Mechanically relevant consequences of the composite laminate-like design of the abdominal wall muscles and connective tissues.

Together, three abdominal wall muscles (external oblique, internal oblique and transversus abdominis) form a tightly bound muscular sheet that has been likened to a composite-laminate structure. Previous work has demonstrated the ability of force generated by these three muscles to be passed between one another through connective tissue linkages. Muscle fibres in each muscle are obliquely oriented with respect to its neighbouring muscles. It is proposed here is that this unique morphology of the abdominal wall muscles functions, through the application of constraining forces amongst the muscles, to increase force- and stiffness-generating capabilities. This paper presents a mathematical formulation of the stress-strain relationship for a transversely isotropic fibrous composite, and establishes a strengthening and stiffening effect when stress can be transferred between the fibrous layers. Application of empirical mechanical properties to this formulation demonstrates this effect for the abdominal wall muscles and, in greater proportion, for the anterior aponeurosis of the abdominal wall. This has implications for increasing the stiffness and passive load bearing ability of the abdominal wall muscles, and has the potential to modulate the whole muscle force-length and force-velocity relationships during contraction.