The vertebrae of the lumbar spine are the largest vertebra in the body because they carry the most weight and are subject to the greatest amount of stress. That being said, it is the area most frequently associated with back and leg pain. The two lowest levels, (L4-L5) and (L5-S1), have the most flexion and extension motion stress, and the highest rate of injury. The intervertebral discs of these two levels also have the highest rate of disc degeneration and herniation. The L5 vertebra is the most common site of spondylosis and spondylolisthesis. The most common lumbar nerves that get pinched or entrapped are the L4, L5, S1 levels.