Mechanical factors are considered to play a dominant role in low back problems. Various spinal structures, including muscles, act in unison to resist the external load, including the body segments. An estimation of the forces in these requires a knowledge of the orientation, location, and area of cross-section of the muscles to complete the information for the formulation of a truly three-dimensional biomechanical mathematical model of the spine in the lumbar region. Computed tomography scans of 10 Japanese patients suffering from chronic low back pain were obtained to determine the geometric data of the abdominal and back muscles from the 12th thoracic vertebral to the first sacral vertebral level. The mean age +/- 1 SD of the group was 40.1 +/- 14.12 years (range, 24-70), 573 +/- 88.5 N of body weight (range, 441-705), and 1.63 +/- 0.09 m tall (range, 1.44-1.74). The geometric parameters quantified were the line of action, and the centroid and physiologic area of cross-section of each muscle as a function of the spinal level. The effective/physiological area of cross-section of each muscle changed along the length of the spine because of the change in the line of action of the muscle. The centroidal approach adopted for quantifying the lines of action of various muscles was found unsuitable for the abdominal muscles, excluding the rectus abdominis, because of the associated anatomic complexities. Alternatives are proposed to complete the data base. The application of the data for the formulation of a truly three-dimensional biomechanical model of the spine at the L3-4 level is briefly presented. Application to nonlinear optimization-based force predictions in various spinal structures is discussed.