Thorecolumlbar fascia can increase the efficiency of the erector spinae muscles

A model is developed which demonstrates that the thoracolumbar fascia can increase the axial stress generated by active contraction of the erector spinae muscles by restricting their radial expansion (bulging). The radial stress in the muscles was calculated from their radii, the radius of the abdominal cavity, and the intra-abdominal pressure generated during lifting. Radii were measured from magnetic resonance images and values of intra-abdominal pressure obtained from the literature. The resulting radial stress was used to calculate the increase in axial stress developed as a result of restricting radial expansion. Values for the axial stress generated by contraction of an unrestricted muscle, which are also required for this calculation, were obtained from the literature. The results indicate that restriction of radial expansion of the erector spinae muscles by the thoracolumbar fascia may increase the stress generated during their contraction by up to about 30%. This would lead to a proportional increase in the extensor moment exerted by these muscles.