• Australopithecus afarensis
• biomechanics
• hominins
• Homo heidelbergensis
• knee
• locomotion
• primates
• tibial spines

There has been no focus in past research concerning the biomechanical function of the tibial spines of primates. This study shows that the relative position of the two tibial spines differs between higher primate taxa. In particular, the line connecting the two spines was found to be parallel to the frontal plane in Homo sapiens, whereas in all other taxa it was inclined 15-25° relative to the frontal plane. Since the tibial spines are the insertion points of the cruciate ligaments on the tibia, we speculate that this peculiar trait restricts mobility of the extended tibia on the femur in Homo sapiens during locomotion, and increases resistance to ground-generated valgus-varus angulation stress and mediolateral rotation at the knee joint while walking with a narrow base of support. Thus, it may constitute an adaptation to habitual bipedalism. We apply the method to fossil hominins to evaluate their degree of human-like bipedalism.