Discs are the pads between the vertebrae of the spine. There are twenty-four mobile vertebrae and five that are fused together to form the sacrum. Discs are vulnerable to injury because they have the difficult job of being flexible enough to move, but strong enough to take enormous stress.

The discs between the lower lumbar vertebrae are the most commonly injured because they support the weight of the entire upper body. The structure of the disc determines the extent of injury, if one occurs.

Disc herniation, rupture, protrusion, and extrusion are all terms that describe this injury. There are subtle differences between these terms, but they all refer to a tear in the outer ring, which allows some of the inner core to escape. A herniation occurs when a portion of the soft inner core (nucleus pulposis) escapes through the outer ring (annulus).

The disc may look like a solid piece of padding between the vertebrae, but it is far more complex. Discs are composed of two parts that work together to carry the load.

The strong outer ring or annulus, is firmly attached to the vertebrae above and below the disc. The ring has strong crosshatched fibers much like the steel belts of a car tire. This outer ring is about 1/4 inch thick, and by itself, doesn't have much compressive strength. If this ring were empty, it would collapse when weight was applied to the bone above. The outer ring holds the nucleus pulposis in place.
The nucleus pulposis is located inside the annulus. This is a soft gelatin-like cartilage with a very high water content which provides cushioning while evenly supporting the weight of the body. This cartilage transmits the load from bone to bone.

There is a delicate balance between the forces trying to push the gelatin-like nucleus out to the sides and the tension of the outer ring holding it in place.

The disc has two functions:

it transmits the weight of the body from the bone above to the bone below.
it allows the spine to be flexible.

The lumbar (lower back) discs must support the weight of the entire upper body plus whatever is picked up or carried. Because the body's center of gravity is in front of the discs, loads on the disc are increased by leverage, much like a nutcracker multiplies force on a nut. Research has shown that picking up a 20 pound weight may increase pressures inside the disc by more than eight times the body's weight. The job of the disc is to transmit this load efficiently and evenly to the vertebra below. The gelatin-like nucleus pulposis distributes the stress evenly across the surfaces of the adjacent bones. The combination of soft inner core and strong outer ring also allows maximum motion between the bone segments. Both of these functions are severely tested in the lumbar spine, where enormous loads frequently occur and physical activities demand wide ranges of motion.

What is a herniated lumbar disc?

The outer ring of the disc is subject to continuous stresses that sometimes cause small tears to develop. If one of these tears is large enough and the stresses inside the disc strong enough, some of the pulposis can leak through the tear. The disc is most vulnerable to injury at the weakest parts of the outer ring, where nerves pass through the spinal canal and exit toward the legs. If disc material compresses one of these nerves, there can be pain both in the back and down the leg . Pain down the leg is often called sciatica,because the most frequently pinched nerves form the sciatic nerve, which runs down the back of the thigh to the lower leg.

A disc can herniate due to either a sudden load or an accumulation of minor injuries over time. At least 50% of patients can't recall a specific event that caused the ruptured disc. When an event can be identified, it often involved lifting and twisting; activities that place severe stresses on the disc.
Discs are named for the bones above and below them. For example, the L4-5 disc is between the 4th and 5th lumbar bones. The L5-S1 disc is between the 5th lumbar and 1st sacral bones. These two discs do the most work and are the most frequently injured. Paralysis rarely occurs with disc herniation. The spinal cord proper ends at the first lumbar vertebra (L1), well above these frequently injured discs.
Other conditions that cause back and leg pain can mimic a ruptured lumbar disc. Some of these conditions are:

arthritis of the hip or knee area.
pulled muscles in the back (lumbar strain).
pulled muscles in the leg and other conditions that cause irritation of the spinal nerves.