and D, exerted by the bilateral application of tension (Tl) to the thoracolumbar facia will be: (Figure 4)

The magnitude of Tl can be estimated from the crosssectional area of transversus abdominis. To determine this the muscle was examined in ten embalmed human adult cadavers.

<img src=’https://novo.pedroprado.com.br/imgs/1990/339-4.gif’>
Figure 4:The geometry of tension forces (Tf) exerted on the midline by lateral tension (Tl) applied to the fibres of the posterior layer of thoracolumbar fascia which are orientated at 0 degrees to the horizontal. Th and Tv represent the horizontal and vertical vectors of Tf.

The transversus abdominis had a mean thickness of 2mm over the 7 cm length along which it attached to the lateral raphe. Therefore the mean cross-sectional area of that part of the transversus that acts on the posterior layer is 14 mm2

Assuming maximum contraction and a generous coefficient of contraction of 10 kg/cm (1 N/mm2 30 he tension (TL) exerted by the two transversus muscles would be:(Figure 4)

From equation (5), the maximum extension force (EF) exerted by the transversus abdominis would be:(Figure 4)

Since this force acts on the spinous processes whose tips lie about 5 cm behind the axes of sagittal rotation, the extension moment (EM) exerted by the transversus would be:(Figure 4)

During erect standing the orientation of fibres in the posterior layer is 300, with respect to the horizontal.

Therefore:(Figure 4)

When the vertebral column is fully flexed, 0 is 40°.

Therefore:(Figure 4)

Discussion

The anatomy of the posterior layer of the thoracolumbar fascia is such that it is capable of transforming a laterally applied tension into an extension moment on the lumbar spine, and because the transversus abdominis attaches to the lateral raphe of the thoracolumbar fascia, this muscle is capable of exerting such an extension moment. In a qualitative sense, this arrangement suggests that the transversus abdominis and the thoracolumbar fascia can act as an internal bracing mechanism providing a definite extensor (anti-flexion) moment on the lumbar spine.

Our calculations, however, reveal that the magnitude of the force that can be exerted in this way on the erect, or even fully flexed, lumbar spine, is relatively small. Given that the maximum extensor moment generated by the back muscles has been determined to be 250-280Nm 31.32 , the moment that can be generated by the thoracolumbar fascia (3.9 ? 5.9 Nm) represents only a small additional moment. Thus. while anatomically feasible and qualitatively attractive, the abdominal mechanism previously described by Gracovetsky et al 4 provides a virtually negligible anti-flexion moment.

The reasons for this discrepancy are several. First, in the original calculations the orientation of the fibres in the thoracolumbar fascia was taken to be 0= 67° which is greater than the observed orientation of 0=30 ? 40°. Secondly, the internal oblique was believed to contribute to the horizontal force on the thoracolumbar fascia but anatomical dissections have now revealed that only transversus abdominis is effectively involved. Moreover, only a limited portion of the transversus abdominis acts on the thoracolumbar fascia. Thus the original calculations over-estimated the number and cross-sectional area of the muscles that could exert lateral tension on the thoracolumbar fascia. Compounded, these various anatomical errors introduced excessive multiplication factors into the calculation of the forces.

Having reappraised the biomechanics of the posterior layer of thoracolumbar fascia in the light of new anatomical data, we are obliged to conclude that despite that suitable arrangement of the fibres of the posterior layer, too few fibres of the transversus abdominis act on the thoracolumbar fascia for it to exert a significant anti-flexion moment. Whatever the role of the thoracolumbar fascia in lifting, it cannot be due substantially to the contraction of transversus abdominis.

While deriving a significant role for the transversus abdominis in the control of flexion, the present results do not exclude other mechanisms whereby the thoracolumbar fascia could exert an antiflexion moment. Our studies show only that transversus abdominis is too small to generate sufficient tension in the posterior layer. The arrangement of the fibres in the posterior layer nevertheless would still allow it to exert an anti-flexion moment, provided sufficiently high tensions were generated in it.

In this regard, another mechanism described to de is the hydraulic amplifier mechanism of Farfan, Gracovetsky and Lamy28. If IS Conceivable that the expansion of the back muscles as they contract could stretch the posterior layer from within, raising the tensi within its fibres. Using the same triangular mechanism as described above, this tension could brace the lower lumbar spinous processes and stabilize them in flexion. On the basis of the anatomy, this mechanism seems qualitatively feasible. However, a quantitative analysis still remains to be explored.

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