Adaptations for bipedal walking: Musculoskeletal structure and three-dimensional joint mechanics of humans and bipedal chimpanzees (Pan troglodytes).

Humans are unique among apes and other primates in the musculoskeletal design of their lower back, pelvis, and lower limbs. Here, we describe the three-dimensional ground reaction forces and lower/hindlimb joint mechanics of human and bipedal chimpanzees walking over a full stride and test whether: 1) the estimated limb joint work and power during the stance phase, especially the single-support period, is lower in humans than bipedal chimpanzees, 2) the limb joint work and power required for limb swing is lower in humans than in bipedal chimpanzees, and 3) the estimated total mechanical power during walking, accounting for the storage of passive elastic strain energy in humans, is lower in humans than in bipedal chimpanzees. Humans and bipedal chimpanzees were compared at matched dimensionless and dimensional velocities. Our results indicate that humans walk with significantly less work and power output in the first double-support period and the single-support period of stance, but markedly exceed chimpanzees in the second double-support period (i.e., push-off). Humans generate less work and power in limb swing, although the species difference in limb swing power was not statistically significant. We estimated that total mechanical positive 'muscle fiber' work and power were 46.9% and 35.8% lower, respectively, in humans than in bipedal chimpanzees at matched dimensionless speeds. This is due in part to mechanisms for the storage and release of elastic energy at the ankle and hip in humans. Furthermore, these results indicate distinct 'heel strike' and 'lateral balance' mechanics in humans and bipedal chimpanzees and suggest a greater dissipation of mechanical energy through soft tissue deformations in humans. Together, our results document important differences between human and bipedal chimpanzee walking mechanics over a full stride, permitting a more comprehensive understanding of the mechanics and energetics of chimpanzee bipedalism and the evolution of hominin walking.

The loss of the 'pelvic step' in human evolution.

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.

Step width and frontal plane trunk motion in bipedal chimpanzee and human walking.

Human bipedalism is characterized by mediolateral oscillations of the center of mass (CoM) between the feet. The preferred step widths and CoM oscillations used by humans likely represent a trade-off of several factors (e.g., stance and swing phase costs). However, it is difficult to assess whether human frontal plane control strategies are unique given few detailed data on frontal plane motion during facultative bipedalism in apes. Here, we collected three-dimensional kinematic and kinetic data in humans and chimpanzees to investigate the relationship between step width, mediolateral CoM motion, frontal plane trunk kinematics, and CoM power during bipedalism. Chimpanzee bipedalism entails mediolateral CoM oscillations and step widths that are (scaled to lower/hind limb length) three times larger than those of humans. Chimpanzees use a combination of linear and angular motion of the trunk and list the entire trunk, and especially thorax, over the stance side foot, generating large mediolateral shifts in the CoM, whereas humans utilize little angular motion within the trunk. Larger mediolateral CoM motions do not have a significant effect on CoM power. Similarities between bipedal chimpanzees and other bipedal non-human primates (macaques and gibbons) indicate that narrow CoM motions are unique to humans and are likely due to our adducted hips and valgus knees. Valgus knees appear early in the human fossil record (∼3.6 Ma), contemporaneous with the Laetoli footprints. However, fossils attributed to Ardipithecus ramidus (∼4.4 Ma) suggest that the earliest hominins may have lacked a hominin-like degree of knee valgus. If correct, this suggests that this species may have used wide steps, larger mediolateral CoM motions, and perhaps larger trunk motions during bipedal walking. Finally, we present a novel means to estimate mediolateral CoM motion from trackway step width, and estimate that the Laetoli G track maker used CoM motions within the human range.

Chimpanzee and human midfoot motion during bipedal walking and the evolution of the longitudinal arch of the foot.

