A revised anatomical model of the abdominal musculature for torso flexion efforts

Models of the abdominal musculature, which used single slice MRI or CT scans to reconstruct the anatomy, were unable to explain magnitudes of abdominal flexion moment which exceed 100 Nm in living subjects. For example, the muscle areas and corresponding flexor moment arms measured from a transverse scan at L4–L5 predict maximum moments less than 50 N m. The architecture of rectus abdominis, external and internal obliques, and transverse abdominis was examined from cadaveric specimens, from which the geometric details were incorporated into a revised model. Scaling of the muscles to obtain physiological cross-sectional areas were obtained using two methods: serial MRI slices; and from several locations of ultrasound measurement. The two major anatomically justified improvements that had dramatic effects on flexor moment production were: the increase in flexor moment arms of muscles (approximately 30%) measured in a standing posture when compared to the supine posture for MRI examination; reinterpretation of the line of action of the obliques which transmit their flexor forces through the linea semilunaris (increasing their effective flexor moment arm) to eventual skeletal attachment at the pelvis.