Pelvic breadth and juvenile locomotor energetics: Implications for the obstetrical dilemma hypothesis across human growth in past and present humans.

The obstetrical dilemma hypothesis (ODH) proposes an evolutionary trade-off between obstetric requirements and locomotor performance. However, whether external pelvic breadth measures influence walking energetics during human growth remains poorly understood. This study quantified gross walking energy expenditure (WalkingEE) in juveniles and examined whether external pelvic breadth measures influence locomotor energetics during growth. Twenty-eight healthy children and adolescents (8-17 years) completed treadmill walking trials at randomized speeds (2-6 km/h). Gross WalkingEE (kcal/24 h) was measured by indirect calorimetry, and anthropometric variables included body mass (BM), height, femur length, tibia length, bi-iliac breadth (BIL), and bi-trochanteric breadth (BITR). Forward stepwise multiple linear regression identified BM, walking speed, and BIL as the only significant predictors of WalkingEE (R2 = 0.89, *p < 0.0001). BM and walking speed were positively associated with energy expenditure, whereas BIL showed a significant negative association, accounting for 43%, 31.5%, and -25.5% of the explained effect, respectively. These findings indicate that greater bi-iliac breadth is associated with lower walking energy expenditure during growth. Moreover, the fact that bi-trochanteric breadth was not retained in the final model, despite being used as a proxy more closely associated with obstetric dimensions, suggests that locomotor energetics and obstetric function may be at least partly independent. Overall, our findings are consistent with a more dynamic interpretation of the obstetrical dilemma hypothesis and provide new empirical support for the view that the human pelvis is a functionally modular structure shaped by multiple selective pressures.