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Sex Differences in Three-Dimensional Muscle Shape: Disentangling Allometry From Sexual Dimorphism Using Statistical
Jun Umehara1,2, Masashi Taniguchi2, Masahide Yagi1,2
1Faculty of Rehabilitation, Kansai Medical University, Hirakata, Japan.
Scandinavian Journal of Medicine & Science in Sports
|March 17, 2026
Summary
This study used statistical shape modeling to analyze human quadriceps femoris muscles. While most sex differences in muscle shape are due to size (allometry), distinct intrinsic shape variations between males and females were identified.
Area of Science:
- Human anatomy
- Biomechanics
- Morphometrics
Background:
- Sexual dimorphism is observed in skeletal muscle size and shape.
- Disentangling allometric effects from sexual dimorphism is crucial for understanding sex-related shape differences in human muscles, but remains underexplored.
Purpose of the Study:
- To identify sex differences in the shape of the human quadriceps femoris muscle.
- To utilize statistical shape modeling for a novel approach to muscle morphology analysis.
- To differentiate between size-driven (allometric) and intrinsic shape variations between sexes.
Main Methods:
- Acquisition of magnetic resonance images from 43 healthy young adults (22 males, 21 females).
- Measurement of muscle size (volume, cross-sectional area, length, width, thickness) for the four quadriceps femoris heads.
- Development of statistical shape models to quantify muscle shape variation and perform sex classification via linear discrimination.
Main Results:
- Males exhibited significantly greater muscle volume, cross-sectional area, length, width, and thickness compared to females.
- Statistical shape models revealed sex-related shape differences, with some distinct variations preserved after accounting for muscle volume (allometry).
- Specific regions in the vastus lateralis and vastus medialis showed unique sex-related shape differences, enabling accurate sex classification.
Conclusions:
- Most observed sex differences in quadriceps femoris shape are attributed to overall size differences (allometry).
- Intrinsic, size-independent shape differences between male and female quadriceps femoris muscles exist.
- Statistical shape modeling is a valuable tool for distinguishing allometric effects from true sexual dimorphism in muscle morphology.
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