Gluteus Maximus Shape Reveals Sex-Specific Associations Between Morphology and Metabolic Dysfunction

Abstract

Background

The gluteus maximus (GM) is a major hip extensor essential for mobility and metabolic health. Previous studies rely on global measures, such as muscle volume or fat fraction, which can overlook spatially localised differences in shape across the GM. Here, we integrate conventional volumetric and fat fraction metrics with 3D mesh-based shape phenotypes to provide a spatially resolved characterisation of GM morphology in relation to anthropometric, lifestyle and cardiometabolic factors, with a focus on type 2 diabetes (T2D) and sex-specific effects.

Methods

We analysed T1 Dixon MRI from UK Biobank participants to quantify GM muscle volume, fat fraction and regional surface morphology using 3D meshes. Statistical parametric mapping was used to assess regional associations with anthropometric, lifestyle and clinical variables. Statistical shape analysis was used to derive principal components (PCs) summarising major modes of GM shape variation, which were evaluated for associations with disease outcomes. Bidirectional causal mediation analyses were performed using GM volumetric and PCs of shape variation. Longitudinal changes in GM composition were assessed in participants with repeat imaging.

Results

GM muscle volume and fat fraction were strongly associated with age, adiposity and physical activity. Shape analysis identified spatially localised remodelling patterns that were not captured by global measures, revealing region-specific surface shrinkage linked to age, BMI, alcohol intake, grip strength, physical activity, frailty, osteoporosis and cardiometabolic disease. T2D exhibited distinct sex-specific patterns: Men showed predominantly inward deformation across the anterior and central GM, whereas women demonstrated outward deformation in corresponding anterior regions and throughout the posterior muscle, with several posterior regions significant only in women. Mediation analyses suggested that T2D-related changes in GM morphology partly mediated increases in fat fraction, indicating that diabetes influences regional muscle shape beyond overall muscle size. PCs representing variation in the central-superior posterior and anterior GM differentiated T2D cases from controls and were associated with incident T2D risk (men: left GM PC6 HR per SD 0.81 [0.70–0.95], false discovery rate [FDR]-adjusted p = 0.038; right GM PC6 HR 0.76 [0.65–0.88], p = 0.002; women: right GM PC5 HR 1.32 [1.08–1.61], p = 0.032).

Conclusions

Integrating 3D GM morphology with conventional muscle composition measures identifies regional shape biomarkers that extend beyond muscle volume and fat fraction. These biomarkers capture spatially specific remodelling associated with anthropometric, lifestyle and cardiometabolic factors, providing improved insight into muscle–fat phenotypes and enhancing risk stratification in population-based imaging studies.