From the extracellular matrix to posture. Is the connective system our true Deus ex machina?

Edited by Dr. Giovanni Chetta

Deep fascia biomechanics

From the biomechanical point of view, the thoraco-lumbar belt has the fundamental task of minimizing stress on the spine and optimizing locomotion. By appropriately considering the band, it will be possible to dispel some common beliefs based on hypotheses, albeit suggestive, never actually demonstrated.
Studies show that the intervertebral disc is rarely destroyed by pure axial compression, as the vertebral body is destroyed long before the annulus fibrosus (Shirazi-Adl et al. 1984). The articular plate of the vertebral body ruptures under axial load. (by pure compression) of about 220 kg (Nachemson, 1970): the pressure of the nucleus of the intervertebral disc causes the fracture of the end-plate in which part of the nuclear material migrates (Schmorl's nodules) and being a damage to the " Cancellous bone can heal quickly. This although the vertebral metamer breaks at about 1,200 kg (Hutton, 1982) and the annulus fibrosus, for a pure axial compression of not less than 400 kg, undergoes only 10% of deformation (Gracovetsky, 1988).

Axial compression, therefore, is not able to create fissures of the annulus (and to create damage to the articular facets) unless violent impacts. Instead, the compression associated with torsion has been shown to be able to damage the fibers of the annulus. and the capsular ligaments of the facet joints; in extreme cases there is a herniation. The damage is localized to the periphery of the disc and being a ligament damage it takes time to repair itself. A disc herniation, with rare exceptions, is therefore actually triggered by shear stresses associated with compression ( Shirazi-Adl et al. 1986). All this suggests that the intervertebral disc is not a sufficient system of cushioning and transmission of loads but, in reality, a energy converter (Gracovetsky, 1986).
On the other hand, however, there is no doubt that the vertebral compression load can reach 700 kg by loading heavy weights (the force applied on L5-S1 lifting a weight flexed to 45 degrees is about 12 times the weight itself).
In the 1940s, Bartelink proposed the idea, still commonly accepted today, that, to lift a weight, the erector spinal muscles act on the spinous processes of the relative vertebrae aided by intra-abdominal pressure (IAP) which, in turn, would push on the diaphragm (Bartelink, 1957). Since it has been verified that the maximum force exerted by the erector muscles corresponds to 50 kg (McNeill, 1979), through a simple calculation it is shown that, according to this hypothesis, by lifting a load of 200 kg the intra -abdominal should reach a value about 15 times the blood pressure (the maximum value of IAP, calculated on a transversal surface of 0.2 m2 is 500 mm Hg - Granhed 1987).

Bartelink's model makes sense if the fascia is introduced. While lifting the weight, flexing the spine with the pelvis in retroversion (i.e. tensioning the fascia as best as possible), the erector muscles do not need to be activated. Lifting occurs mainly by the action of the extensor muscles of the thigh on the hips (hamstring and gluteus maximus) and of the fascia. In Olympic champions it was found that the effort is divided into 80% fascia and 20% muscles (Gracovetsky, 1988). It is therefore the collagen that does most of the work, since, acting as a cable, it consumes practically no energy; moreover, thanks to its insertion of the iliac crests-spinous apophysis, it is positioned practically outside the body, presenting the advantage to be away from the fulcrum of the lifting lever (major lever arm) This is a forced evolutionary choice, as erector muscles to be able to lift more than 50 kg would have to increase their mass thus occupying the entire abdominal cavity. The strength supplements (muscles and fascia) were therefore placed outside the abdominal cavity.
The erector muscles (multifidus) and intra-abdominal pressure, together with the psoas muscles, actually regulate lumbar lordosis three-dimensionally, thus assuming an important role as modulators of the transfer of forces between muscles and fascia.
In fact, internal abdominal pressure does not significantly compress the diaphragm; in reality, it acts on the lumbar lordosis and therefore on the transmission of forces between muscles and fascia. In fact, the intra-abdominal pressure flattens the fascia causing the transverse abdominal muscles (which constitute the active part of the dorsal-lumbar fascia as its fibers are attached free edges of it) to traction on the same plane as the fascia. When the intra-abdominal pressure is low this mechanism is disabled and any action of the abdominal muscles (of the rectus muscle in particular) leads to a flexion of the trunk. In other words, if the tension of the internal abdominal muscles is high, the lumbar region goes into hyperlordosis by extending, while if the pressure in the abdomen is low the spine can flex with the pelvis in retroversion, thus stretching the fascia (retrovertere the pelvis before starting the lifting in flexion is a typical attitude of people who lift weights without problems. In this latter condition there is also less opposition to the systolic blood pressure, so the blood flows better towards the extremities (in some way our muscular system). skeletal means that there is no excessive internal abdominal pressure so as to preserve peripheral blood circulation.) Therefore the fascia can make its important contribution during the flexion of the spine if abdominal tension is decreased (Gracovetsky, 1985).