Experiments on cadaver spines have quantified the bending stresses (bending moment) required to cause overload injury or fatigue failure to the lumbar spine. This paper reviews subsequent experiments in living people which sought to identify the circumstances that cause spinal bending to rise to potentially damaging levels in life. Healthy volunteers performed various bending and lifting activities in the laboratory, and peak bending moment was quantified from measurements of lumbar flexion, obtained using the 3-Space Isotrak. Bending stresses approached or even exceeded estimated fatigue limits in some subjects when they performed lifting tasks in the early morning (when the spine's bending stiffness is increased), and following fatigue of the back muscles. Individuals with a small range of lumbar flexion were particularly likely to exceed safe limits. These findings may explain why repeated bending and lifting are so closely associated with low back problems.
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