Comparing contractile properties within an integrated group of muscles: The abdominal wall

The abdominal wall (external oblique (EO), internal oblique (IO), rectus abdominis (RA), and transverse abdominis (TrA)) is a functionally and anatomically integrated group of muscles. While the passive mechanical properties of the individual abdominal muscles have been studied previously, their contractile properties have yet to be described. Muscle samples were taken from the EO, IO, RA, and TrA of 6 Sprague-Dawley rats. Single muscle fibres were isolated from each sample, chemically permeabilized and tested in a Ca²⁺ (pCa 4.2) bath to determine their contractile properties: specific force, active modulus, unloaded shortening velocity, and rate of force redevelopment. Myosin heavy chain (MHC) isoforms were identified by gel electrophoresis to determine the fibre type of each tested fibre, as well as larger bundles of fibres. No type I fibres and only two type IIa fibres were tested, therefore type IIx and IIb fibres were combined for statistical analysis. There were no significant differences between muscles for specific force, active modulus, and unloaded shortening velocity (p > 0.05). Rate of force redevelopment was statistically significant (p = 0.029), with TrA being 62 % greater than EO, suggesting faster cross-bridge transitioning between low and high force-generating states in the TrA. The functional significance of this difference is unclear and will need to be studied further.