herniated disc (disc bulge) is a common cause of lower back pain. The two bottom discs in the lumbar spine, those between the 4th and 5th vertebrae (L4-5) and between the 5th lumbar and 1st sacral vertebrae (L5-S1) are the discs that are most prone to bulges or herniations.

A herniation happens when the thick outer layer (annulus fibrosis) of an intervertebral disc bulges or ruptures. When this happens the gel-like interior of the disc (nucleus pulposus) can break through the outer annulus and protrude out into the intervertebral space where it can press on a spinal nerve causing leg pain, leg numbness or cramped or weak leg muscles.