Lumbar Disc Herniations

Between each vertebra of the spine sits an Intervertebral disc. These discs act as a shock absorber (similar to shocks in a car) to protect the bones of the spine from pushing against each other. They also allow movement to occur. It is comprised of two parts, an outer layer called the annulus fibrosis, and an inner layer called the nucleus pulposus.
 
An intervertebral disc herniation can occur traumatically, all at once, say a deadlifting injury at the gym, or it can happen, and most commonly does happen, slowly from prolonged sitting. Over time the nucleus pulposus delaminates the layers of the annulus fibrosus and it is the nucleus pulposus that herniates out of place.
 
This leaked out material can either put direct pressure on the nerve root or spinal cord, causing a variety of symptoms including but not limited to: pain radiating down the leg, parasthesias, numbness and in some cases even weakness in the leg.

Predisposing Factors
A co-morbid factor of this condition is often a desk job that involves sitting for most of the day. This type of patient usually has poor sitting posture, which can lead to a weakening of your spinal erectors and destabilize the spine, contributing to the condition.