The purpose of this study was to add to the growing database of cross-sectional areas and moment arm lengths of trunk musculature using the methods of computerized tomographic scanning. An attempt was also made to estimate muscle force and moment generating capacity under various reported values of muscle force per unit cross-sectional area. The data were obtained on 13 active men 40.5 ± 11.9 years of age, 173.8 ± 5.9 cm tall and 89.1 ± 11.7 kg body mass. Transverse CT scans were taken at the level of the L4/L5 disc with the subjects supine. Muscle cross-sectional areas were measured from 35 mm slides of the scans using a planimeter and moment arm length in the transverse plane were taken from the centroid of the L4/L5 disc to the centroid of the muscle section. Prior to estimating force and moment generating capacity, areas were corrected, where necessary, for fibre pennation angle to produce a physiological cross-sectional area.
The physiological cross-sectional areas (cm²) for one side of the body were (mean ± S.D.): sacrospinalis (SS) 15.9 ± 2.5: multifidus (Mu) 4.2 ± 0.7: psoas (Ps) 17.6 ± 4.0: rectus abdominis (RA) 7.9 ± 2.5: external oblique (EO) 9.4 ± 2.7: internal oblique (IO) 8.1 ± 2.3; transverse abdominus (TA) 2.9 ± 1.3. The anterior posterior moment arm lengths were: erector mass (SS and Mu combined) 5.90 ± 0.52: Ps 0.58 ± 0.40; R.A. 10.28 ± 2.07; E.O. (anterior portion) 5.94 ± 1.39; E.O. (posterior portion) 2.08 ± 1.39; I.O. (anterior portion) 6.92 ± 1.63; I.O. (posterior portion) 3.85 ± 1.54. The corresponding lateral moment arm lengths were: 3.26 ± 0.36; 4.88 ± 0.36; 4.35 ± 1.31; 12.86 ± 1.93; 13.95 ± 1.16; 10.77 ± 2.02; 12.52 ± 1.26.
The maximum force per unit cross-section that human muscles are capable of generating is not well defined. However, assuming an intermediate value of 50 N cm⁻² of physiological cross-section, the erector musculature observed at the L4/L5 level should be capable of generating an extensor moment of about 118 N.m. At a muscle stress of 30 or 90 N cm⁻², values also reported on human muscle, the moment would be 71 and 213 Nm, respectively. It must be remembered, however, that muscles not observable at the L4/L5 level can create moments around that center of rotation. Iliocostalis lumborum and longissimus thoracis insert onto what we have called the erector mass common tendon. This superficial dorsal tendon was measured in this study to have a moment arm of 8.49 ± 0.68 cm in the sagittal plane. These additional muscles with their relatively long moment arm of 8.5 cm, which seems more anatomically justifiable than 5.9 cm measured to the erector mass centroid, would elevate the extensor moment potential from muscular sources to values as high as 490 Nm observed during some back extensor tests.