Sex Differences in Three-Dimensional Muscle Shape: Disentangling Allometry From Sexual Dimorphism Using Statistical...

Sexual dimorphism in skeletal muscles is evident in both size and shape. As most morphological traits scale with size, disentangling allometric effects from sexual dimorphism is essential for revealing sex-related shape differences; however, this remains underexplored in human muscles. This study aimed to identify sex differences in muscle shape within the human quadriceps femoris, with a novel approach based on muscle statistical shape modeling. Magnetic resonance images of 43 healthy young adults (22 males and 21 females) were obtained. Muscle size of the four heads of quadriceps femoris was measured, and statistical shape models were developed to quantify the muscle shape variation. Muscle size and shape were compared between males and females, and shape-based sex classification was performed using linear discrimination. While males showed significantly greater volume, anatomical cross-sectional area, length, width, and thickness in all muscles, the statistical shape models demonstrated sex-related shape differences. When adjusting for muscle volume, the primary sex-related shape differences were largely explained by allometry. However, some distinct sex differences were preserved, specifically for the bulging and shrinkage at different regions in the vastus lateralis and the anterior edge shrinkage, distal bulging, and thickness variation within the vastus medialis. These shape differences distinguished male and female with high accuracy. Our findings revealed that although most sex differences in quadriceps femoris shape are driven by size (allometry), intrinsic shape differences between sexes exist. This study highlights the utility of statistical shape modeling in distinguishing allometric effects from true sexual dimorphism in muscle morphology.