The trunk muscles of the lumbar region were studied using magnetic resonance imaging (MRI) in five male volunteers placed in neutral and in two twisted postures. Using a CAD digitizing system, the positions of trunk muscle centroids and the cross-sectional areas of these muscles were measured from the transverse scans at 1 cm intervals from L2 to S1. Muscle lines of action were created by connecting the muscle centroids from the successive sections. Changes in cross-sectional areas and displacements of centroids in a three-dimensional coordinate system and the local two-dimensional coordinate system were estimated at each disc level. In the three-dimensional coordinate system, all muscle centroids were displaced when twisting. Thus, the locations and orientations of all muscle lines of action changed. In the local two-dimensional coordinate system, only the muscle centroids of four abdominal muscles were displaced. Most of these displacements occurred in the first 25° of twisting. Higher up in the lumbar spine, the displacement was greater. The changes in moment arms were sometimes as great as twofold. Only the abdominal oblique muscle (AOM) changed their cross-sectional area significantly; the area of the right AOM ipsilateral to the side of twisting increased, while that of the left AOM was decreased.
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