Lower Back - Lumbar Spine

The lumbar spine is essentially a series of five building blocks called vertebra. They stack up, one on top of the other. They sit on the sacrum, the triangular bone wedged into the pelvis. The sacrum is the "base bone" of the spine. The vertebrae are separated in front by the discs. The discs are the shock absorbers of the spine. They allow motion and yet absorb impact.

The back part of the spine contains the joints called the facets. These facets are the governors of spinal motion. They will allow bending forward, backward and side to side but resist rotation. They also act as door stops to prevent one vertebra from sliding forward on the one below.

The transverse processes protrude from the sides of the vertebrae and are essentially lever arms that muscles attach to. These levers allow smaller muscles to move the vertebral segments more easily. The spinous processes which project out the back of the vertebra are also lever arms. Strong ligaments attach that prevent the spine from bending too far forward and when bending backwards, these processes abut, preventing too much backwards motion. These spinous processes are the bumps seen on the back. 

To better understand these conditions, please read Anatomy of the Lumbar Spine first.