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Effects of torso flexion on fatigue failure of lumbar motion segments.

Gallagher-S; Marras-WS; Litsky-AS; Burr-D
American Industrial Hygiene Conference and Exposition, May 8-13, 2004, Atlanta, Georgia. Fairfax, VA: American Industrial Hygiene Association, 2004 May; :31
Bone mineral content, density, and vertebral body area measurements were made of 12 cadaver lumbar spines, whose motion segments (24 were studied) were later subjected to a fatigue loading regimen simulating lumbar loads when lifting a 9-kg box in three angles of torso flexion (0, 22.5, and 45 degrees). Motion segments were loaded every 3 seconds using a loading profile (compression, shear, and load rate) appropriate for each torso flexion angle. A maximum of 10,020 cycles was employed. Specimens surviving the entire number of cycles were treated as censored observations. A survival analysis (Cox regression analysis) was used to evaluate the impact of torso flexion and bone mineral characteristics of the motion segments on the number of cycles to failure. Results of the Cox regression analysis demonstrated that, compared to the referent condition (0 degrees torso flexion), 22.5 degrees of torso flexion had a relative risk of 15.1 (95% CI: 2.7-83.5), while the 45 degree torso flexion condition had a relative risk of 29.4 (95% CI: 4.6-185.9). Bone mineral content was found to have a protective influence with respect to the number of cycles to failure (RR = 0.917; 95% CI: 0.85-0.99). Results suggest that torso flexion, especially extreme flexion, leads to rapid fatigue failure of lumbar motion segments. Increases in bone mineral content were found to extend the fatigue life of motion segments. Findings suggest that torso flexion should be avoided in lifting tasks, and that aging workers may be particularly vulnerable to fatigue failure of spine tissues, due to the loss of bone mineral content (particularly among women) that occurs with aging.
Fatigue; Motion-studies; Bone-disorders; Bone-structure; Risk-analysis; Demographic-characteristics; Age-factors; Manual-lifting; Injuries; Back-injuries; Statistical-analysis
NIOSH Pittsburgh Research Laboratory, P.O. Box 18070, Pittsburgh, PA 15236
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American Industrial Hygiene Conference and Exposition, May 8-13, 2004, Atlanta, Georgia