Computed tomography is valuable in obtaining in vivo length of muscular moment arms, which can be used in biomechanical models. Using computed tomography, the authors measured lengths of the muscular moment arms of the erector spinae, psoas, oblique abdominal, and rectus abdominis muscles to the bilateral motion axis (about which flexion/extension movements occur) and to the anteroposterior motion axis (about which lateral bending occurs), both axes running through the lumbosacral joint. The subjects were 21 patients. Examinations were performed on a General Electric CT/T 7800 scanner. The average erector spinae moment arm to the bilateral motion axis was 68 mm, which is greater than the moment arm used earlier. The results also revealed significant differences in moment arm length between men and women for several muscles. The authors conclude that for some muscles different moment arm lengths should be used for men and for women. The moment arms reported are proposed for use in biomechanical models for calculating lumbosacral joint load.