Forward bending and lifting may provoke lumbar disc herniations.
Herniated lumbar discs are quite common and can be very painful and disabling. While it may not be possible to prevent all lumbar disc herniations, knowing the likely mechanisms of injury and applying that knowledge can reduce the odds of sustaining this unpleasant injury. A number of factors may contribute to lumbar disc damage. The mechanism of injury most commonly implicated in lumbar herniation involves forward bending, or flexion, of the lumbar spine.
The discs of the lumbar spine are specialized connective tissue structures that serve as shock absorbers between the spinal bones. Discs are made springy by a moist, jelly-like center portion called the nucleus pulposus.
The nucleus pulposus is surrounded and contained by more dense, layered, fibrous tissue that makes up what is called the annulus fibrosis. Discs "herniate" when damage to the annulus fibrosis allows the nucleus pulposus material to escape its confines and protrude outward from the disc.
Although lumbar disc herniations commonly occur, they are not easily reproduced in a research setting. However, it is notable that in published studies in which disc herniations were produced in animal or human lumbar spine specimens, most required some component of spine flexion. For this reason, Stuart McGill, Ph.D., author of the book "Low Back Disorders," concludes that repeated or prolonged spine flexion is the primary mechanism leading to lumbar disc herniation.
In a laboratory setting, McGill and his colleagues found that the easiest way they could produce herniations in spine specimens was to place a load, or weight, onto to the spine while repeatedly bending it forward into flexion. These conditions are consistent with what would occur with repetitive lifting activities in which the spine is flexed forward.
A study presented in 2001 in the journal "Clinical Biomechanics" demonstrated a link between compression of the spinal joints -- technically known as vertical loading -- and disc herniation. Although the study used spine sections taken from the necks of pigs, the researchers attempted to reproduce loading patterns common to the lumbar spines of humans.
In the study, larger and more frequent disc herniations resulted when compression forces on the spinal bones were increased. However, relatively small compression forces were necessary to herniate discs that were exposed to high repetitions of forward and backward bending. The authors concluded that repeated flexion and extension motions probably play a larger role than compression alone in causing disc herniations. In other words, the spine can withstand large compression forces, like lifting, when in a neutral upright position. Adding a load while bending the spine forward and backward can quickly spell trouble.
Another study published in 2010 in "Cinical Biomechanics" examined the role of axial torque, or twisting, in disc herniation. Using an animal model, the researchers found that isolated twisting did not cause disc herniation. However, twisting did damage the annulus fibrosis.
With damage to the annulus, the discs were much more susceptible to herniation when exposed to repetitive flexion movements. This suggests that, even if the mechanism of a disc herniation is flexion, damage caused by spinal twisting may contribute to disc herniations by weakening the disc.
A study published in 1987 in the "Journal of Chronic Diseases" examined the risk of lumbar disc herniations with occupation. The researchers reported that the risk for this condition is greater among men in blue-collar jobs compared to those in white collar-jobs. This likely relates to increased bending and lifting activities among blue-collar workers compared to white-collar workers. The authors noted less variation by occupational groups among women. The risk of lumbar disc herniations, however, was higher among women who described their work as strenuous.
An additional finding that may relate to the spinal flexion mechanism is that the risk of lumbar disc herniations is high among people whose work involves driving a motor vehicle. Sitting while driving places the lumbar spine in a sustained posture of slight flexion.