The longitudinal arch of the human foot is commonly thought to reduce midfoot joint motion to convert the foot into a rigid lever during push off in bipedal walking. In contrast, African apes have been observed to exhibit midfoot dorsiflexion following heel lift during terrestrial locomotion, presumably due to their possession of highly mobile midfoot joints. This assumed dichotomy between human and African ape midfoot mobility has recently been questioned based on indirect assessments of in vivo midfoot motion, such as plantar pressure and cadaver studies; however, direct quantitative analyses of African ape midfoot kinematics during locomotion remain scarce. Here, we used high-speed motion capture to measure three-dimensional foot kinematics in two male chimpanzees and five male humans walking bipedally at similar dimensionless speeds. We analyzed 10 steps per chimpanzee subject and five steps per human subject, and compared ranges of midfoot motion between species over stance phase, as well as within double- and single-limb support periods. Contrary to expectations, humans used a greater average range of midfoot motion than chimpanzees over the full duration of stance. This difference was driven by humans' dramatic plantarflexion and adduction of the midfoot joints during the second double-limb support period, which likely helps the foot generate power during push off. However, chimpanzees did use slightly but significantly more midfoot dorsiflexion than humans in the single limb-support period, during which heel lift begins. These results indicate that both stiffness and mobility are important to longitudinal arch function, and that the human foot evolved to utilize both during push off in bipedal walking. Thus, the presence of human-like midfoot joint morphology in fossil hominins should not be taken as indicating foot rigidity, but may signify the evolution of pedal anatomy conferring enhanced push off mechanics.

Chimpanzee ankle and foot joint kinematics: Arboreal versus terrestrial locomotion.

OBJECTIVES: Many aspects of chimpanzee ankle and midfoot joint morphology are believed to reflect adaptations for arboreal locomotion. However, terrestrial travel also constitutes a significant component of chimpanzee locomotion, complicating functional interpretations of chimpanzee and fossil hominin foot morphology. Here we tested hypotheses of foot motion and, in keeping with general assumptions, we predicted that chimpanzees would use greater ankle and midfoot joint ranges of motion during travel on arboreal supports than on the ground. METHODS: We used a high-speed motion capture system to measure three-dimensional kinematics of the ankle and midfoot joints in two male chimpanzees during three locomotor modes: terrestrial quadrupedalism on a flat runway, arboreal quadrupedalism on a horizontally oriented tree trunk, and climbing on a vertically oriented tree trunk. RESULTS: Chimpanzees used relatively high ankle joint dorsiflexion angles during all three locomotor modes, although dorsiflexion was greatest in arboreal modes. They used higher subtalar joint coronal plane ranges of motion during terrestrial and arboreal quadrupedalism than during climbing, due in part to their use of high eversion angles in the former. Finally, they used high midfoot inversion angles during arboreal locomotor modes, but used similar midfoot sagittal plane kinematics across all locomotor modes. DISCUSSION: The results indicate that chimpanzees use large ranges of motion at their various ankle and midfoot joints during both terrestrial and arboreal locomotion. Therefore, we argue that chimpanzee foot anatomy enables a versatile locomotor repertoire, and urge caution when using foot joint morphology to reconstruct arboreal behavior in fossil hominins.

Laetoli footprints reveal bipedal gait biomechanics different from those of modern humans and chimpanzees.

Bipedalism is a key adaptation that shaped human evolution, yet the timing and nature of its evolution remain unclear. Here we use new experimentally based approaches to investigate the locomotor mechanics preserved by the famous Pliocene hominin footprints from Laetoli, Tanzania. We conducted footprint formation experiments with habitually barefoot humans and with chimpanzees to quantitatively compare their footprints to those preserved at Laetoli. Our results show that the Laetoli footprints are morphologically distinct from those of both chimpanzees and habitually barefoot modern humans. By analysing biomechanical data that were collected during the human experiments we, for the first time, directly link differences between the Laetoli and modern human footprints to specific biomechanical variables. We find that the Laetoli hominin probably used a more flexed limb posture at foot strike than modern humans when walking bipedally. The Laetoli footprints provide a clear snapshot of an early hominin bipedal gait that probably involved a limb posture that was slightly but significantly different from our own, and these data support the hypothesis that important evolutionary changes to hominin bipedalism occurred within the past 3.66 Myr.

Bone shaft bending strength index is unaffected by exercise and unloading in mice.

