Human bipedalism entails relatively short strides compared with facultatively bipedal primates. Unique non-sagittal-plane motions associated with bipedalism may account for part of this discrepancy. Pelvic rotation anteriorly translates the hip, contributing to bipedal stride length (i.e. the ‘pelvic step’). Facultative bipedalism in non-human primates entails much larger pelvic rotation than in humans, suggesting that a larger pelvic step may contribute to their relatively longer strides. We collected data on the pelvic step in bipedal chimpanzees and over a wide speed range of human walking. At matched dimensionless speeds, humans have 26.7% shorter dimensionless strides, and a pelvic step 5.4 times smaller than bipedal chimpanzees. Differences in pelvic rotation explain 31.8% of the difference in dimensionless stride length between the two species. We suggest that relative stride lengths and the pelvic step have been significantly reduced throughout the course of hominin evolution.

All human kinematic data were collected using a 12-camera Vicon motion capture system (Vicon Motion Systems Ltd., Oxford, UK). Motion data were collected at 150 frames s⁻¹ using the full-body Plug-In Gait marker set (based on the Newington/Helen Hayes gait model; Davis et al., 1991; Kadaba et al., 1990). Marker positions
were filtered using a generalized cross-validatory spline (i.e. ‘Woltring filter’; Woltring, 1986). Pelvic rotation was calculated using the default Plug-In Gait analysis and were taken directly from Vicon Nexus (v2.9, Vicon Motion Systems). Chimpanzee data were collected on a 4-camera Xcitex motion capture system (Xcitex Inc., Woburn, MA, USA) at 150 frames s−1. Marker clusters (≥3 markers) were placed on the pelvis and rotations were calculated in the global coordinate system using Cardan angles and a standard tilt, list, rotation sequence of rotations. Bi-acetabular breadth was taken as the distance between the estimated hip joint centers as calculated using the Plug-In Gait/Newington–Gage model for humans. For chimpanzees, biacetabular breadth (between the centers of each femoral head) was measured from pelvic X-rays that were taken within 6 months of data collection and increased by a small amount representing predicted growth.

Anterior translation of the the swing-side hip over the stride was calculated using the pevlic angle at the contralateral and second ipsilateral touchdowns. The contribution that this distance made to the stride was calculated by dividing it by stride length and multiplying by 100. Stride length in both species was calculated by taking the fore–aft
distance between the positions of a heel marker at two consecutive ipsilateral heel strikes. It was made dimensionless by dividing by effective limb length (ELL). ELL was calculated as the distance of the greater trochanter from the ground during standing (humans) or when the greater trochanter was vertically positioned above the foot during a stride (chimpanzees).

For additional information, refer to the related manuscript: https://doi.org/10.1242/jeb.240440