The biomechanics of the lower back was reviewed as a basis for establishing a load handling limit. Topics included occupational biomechanics of load lifting, biomechanical basis for back injury, compressive strength of lumbar spinal column, biomechanical models, effect of dynamic factors on spinal stress, intraabdominal pressure, asymmetric lifting, effect of task variables on stresses to spine, vertical location of load, horizontal location of load, speed of lifting, lifting straight up, and lifting technique. Studies have indicated that lifting compact objects of moderate weights close to the body can create compressive forces sufficient to cause damage to some lumbar intervertebral discs. The compressive force is directly related to the horizontal moment arm of the load and thus even light loads should be lifted close to the body through proper workplace design and education and training of the workers. Both the intradiscal pressure and compressive force increase with an increase in angle of trunk flexion. Asymmetric lifting results in significantly lower maximum voluntary muscle strength and higher compressive force, intradiscal pressure, myoelectric activity and antagonistic activity of trunk muscles. Frequent lifting of moderate to heavy loads can result in fatigue fracture of the lumbar intervertebral joints plus cardiovascular stresses and localized muscle fatigue. Rapid or jerking motions can impart substantially higher and potentially dangerous stresses to the low back structures.
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