Normality of sagittal spinal alignment parameters reveals evolutionary signals in healthy adults across five...

The evolution of upright bipedalism required coordinated modifications in spinal curvature, pelvic orientation, and lower limb structure. However, it remains unclear whether sagittal alignment traits in modern humans have reached evolutionary stabilization or continue to exhibit developmental variability across populations. We hypothesize that certain sagittal alignment traits have undergone canalization-an evolutionary process that buffers against phenotypic variation-resulting in normal (Gaussian) distributions across populations. Conversely, traits under ongoing biomechanical or developmental constraints may deviate from normality. This study aimed to determine the distribution characteristics of key spinal and pelvic alignment parameters in healthy adults, and to assess whether these distributions reflect evolutionary stabilization or variability. Using high-resolution EOS imaging, we measured ten sagittal alignment parameters in 261 healthy adults under 40 years old across five countries. We applied the Shapiro-Wilk test to assess normality, and evaluated skewness, kurtosis, and modality to characterize distributional shapes. In the full cohort, three parameters-pelvic incidence (PI), lower lumbar lordosis (LL4-S), and spinal 3D length (S3DL)-significantly deviated from normality. PI showed near-symmetric but platykurtic distribution and remained non-normal across sex and age subgroups, while displaying normality within each country. LL4-S showed positive skew and elevated kurtosis. Most other parameters showed unimodal, mesokurtic distributions consistent with evolutionary canalization. Our findings support the hypothesis that sagittal alignment traits in modern humans exhibit varying degrees of evolutionary stabilization. Traits such as PI may reflect both canalization and residual variability, highlighting their relevance in biomechanical adaptation and spinal vulnerability.