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Compression and Tension

How can Compression and Tension coupled forces of the spine lead to health?

Saul Yudelowitz BSc(Hons)

The spine is a marvel of mechanical engineering. It is subjected to many different types of forces during a lifetime. This article deals with just two of those forces, compression and tension. To start the definition of compression is when a force acts on a tissue thereby shortening its length relative to the original length while tension is the opposite, a force acting on a tissue that causes lengthening relative to its original length.

An example of each would be when the bicep muscle contract concentrically to flex the elbow, this would be compression of the muscle, while bending down to touch your toes with straight legs would be tension of the hamstring muscles. The latter is commonly confused as a stretch however muscle canít stretch and contract at the same time so this would be an eccentric contraction and therefore not a stretch.

It is a well known engineering principle that compression and tension forces are always perpendicular to each other. A simplified formula to calculate the tension force would be: Ft = Fc.Cos∂

Where Ft is the tension, Fc is the applied force and Cos∂ is the angle of the applied force. Note that Cos 0 = 1 so an applied force that is perpendicular is equal in compression and tension. An example of natures brilliance is an egg. If an egg were placed perpendicular on a flat surface with a force of 10N applied perpendicular to the apex of the curve the tension around the diameter of the egg would be 10N. An egg is structurally very strong in the above example, one reason for this is the curve of the shell. As the egg is compressed it changes shape from an oval shape to a circular shape. This change is very small but it is, once again a well established engineering principle that the apex of a parabola curve is the strongest point, this is actually what makes a circle an ingenious structure as any point on its circumference is its strongest.

The discs of the spine consists of two basic parts, the nucleus pulpous and the annulus fibrosis. It could be seen as a balloon filled with water with an additional balloon inside the first filled with oil. Due to the different properties of the fluid

inside the balloons they respond to compression in different ways. There is an additional point of interest with the nucleus pulpous. The fibres are arranged in a similar fashion to the curve of the egg. The arrangement is more complicated as they crisscross and have an internal concave shape and external convex shape. The curve is only slight in comparison to the egg. This is one of the reasons that the disc can absorb a high degree of compression. Of course all tissue has a limit and there are disc prolepses, when the internal balloon breaks through the outer balloon.

This now leads to the coupled forces acting on the spine. Many texts suggest the apex of the lumbar lordosis to be at L3.

Moving the compression down to the L/S junction allows more biomechanical efficiency. This is due to the following reasons.

The disc of the L/S junction is wedge shaped being thinner at the posterior and thicker at the anterior aspect. This allows the compression force to be reduced over the larger anterior part. This is why I refer to a basic formula above as it does not take into account the change of surface area. In a simplistic mechanical approach, energy is never destroyed. Force applied to a large surface area that is channelled to a smaller surface area will increase as well as the opposite being true. The second reason for the compression being applied to the L/S junction is that the tension that is produced across that anterior surface of the disc will allow the spine above the L/S junction and below, the sacrum to be able to accept higher amounts of force due to the increased flexibility they now achieve. This is mainly due to the fascia: muscles and ligaments. This increased flexibility needs to be controlled and this is where the saying: segmental spinal stability comes from. If too much force compresses the L/S junction then the facet joints will be approximated. Over time, if this force is too high it could start the degeneration of the joints and lead to degenerative joint disease. In extreme cases a spondilosis or spondilolithesis could result.

Reason number three. In engineering if you wanted to calculate the strength of a curved bar compared to a straight bar the following formula would do it for you.

S = (n)2 + 1

Where S = the strength, n is the numbers of curves in the bar. The spine has four curves, so the spine is 17 times stronger as opposed to a straight spine. What is

common to find in the clinical setting is a flexed L/S junction with the L4/5 flexed and then extension of the spine up into the lower thoracic area. This straightens the spine and will make the neck and shoulder muscles very tight. This is one of the most common reasons for chronic tight neck and shoulder muscles. The sacrum and thoracic are primary curves while the lumbar and cervical are secondary. The next reason for the compression at the L/S junction is the spine could be closely compared to a fishing rod from a mechanical point. The handle of the rod would be the thicker lumbar spine while the end of the rod would be the thinner cervical spine. Just like a fishing rod, a small input from the hand produces and exponential force at the end of the rod. The difference with the spine is that we have a weight of 7-8% of body weight attached to the top of the spine with many muscles holding this weight onto the spine. Having the compression of the spine higher up the lumbar spine would be the same as holding the fishing rod closer towards the end of the rod, reducing power transmission. Since energy is never destroyed it shall be absorbed by the spine leading to degenerative joint disease.

So what can we do to correct the chronic tight muscles of the neck and shoulders? Easy, by maintaining your neutral L/S junction extension when you sit you will increase the flexibility of the spine and allow the cervical spine to move into its neutrally aligned position. This could be explained in an easy example. If you sit with your lower back rounded out away from your abdomen then your neck and shoulders muscles will tighten up. To understand this incorrect sitting posture imagine you were looking at a cyclist in the Tour de France from the side as they rode by. You will see that the lower back or lumbar spine is rounder away from the abdomen. If the cyclist didnít want to tighten up the muscles of the neck and shoulder they would need to look straight down at the road where the wheel made contact with the road. This is dangerous, as you could not see where you are going, so they then have to tip the head backward towards the spine to look forward. This leads to tight muscles of the neck and shoulder as well as the joints of the spine becoming weight bearing, something they were not designed for. Degenerative joint disease would result if this position was maintained for long periods. If I am correct then many professional cyclists will have cervical spine symptoms ranging from mild to sever. The

implications of this type of posture are far reaching and have a significant effect on the entire body. One point of note here is the effect on the digestive system. Many parts of the digestive system are retroperitoneal. A flexed lumbar spine increases the tension in these retroperitoneal attachments thereby reducing the mobility of the gastrointestinal tract. This ultimately leads to food putrefying and setting up a condition of dysbiosis. I leave you with this thought. Biomechanics is the marriage of western and eastern medicine. With good biomechanics, the viscera are allowed to have the mobility and motility they require to function. Poor biomechanics impedes their function. I have only given one example however there are many examples for every viscera of the body. The human body is an incredible creation, using the body in the manner in which it was designed to function is what will lead to health.

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