Influence of lumbar and hip mobility on the bending stresses acting on the lumbar spine.

Bending and lifting activities are associated with injury to the lumbar discs and ligaments, and cadaveric experiments suggest that this damage is most attributable to a high bending moment (bending stress) acting on the osteoligamentous spine. We examined the hypothesis that people with poor sagittal mobility in the lumbar spine and hips apply higher bending stresses to their spines during everyday lifting activities. Forty-nine subjects performed a series of simple forward bending and lifting exercises while their lumbar flexion was measured continuously using a skin-surface technique (3-space isotrak). Peak flexion angles were compared with the bending properties of cadaveric osteoligamentous spines in order to calculate the peak bending moment (bending stress) acting on the lumbar spine during each exercise. All subjects flattened or reversed their lumbar lordosis when lifting, and most came close to or exceeded their static in vivo limit of lumbar flexion in many of the activities. The bending moment acting on the lumbosacral junction rose to about 30 Nm, which is about 50% of that required to cause injury in a single lift. Bending moments were significantly lower in subjects who had good sagittal mobility in the lumbar spine. Good hip mobility was similarly associated with a reduction in bending moment, but this reached significance only in subjects who reported a history of low back pain.