In a recent paper published in the Proceedings of the Royal Society B, KLI senior fellow Katya Stansfield and KLI president Philipp Mitteroecker, together with colleagues from the University of Vienna, discuss the influence of muscle fatigue on pelvic evolution.

Human childbirth is uniquely challenging among primates—a consequence of our evolution for upright walking. For decades, scientists attributed this to the "obstetrical dilemma": natural selection supposedly kept the pelvis narrow to reduce the energy cost of walking, despite making childbirth more dangerous. However, experimental studies from about 10 years ago found no difference in total energy consumption between people with wider and narrower pelves, leading some researchers to declare the obstetrical dilemma "dead". Now, a new computational study resolves this paradox. Using detailed musculoskeletal simulations, researchers systematically varied pelvic dimensions to isolate their effects on walking energetics. The results confirm that while a wider pelvis barely affects whole-body energy consumption (only ±1% for a ±20% width change), it substantially increases the workload on the hip abductor muscles—the gluteal muscles on the sides of the pelvis—by approximately ±10% for the same change in width.
The key insight is that evolutionary constraints may have operated through muscle-specific mechanisms rather than total energy economy. Even if total energy expenditure remains nearly constant, forcing the abductor muscles to work harder with every step could lead to faster fatigue and reduced endurance over the long distances our ancestors traveled.  This refined understanding suggests that muscle fatigue, not whole-body metabolism, may have been the critical factor keeping the human pelvis relatively narrow despite obstetric challenges.

Publication:
E Stansfield, C Egner, P Mitteroecker, H Kainz; Did energy costs of walking limit the evolution of a larger human birth canal?. Proc Biol Sci 1 February 2026; 293 (2064): 20252895. https://doi.org/10.1098/rspb.2025.2895