The biomechanics of the lumbar multifidus.

The possible actions of the lumbar multifidus were determined by plotting the points of attachment and orientation of each of its component fascicles on radiographs of 5 cadavers and 21 living subjects. Subsequent analysis revealed that the principal action of multifidus is posterior sagittal rotation (extension without posterior translation) of the lumbar vertebrae. It has no translatory action. Any axial rotation exerted by the lumbar multifidus is only a minor, secondary action which must be coupled with posterior sagittal rotation. This extension balances the flexion moment generated by the abdominal muscles which rotate the trunk. The constancy of the sites of attachment of the multifidus allows each of its fascicles to be plotted accurately on radiographs or computer diagrams which can be used to produce highly detailed analyses or models of the forces exerted by the multifidus on the lumbar spine.