A method is presented to obtain lines of action of nine major trunk muscles acting on the lumbar spine. The muscles are modeled as one or several linear or curvilinear lines of action obtained by digitizing the origins and insertions of main bundles of muscles. For curvilinear lines of action, points were also digitized along the length of the bundles, and a curve-fitting method was used. Lines of action are displayed graphically for one embalmed cadaver. Tabulated data for unit force vectors and unit moment vectors about the L3-L4 and L4-L5 discs are also presented.