A modeling study was undertaken to determine the effects of flexion on the forces exerted by the lumbar back muscles. Twenty-nine fascicles of the lumbar multitidus and erector spinae were plotted onto tracings of radiographs of nine normal volunteers in the flexion position. Moment arms and force vectors of each fascicle were calculated. The model revealed that moment arms decreased slightly in length resulting in no more than an 18% decrease in maximum extensor moments exerted across the lumbar spine. Compression loads were not significantly different from those generated in the upright posture. However, there were major changes in shear forces, in particular a reversal from a net anterior to a net posterior shear force at the L5/S1 segment. Flexion causes substantial elongation of the back muscles, which must therefore reduce their maximum active tension. However, if increases in passive tension are considered it emerges that the compression forces and moments exerted by the back muscles in full flexion are-not signi ficantly different from those produced in the upright posture.