Steady bipedal locomotion with a forward situated whole-body centre of mass: the potential importance of temporally asymmetric ground reaction forces

Bipedalism has repeatedly evolved in many independent lineages throughout tetrapod history. Despite being widespread, the fundamental biomechanical factors involved in bipedalism remain unclear. This study experimentally investigated bipedalism in facultatively bipedal lizards and obligatorily bipedal birds to explore temporal asymmetry in the vertical component of the ground reaction force (F-z). Both lizards and birds showed significant temporal asymmetry - with higher vertical forces exerted earlier in the stance - as indicated by three different measures computed from force-time profiles. This result parallels those reported previously for other bipedal animal groups that have a forward situated whole-body centre of mass (COM), such as kangaroos and non-human primates. Humans, in contrast, exhibit an orthograde posture with the COM close the hips, and show little temporal asymmetry in F-z, particularly during walking. Across a wide range of quadrupedal animals, temporal asymmetry is quite variable. Collectively, these results suggest that an early-skewed' F-z may be an important feature of steady bipedal locomotion when the COM is forward of the hips, although an exact mechanism of cause-and-effect, if one exists, remains to be established. This finding has relevance for attempts at better understanding bipedal locomotion in extinct animals that likely had a COM located forward of the hips, such as carnivorous dinosaurs.