Regional lumbar alignment, physiological variation, and pelvic incidence in healthy adults: Emphasis on the lumbosacral junction

The acquisition of habitual bipedal posture required substantial reorganization of the human spine and pelvis, including the development of lumbar lordosis. While global lumbar alignment has been extensively studied, the anatomical relationship between regional lumbar lordosis and pelvic incidence in healthy adults remains incompletely understood. In this cross‐sectional study, 154 rigorously screened healthy adults aged 20–76 years underwent standing whole‐body EOS imaging, simultaneous force plate balance assessment, and body composition analysis. Sagittal alignment parameters, including segmental lumbar lordosis, pelvic incidence, balance metrics, and body composition variables, were analyzed for distributional characteristics and age‐ and sex‐related associations. Segmental lumbar lordosis parameters, including lower lumbar lordosis (LL4–S1), demonstrated approximately normal and unimodal distributions with preserved central tendencies across adulthood. Pelvic incidence showed a consistent positive association with upper lumbar lordosis (LL1–4), whereas its relationship with lower lumbar lordosis (LL4–S1) was weak or absent in pooled analyses and emerged only in sex‐stratified analyses, particularly among males. In contrast to sagittal alignment parameters, balance and body composition variables demonstrated greater variability and clearer age‐ and sex‐related modulation. These findings support the concept that the lumbar spine is regionally differentiated rather than functionally uniform. Lower lumbar alignment demonstrated weaker and, in pooled analyses, often absent associations with pelvic incidence than the upper lumbar spine, suggesting differential biomechanical regulation of the lumbosacral junction. Although broader evolutionary implications remain speculative, the present findings provide an anatomical and biomechanical framework for future investigations of human upright posture and lumbosacral disorders.