Muscle force analysis of the lumbar spine.

A mathematical analysis of the complex musculature controlling the lumbar spine was performed, using computer analysis of muscle areas seen on serial cross sections through two fresh human cadaver trunks. Physiologic cross sectional areas were used to approximate maximal potential forces generated, and three dimensional centroid lines of muscles were used to approximate their direction of action. Results indicate that the total extensor moment exceeds the total flexor moment only slightly; the rectus abdominis, internal oblique, and external oblique muscles contribute approximately one third to total flexor moment; the erector spinae group of muscles contributes approximately one half of the total extensor moment; and in rotation (twisting) the large abdominal oblique muscles dominate over small "rotator" muscles of the spine. The maximal calculated total extensor moment was 2860 kg.-cm. The magnitude of flexor and extensor moments suggests that the rise in intra-abdominal pressure during lifting is produced by the transverse and posterior oblique abdominal muscles, and not by the rectus abdominis.