Geometry of adult rectus abdominis and erector spinae muscles*.

Mathematical modeling of human joints requires accurate information concerning anthropometric parameters such as muscle cross-sectional areas and muscle moment arms. The purpose of this study was to calculate the average crosssectional area, average volume, and length of the muscle moment arms about the spinal column for the rectus abdominis (RA) and the erector spinae (ES) for those portions of the muscles that are involved in spinal flexion and extension. Materials for this study came from archival computed tomography scans of 28 living subjects. The muscle outlines were digitized and the moment arms calculated as a percentage of anteroposterior trunk depth. The average cross-sectional area of the RA was 10.5 cm2, and 31.75 cm2 for the ES. The average volumes for the RA and ES were 356.16 cm3 and 843.46 cm3, respectively. The muscle moment arm ratio was 0.29 for the ES, while the males and females had ratios of 0.50 and 0.40, respectively, for the RA. Computed tomography scanning is a method that allows the accurate calculation of internal anthropometric parameters that will be of use in mathematical modeling. J Orthop Sports Phys Ther 1985;6(5):278-280.