"Oh, my aching back"

Eighty percent of human beings experience lower back pain at some time during their lifetimes. This statement probably does not fully reflect the greater number of people who have backaches that do not prevent them from working or performing everyday activities but make their daily activities painful.
The spine is a mechanical structure that supports the individual from birth troughout her or his life. The spine is known as the vertebral column and is essentially a column of one functional unit upon another. These functional units, posioned one upon another and balanced upon the sacrum, keep the column erect and in perfect balance against gravity.
The lumbar spine contains usually five sometimes there is four or six vertebrae and forms a normal curve in the erect posture called lordosis. Between the vertebrae and the discs emerge the nerves that descend into the legs. Every pair of vertebrae of the spinal column is separated by a disc. The disc is a hydraulic system that keeps the vertebrae apart. It acts to cushion any balance or pressure and permits the functional unit to move in flexion to the front, extension to the back, and to the side (lateral flexion).
The disc is made of two separate parts: an outer layer that is termed the annulus fibrosus and a central core termed the nucleus pulposus.
A central core, a transparent jelly, contains 88 percent water. No blood vessels or nerves penetrate the nucleus which is tightly bounded peripherally by fibrous tracts. An outer layer is made up of concentric fibres, which are made of tissue called collagen. These fibres are layered so that the first layer of fibres crosses in an oblique direction from one vertebra to the other. The next inner layer of fibres crosses from one vertebra to the other in the opposite direction, thus causing these fibres to criss-cross and intertwine. Each layer of fibres goes in an opposite direction. This arrangement gives strength to the disc annulus, yet permits the vertebrae to move in any direction. The fibers of the annulus can strech to a limited degree; therefore, when the vertebrae are compressed together, the fibres stretch but do not tear. When the vertebrae bare on each other, the fibres can stretch enough to allow bending but do not tear. When the fibres are twisted the fibres become stretched beyond their limits and thus tear.

A site of low back and leg pain

A nerve root emerges through the foramen of each functional unit. There is a nerve root emerging between each pair of lumbar vertebrae, that is: L1-2, L2-3, L3-4, L4-5 and L5-S1; and several nerve roots emerge through the foramen of the sacrum. A small branch from each nerve root arches backward to the back muscles, to the joints, to the skin, and to the ligaments that carry sensation of the lower back control the back muscles.
As they emerge from the spinal canal through the intervertebral foramen, the nerve roots are contained with a dural sac. This dural sac is well supplied by nerves, making it sensitive. The dural sac is a site of pain when irritated, inflamed, or injured!

Back pain caused by bad posture.

Increasing the lumbosacral angle by changing the angle of the sacrum changes the curvature of the lumbar spine. In this drawing, the lordosis is increased, causing possibly lower backache. Bad sleeping habits, especially lying on a soft, sagging bed or sleeping on one's stomach, cause lower backache from exessive lordosis.
Faulty sitting positions may cause lower backache. A chair that causes or allows exessive lordosis is worse than useless.
"Sway back" from pregnancy or high heels increases lordosis and can cause lower backache. Not all sway backs are painful. It is oly when lordosis is exessive, or accentuated, that pain may result. Once the body has bent forward 45 degrees from an erect posture, the lumbar spine no longer can bend. From there on the pelvis rotates to allow the body to bend farther down. In order to allow the pelvis to rotate, the muscles behind the thighs and in the buttocks also have to be flexible. If the long muscles behind the tingh, meaning the hamstring muscles are exessively tight, the pelvis is prevented from rotating long before it has accomplished its potential movement. The reminder of bending becomes imposed upon the lower back, the lumbar spine. As the lumbar spine can bend only approximately 8 to 10 degrees at each functional unit if the pelvis is stopped midway in forward rotation, each functional unit must then bend exessively. The tissues of the lower back now must exeed the 8 to 10 degrees of normal flexion. This results in the tissues becoming overstreched and causing lower back pain!
As regards lower back lordosis, abrupt lifting and with unbent knees causes lower back pain!
Should the muscles on only one side of the spine become irritated and cause a spasm, the spine becomes pulled to one side. This twisting of the spine to one side is known as acute scoliosis.
Lower back pain resulting from spinal column stress can occur from activities as innocuous as sneezing, stepping down steps that are not there, stepping into a hole, coming up unexpectedly under an overhead beam, and hitting your head, etc.
Such activities put mechanical stress upon the spine that at the instant of stress, may be in an awkward position, such as while bending, twisting, and arching. If repeated incidences of these occur, the back can be weakened and become more susceptible to subsequent minor injuries.
When these activities occur,the powerful muscles of back perform powerful contractions in a very small distance. They literally jam the joints together; these joints have been specified as being sensitive, and thus inflamation results from this injury.
Inflamation is merely the reaction of the abused tissues. When these tissues are irritated by inflamation the muscles around them immediately go into protective spasm.

Lower back pain with Leg pain: The Ruptured Disk

If the disk bulges, ruptures, slips, or herniates, whatever term is used, the nucleus either ruptures out of its annular container or pushes the remaining untorn annular fibers out into the spinal canal or into the intervertebral foramen. The nucleus is under a great deal of pressure, held between the opposing end plates of the vertebrae and surrounded by annular fibres. If the fibres tear, the pressure within the nucleus exerts force in an attempt to allow the nucleus to escape. It cannot escape through the end plates, so it forces its way outwards to the periphery.
A patient may have leg pain originating from the lumbar spine without back pain or may have both back pain and leg pain. Leg pain existing and originating from the back is medically termed sciatica. Sciatica, which is sciatic nerve pain, is considered to result from irritation or inflammation of the nerve roots. Nature always responds to a painful inflamed tissue and prevents it from being irritated. Therefore, the muscles of the legs will go into spasm to prevent the leg from becoming elevated, or bent at the hip. The back muscles will go into spasm to prevent the back bending, which also stretches the nerve.
The pain can be nagging, aching, stabbing, shooting, or a burning sensation felt in the leg in the distribution of the specific nerve root.
It is only when the herniating nucleus presses against the deep surface of the posterior longitudinal ligament that the nerve endings of the ligament are stretched causing lower back pain (lumbago). Finally compression of the nerve roots by the herniating disc causes nerve root pain, i.e. scitica.