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Effect of torso flexion on the lumbar torso extensor muscle sagittal plane moment arms.

Jorgensen-MJ; Marras-WS; Gupta-P; Waters-TR
Spine J 2003 Sep-Oct; 3(5):363-369
Accurate anatomical inputs for biomechanical models are necessary for valid estimates of internal loading. The magnitude of the moment arm of the lumbar erector muscle group is known to vary as a function of such variables as gender. Anatomical evidence indicates that the moment arms decrease during torso flexion. However, moment arm estimates in biomechanical models that account for individual variability have been derived from imaging studies from supine postures. The purpose of this study is to quantify the sagittal plane moment arms of the lumbar erector muscle group as a function of torso flexion, and identify individual characteristics that are associated with the magnitude of the moment arms as a function of torso flexion. The study designed was utilization of a 0.3 Tesla Open magnetic resonance image (MRI) to image and quantify the moment arm of the right erector muscle group as a function of gender and torso flexion. Methods used were Axial MRI images through and parallel to each of the lumbar intervertebral discs at four torso flexion angles were obtained from 12 male and 12 female subjects in a lateral recumbent posture. Multivariate analysis of variance was used to investigate the differences in the moment arms at different torso flexion angles, whereas hierarchical linear regression was used to investigate associations with individual anthropometric characteristics and spinal posture. The results showed the largest decrease in the lumbar erector muscle group moment arm from neutral to 45-degree flexion occurred at the L5-S1 level (9.7% and 8.9% for men and women, respectively). Measures of spinal curvature (L1-S1 lordosis), body mass and trunk characteristics (depth or circumference) were associated with the varying moment arm at most lumbar levels. The study concludes that the sagittal plane moment arms of the lumbar erector muscle mass decrease as the torso flexes forward. The change in moment arms as a function of torso flexion may have an impact on prediction of spinal loading in biomechanical models.
Muscle-function; Biomechanical-modeling; Spinal-cord; Human-factors-engineering; Biodynamics; Body-mechanics; Anthropometry; Men; Women; Musculoskeletal-system
Wichita State University, Industrial and Manufacturing Engineering Department, 120 Engineering Building, Wichita, KS 67260-0035
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Journal Article
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NIOSH Division
Priority Area
Disease and Injury: Low Back Disorders
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The Spine Journal