The effect of size, sex and running discipline on the shape of rectus abdominis muscle: A 3D geometric morphometric approach

The morphology of the rectus abdominis muscle is closely related to its biomechanical functions in trunk stabilization and breathing across different activities. However, the three-dimensional morphological characteristics of this muscle remain poorly documented in vivo. Therefore, the present observational cross-sectional study aims to evaluate how size, sex and running discipline influence the 3D shape of this muscle, hypothesising that there are allometric effects, sexual dimorphism effects and that the explosive demands of sprinting require a more specialised morphology than endurance running. This work models the in vivo shape of this muscle at rest in a sample of athletes specialised in endurance running or sprinting. The rectus abdominis muscles were measured directly from CT scan slices, and 3D geometrical models of the muscle were created from the triangulation of 3D coordinates. The shape of the muscle was studied through 3D geometric morphometric analyses. The study revealed the significant effect of sex and running discipline in the 3D shape of the rectus abdominis. The results highlight distinct morphological features in males and females and in sprinters and endurance runners. Males show a more triangularly shaped rectus abdominis (wider in its superior part) compared to females, who show a more rectangular muscle. Similarly, sprinters show thicker and more rectangular muscles, compared to endurance runners. These findings suggest that the functional anatomy of the rectus abdominis muscle could differ depending on sex and running discipline, pointing to potential differences in biomechanics and respiratory performance. This novel approach opens the door to further exploration of in vivo muscle morphology and its implications in physiology applied not only to sports science, but also in clinical settings.