How Did the Pelvis and Vertebral Column Become a Functional Unit during the Transition from Occasional to Permanent Bipedalism?

The functional linkage between pelvis and spine remained long hidden to science. Here, we recount the history of research that led in 1992 to the discovery of the "angle of sacral incidence" by the team of G. Duval-Beaupère. This angle, formed between a ray from the hip joint center to the superior sacral surface and the perpendicular to the sacral surface, was later called pelvic incidence. Specific to each individual, pelvic incidence is tightly correlated with the degree of lumbar lordosis. It is each individual's "signature" for an efficient sagittal balance since it represents the sum of two positional parameters, sacral slope and pelvic tilt. The simultaneous experimental determination of the trunk line of gravity permitted Duval-Beaupère's team to elucidate the conditions of an efficient sagittal balance of the trunk on the lower limbs. We present an in vivo EOS study of eight spino-pelvic parameters describing the sagittal balance in 131 adults. We observe a chain of correlations between the six angular parameters and discuss the functional significance of these results. We show that pelvic incidence increases and lumbar lordosis develops when the infant learns to walk, leading to a correlation between these parameters. This process of association between pelvis and spine might have acquired a solid genetic basis during hominid evolution by natural selection acting on both pelvis and spine. We suggest that this process of functional integration was only possible in the context of bipedal locomotion becoming permanent and stereotyped, expressed by a relatively invariant, periodic walking cycle. Anat Rec, 300:912-931, 2017.