From pelvic geometry to modal compliance: sex-specific deformation pathways in the human pelvic ring.

Walking, posture and childbirth all depend on the pelvic ring, yet human pelvic dimorphism is usually interpreted from static birth-canal dimensions. We asked whether sex differences in pelvic function are explained by overall size and static pelvimetry or by available mechanical deformation pathways. We analysed 278 anatomically standardized lumbopelvic finite-element models built from clinical computed tomography scans. In each model, modal decomposition separated pelvic deformation into a shared, ranked set of pathways, allowing us to compare pathway stiffness, load recruitment and sensitivity to ligamentous and cartilaginous constraints. Overall pelvic size accounted for much of the baseline stiffness spectrum, confirming a strong size-dependent component. However, size- and age-adjusted sex differences remained: male pelves showed approximately 20% higher eigenvalues than female pelves in modes 2 and 9. Loading probes showed a common backbone in both sexes during habitual support, whereas inlet opening revealed a different response organization, with female pelves distributing deformation across a broader, coordinated set and a higher effective number of participating modes. The principal inlet-opening pathway was most sensitive to the pubic symphysis and anterior sacroiliac ligamentous constraints. Pelvic dimorphism, therefore, lies not only in canal shape, but in the organization of mechanical pathways that balance support with deformation.