Simulated transversus abdominis muscle force does not increase stiffness of the pubic symphysis and innominate bone: an in vitro study

BACKGROUND:
The transversus abdominis muscle is thought to exert a stiffening effect on the sacroiliac joints. However, it is unknown whether this muscle is capable of increasing pubic symphysis and innominate bone stiffness during load exerted on the pelvis. The objective of this study is to investigate whether in vitro simulated force of transversely oriented fibres of the transversus abdominis increases stiffness of the pubic symphysis and innominate bone.
METHODS:
In 15 embalmed specimens an incremental moment was applied in the sagittal plane to one innominate with respect to the fixated contralateral innominate. For pubic symphysis motion and innominate bone deformation load-deformation curves were plotted and slopes of adjusted linear regression lines were calculated. The slopes are considered to be a measure of pubic symphysis and innominate bone stiffness. Slopes were tested for significant differences before and after simulation of the transversus abdominis force.
FINDINGS:
Stiffness of pubic symphysis and innominate bone does not change under influence of simulated force of the transversus abdominis. For pubic symphysis, the slope of the regression line hardly changes, from 0.0341mm/Nm (SD 0.0277) before transversus abdominis force simulation to 0.0342mm/Nm (SD 0.0273) during simulation. For innominate bone, the mean slope increases minimally, from 0.0368mm/Nm (SD 0.0369) to 0.0413mm/Nm (SD 0.0395), respectively.
INTERPRETATION:
Simulation of the force of a single muscle - transversus abdominis - does not increase stiffness of the pubic symphysis and innominate bone. The hypothesized stiffening influence of the transversus abdominis on the pelvic ring was not confirmed in vitro