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Transmission of Muscularly Generated Force and Stiffness Between Layers of the Rat Abdominal Wall

Study Design. In situ testing of the rat abdominal wall. Objective. To test the ability of muscularly generated force and stiffness to be transmitted between the layers of the abdominal wall. Summary of Background Data. The abdominal wall is comprised of 3 obliquely oriented sheet-like muscles bound together through a connective tissue network. This anatomic arrangement would seem ideal to facilitate myofascial force transmission, which if present would indicate shear connections between the muscle layers that could have important mechanical consequences. Methods. In 10 Sprague-Dawley rats, the 3 layers of the abdominal wall were isolated together and attached to a servomotor force/displacement system. The abdominal wall was stimulated via electrodes over the surface of the transverse abdominis, and measures of force and stiffness were obtained. The aponeurosis attaching the transverse abdominis to the rectus sheath was then cut and the wall was restimulated and the same measures were again obtained. Results. Active force and stiffness were both reduced in the cut aponeurosis state. These drops were much lower (10.6% and 10.7%, respectively) than would be expected if the transverse abdominis were completely removed. Furthermore, a control group (5 rats), in which the aponeurosis was not cut, but a similar amount of time to that necessary to perform the aponeurosis surgery was allowed to elapse, showed reductions in active force and stiffness (7.9 and 8.2, respectively) nearing that seen in the cut state. This indicates that at least a portion of this drop was due to the passage of time in the compromised surgical state. Conclusion. It was concluded that the majority of the force and stiffness generated by the transverse abdominis was transferred through the connective tissue network adhering to the internal oblique muscle. This indicates the presence of strong shear connections between the muscular layers, which suggests a composite stiffening function of the architectural design.