The lower back, also known as the lumbar spine, includes the vertebral bones that doctors label as L1 to L5, and it is injuries to this area of the spine that most often cause lower back pain and low back pain.
The lower back in visual terms may describe the bottom of the back where you can see and feel a forwards curve. In anatomical terms the lower back may correspond to the lumbar spine, which includes the last five vertebrae (spinal bones) as well as the intervertebral discs that separate them and other associated muscles and soft tissues. The terms "lower back" and "lumbar spine" are used interchangeably.
The basic element of the lumbar spine is the 5 lumbar vertebrae. These discs, when described by doctors or in X-Rays, are labeled L1-L5. The intervertebral discs that separate the vertebrae are labeled according to the level of the spine that they occupy. For example, the disc between the 12th Thoracic Vertebrae and the first Lumbar vertebra is labeled T12-L1, and the disc between the last lumbar vertebra and the sacrum is labeled L5-S1.
The lower back begins at about 5 inches below the shoulder blades and connects to the sacrum at L5-S1. Together. The sacrum, which is located within the cavity of the pelvis, is held in its position by the sacroiliac joints. The Sacroiliac joints themselves attach to the Ilia of the pelvis, and maintain the stability of the bottom of the spine while allowing for some movement.
The bones and joints of the lumbar spine are weight bearing structures that take on a lot of gravities energies when we do things like squat down to pick up weights, land on hard surfaces from jumps, and sit for long periods of time. Due to the unique architecture of our upright vertebral spine, and its location near our center of gravity, our lower backs sustain more pressure than the other sections. Yet, these structures are built thicker and denser to meet these challenges. The body of the lumbar vertebrae is larger, and the laminae and pedicles are thicker and stronger. The discs of the lumbar spine are also wider and thicker to absorb shock and take on substantial loads.
The lumbar spine meets the sacrum at the L5-S1 disc, which allows for some rotation, so that the hips and pelvis may swing when we walk and run.
Though the structures of the back are described being part of the lumbar spine, the spinal cord itself actually ends at about T12 and L1. Past this point, the nerves continue and accumulate into a bundle of nerves called the "cauda equina", names because of it's similar appearance to a horse's tail. These nerves extend past the cauda equine and through to the hips, buttocks, legs, and feet. Because the spinal cord does not extend past the top of the lumbar spine, injuries and degenerative conditions in the lumbar spine do not typically cause paralysis. Though it should be noted that conditions such as nerve impingement of the lumbar and sacral nerve roots and the cauda equine could cause the loss of bowel and bladder control, and the loss of conscious motor movement of the legs or feet. Conditions such as these are the ones that require immediate emergency treatment or back surgery.
The two lowest discs, the L4-L5 and L5-S1 discs bearing the most weight and are therefore the most vulnerable to injury. Back pain conditions such as sciatica, and low back pain with radiculopathy are often associated with spondylolysis of these bottom intervertebral discs.