What Causes Low-Back Pain

By Steve McCaw

Part of Biomechanics For Dummies Cheat Sheet

Low-back pain affects many people. It’s often said that a person with low-back pain suffers from a “slipped disk,” but the better term is a bulging disk. Regardless of what it’s called, low-back pain is very debilitating, causing both pain and muscle weakness.

The spine is the backbone of the body. It consists of 24 individual bones called vertebrae. Each vertebra is adapted to provide support, protection, and sites for muscle attachment. Between each pair of vertebrae is an intervertebral disk, a structure made of layers of tough connective tissue called the annulus fibrosus. The annulus surrounds a gelatinous center called the nucleus pulposus. The structure of the intervertebral disks is uniquely adapted to help keep the vertebrae in alignment while allowing for limited motion between each pair of vertebrae. The motion of the spine reflects the combined motion between the pairs of adjacent vertebrae.

The spinal cord runs from the brain down the length of the spine within a protective channel formed by the vertebrae. A pair of nerves branch off the spinal cord and pass out of the spine between each pair of vertebrae, one to the left and one to the right. A nerve contains both motor neurons (which send signals away from the spinal cord) and sensory neurons (which bring signals to the spinal cord). Each nerve goes to a specific region of the body. The nerves in the low back, the lumbar portion of the spine, bring sensation from and control muscles in a region of the leg.

Low-back pain can develop when the tough outer layer, the annulus, breaks down and the gelatinous center, the nucleus, pushes it out, creating a bulge. The vertebral disk is sort of like a jelly-filled donut. If you step on one side of a jelly donut, the gooey center squishes out the opposite side of the donut. A bulge in the disk occurs similarly, although not quite as dramatically. When the spine bends forward, the vertebrae squeeze (apply a compressive load) on the front of the disk and pull (apply a tensile load) on the back of the disk. The compressive load on the front pushes the nucleus pulposus toward the back of the disk, where the annulus fibrosus has been stretched. If there is a weakness in the annulus fibrosus, from a congenital defect or from a breakdown of the connective tissue, the repetitive pushing of the nucleus pulposus can eventually cause the connective tissue to bulge out and push on the nerve (sometimes called a pinched nerve). The push disrupts the signal transmission along the nerve, leading to muscle weakness, pain, and numbness in the area of the body served by the nerve.

A bulging disk can occur from a single incident, such as a fall or violent collision that loads the back. However, the most common mechanism of a bulging disk is repetitive forward flexion of the spine. This form of overuse can lead to a gradual breakdown of the annulus, and then an identifiable event (leaning forward to pull an item out of the trunk of the car) triggers the rupture of the annulus and produces the bulge that presses on the nerve. Maintaining the inward curve of the low back while standing and sitting, and particularly while lifting with the arms, is a valuable preventive step to avoid low-back pain.