A versatile gluteus medius drove a seamless transition to bipedalism.
Level 5 - mechanism / opinion, no new human data
Biomechanical simulation and kinematic modeling in an animal model without human clinical data
PubMed 42241984 · doi:10.1016/j.jhevol.2026.103856
What was done
Researchers investigated the functional mechanics of the gluteus medius during quadrupedal and bipedal walking using a moment arm analysis and model-kinematics matching approach in Japanese macaques (*Macaca fuscata*), a primate model that lacks morphological adaptations for bipedal locomotion.
What was found
The abstract reports no numerical values or statistical estimates. Directionally, changing posture from quadrupedal to bipedal walking caused the primary mechanical action of the gluteus medius to shift from medial rotation to abduction, driven by changes in the geometric alignment between the muscular force vector and femoral orientation.
Why it matters
This demonstrates that hip abductor function can switch dynamically based on posture alone, suggesting that behavioral changes could have provided mediolateral stability during early bipedalism before specialized pelvic skeletal adaptations evolved.
Limits
The abstract does not state the number of animals studied (n), specific moment arm measurements, or statistical variances. Findings rely on a computational biomechanical model in an extant quadrupedal monkey species and infer evolutionary transitions indirectly.