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establish confidence in causal inference, but alone, the biomechanical studies do not provide the mechanism.

The basic science studies that document damage and injury mechanisms provide biological plausibility for the associations from the biomechanical studies and epidemiologic studies. The biomechanical studies alone, while elegant, have limitations in that the measurements are more intrusive and obvious than observations in the casual work environment. This intrusiveness limits the generalizability of measures and the implication of findings for the ambient work environment. The epidemiologic findings based on broad population data, however, provide context, indicating that the data from the biomechanics studies are neither isolated nor irrelevant. Although each type of study can be criticized as limited, it is the pattern across the different study designs and types of measures that affects the ability to consider the relationship as causal. Moreover, the strength of the evidence is significantly enhanced by the consistency associated with the pattern of evidence. For example, low back disorder risk has been established through epidemiologic studies for work that involves heavy lifting as well as other risk factors. The relative risks have been derived from a rigorous evaluation of the literature and have been found to be reasonably strong and consistent. Furthermore, the studies control for confounding and indicate a presence of temporal association and dose-response relationships.

The epidemiologic literature that specifically quantifies heavy lifting shows that risk of injury is greatest when loads are lifted from low heights, when the distance of the load from the body is great, and when the torso assumes a flexed, asymmetric posture. Thus, risk estimates significantly increase when work risk factors can be classified with greater precision. These studies typically have relied on more precise endpoints (such as injury logs), but they have limitations to the extent that they have not involved prospective assessments. A number of such studies have the limitation that the results are based on self-reports of injury.

Collectively, a pattern emerges demonstrating that risk is associated with specific characteristics of the workplace. By exploring the biomechanical basis for this risk, the pattern of evidence becomes even stronger. Biomechanical studies have shown that the loading of the spine is increased dramatically when a load is lifted from a low height in an asymmetric posture. According to the basic biology tolerance literature, the spinal load tolerance is also greatly reduced when the torso is positioned in a flexed posture or torsion is applied along the axis of the spine. Basic biological research has also described how spinal loading can lead to cell death within the disc. Furthermore, the basic science literature has described pathways for the perception of pain when specific structures in the spine are loaded under these conditions. Finally, intervention studies



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