DOI: 10.5061/dryad.0p2ngf21x
title: The loss of the ‘pelvic step’ in human evolution

description: 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.
description: 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<sup>−1</sup> 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<br> 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<br> 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