Architectural and morphological assessment of rat abdominal wall muscles: comparison for use as a human model

Abstract

The abdominal wall is a composite of muscles that are important for the mechanical stability of the spine and pelvis. Tremendous clinical attention is given to these muscles, yet little is known about how they function in isolation or how they interact with one another. Given the morphological, vascular, and innervation complexities associated with these muscles and their proximity to the internal organs, an appropriate animal model is important for understanding their physiological and mechanical significance during function. To determine the extent to which the rat abdominal wall resembles that of human, 10 adult male Sprague-Dawley rats were killed and formalin-fixed for architectural and morphological analyses of the four abdominal wall muscles (rectus abdominis, external oblique, internal oblique, and transversus abdominis). Physiological cross-sectional areas and optimal fascicle lengths demonstrated a pattern that was similar to human abdominal wall muscles. In addition, sarcomere lengths measured in the neutral spine posture were similar to human in their relation to optimal sarcomere length. These data indicate that the force-generating and length change capabilities of these muscles, relative to one another, are similar in rat and human. Finally, the fiber lines of action of each abdominal muscle were similar to human over most of the abdominal wall. The main exception was in the lower abdominal region (inferior to the pelvic crest), where the external oblique becomes aponeurotic in human but continues as muscle fibers into its pelvic insertion in the rat. We conclude that, based on the morphology and architecture of the abdominal wall muscles, the adult male Sprague-Dawley rat is a good candidate for a model representation of human, particularly in the middle and upper abdominal wall regions.

Fig. 1

Schematic side view of the external oblique, internal oblique and transversus abdominis, and anterior view of the rectus abdominis, to illustrate the regional division of muscles for architectural analysis. Thick black lines represent regional divisions (shown as 1–3) and thin red lines represent approximate muscle fiber lines of action.

Fig. 2

Scatterplot of physiological cross-sectional area (PCSA) vs. normalized fascicle length for abdominal wall muscles of the rat. A large PCSA indicates large isometric force-generating ability, and a long normalized fascicle length indicates the ability to generate force across a wide range of lengths. Data are plotted as mean ± SE. RA, rectus abdominis; EO, external oblique; IO, internal oblique; TrA, transversus abdominis.

Fig. 3

Scatterplot of physiological cross-sectional area (PCSA) vs. optimal fascicle length for abdominal wall muscles of rat and human. For comparison between the two, the mean PCSA and optimal fascicle length are normalized to the means of the rectus abdominis (RA) within each species. Human data taken from Brown et al. (in press) (n = 11). EO, external oblique; IO, internal oblique; TrA, transversus abdominis.

Fig. 4

Mean (± SE) sarcomere lengths for the abdominal wall muscles of rat and human. Bold horizontal green line represents the optimal sarcomere length for each species. Numerical values corresponding to each bar represent the percent difference from optimal length. Human data taken from Brown et al. (in press) (n = 11). EO, external oblique; IO, internal oblique; RA, rectus abdominis; TrA, transversus abdominis.

Fig. 5

Diagram of the anterior view of the rat abdominal wall representing the approximate fiber lines of action of the external oblique (EO), internal oblique (IO) and transversus abdominis (TrA). Note the composite laminate-like structure formed by the overlying of these three muscle layers.

Fig. 6

Mean (± SE) abdominal wall muscle masses normalized to body mass for both rat and human. Human data taken from Brown et al. (in press). EO, external oblique; IO, internal oblique; RA, rectus abdominis; TrA, transversus abdominis.