Anthropologists frequently use the shaft bending strength index to infer the physical activity levels of humans living in the past from their lower limb bone remains. This index is typically calculated as the ratio of bone shaft second moments of area about orthogonal principal axes (i.e. I(max)/I(min)). Individuals with high I(max)/I(min) values are inferred to have been very active, whereas individuals with low values are inferred to have been more sedentary. However, there is little direct evidence that activity has a causal and predictable effect on the shaft bending strength index. Here, we report the results of two experiments that were designed to test the model within which anthropologists commonly interpret the shaft bending strength index. In the first experiment, mice were treated daily with treadmill exercise for 1 month to simulate a high-activity lifestyle. In the second experiment, in an attempt to simulate a low-activity lifestyle, functional weight-bearing was removed from the hindlimbs of mice for 1 month. Femoral mid-shaft structure was determined with µCT. We found that while exercise resulted in significant enhancement of I(max) and I(min) compared with controls, it failed to significantly increase the I(max)/I(min)index. Similarly, stunted bone growth caused by unloading resulted in significantly diminished I(max) and I(min) compared with controls, but low activity did not lead to significantly decreased I(max)/I(min)compared with normal activity. Together, these results suggest that caution is required when the bone shaft bending strength index is used to reconstruct the activity levels of past humans.

Three-dimensional kinematics of the pelvis and hind limbs in chimpanzee (Pan troglodytes) and human bipedal walking.

The common chimpanzee (Pan troglodytes) is a facultative biped and our closest living relative. As such, the musculoskeletal anatomies of their pelvis and hind limbs have long provided a comparative context for studies of human and fossil hominin locomotion. Yet, how the chimpanzee pelvis and hind limb actually move during bipedal walking is still not well defined. Here, we describe the three-dimensional (3-D) kinematics of the pelvis, hip, knee and ankle during bipedal walking and compare those values to humans walking at the same dimensionless and dimensional velocities. The stride-to-stride and intraspecific variations in 3-D kinematics were calculated using the adjusted coefficient of multiple correlation. Our results indicate that humans walk with a more stable pelvis than chimpanzees, especially in tilt and rotation. Both species exhibit similar magnitudes of pelvis list, but with segment motion that is opposite in phasing. In the hind limb, chimpanzees walk with a more flexed and abducted limb posture, and substantially exceed humans in the magnitude of hip rotation during a stride. The average stride-to-stride variation in joint and segment motion was greater in chimpanzees than humans, while the intraspecific variation was similar on average. These results demonstrate substantial differences between human and chimpanzee bipedal walking, in both the sagittal and non-sagittal planes. These new 3-D kinematic data are fundamental to a comprehensive understanding of the mechanics, energetics and control of chimpanzee bipedalism.

Focal enhancement of the skeleton to exercise correlates with responsivity of bone marrow mesenchymal stem cells rather than peak external forces.

Force magnitudes have been suggested to drive the structural response of bone to exercise. As importantly, the degree to which any given bone can adapt to functional challenges may be enabled, or constrained, by regional variation in the capacity of marrow progenitors to differentiate into bone-forming cells. Here, we investigate the relationship between bone adaptation and mesenchymal stem cell (MSC) responsivity in growing mice subject to exercise. First, using a force plate, we show that peak external forces generated by forelimbs during quadrupedal locomotion are significantly higher than hindlimb forces. Second, by subjecting mice to treadmill running and then measuring bone structure with µCT, we show that skeletal effects of exercise are site-specific but not defined by load magnitudes. Specifically, in the forelimb, where external forces generated by running were highest, exercise failed to augment diaphyseal structure in either the humerus or radius, nor did it affect humeral trabecular structure. In contrast, in the ulna, femur and tibia, exercise led to significant enhancements of diaphyseal bone areas and moments of area. Trabecular structure was also enhanced by running in the femur and tibia. Finally, using flow cytometry, we show that marrow-derived MSCs in the femur are more responsive to exercise-induced loads than humeral cells, such that running significantly lowered MSC populations only in the femur. Together, these data suggest that the ability of the progenitor population to differentiate toward osteoblastogenesis may correlate better with bone structural adaptation than peak external forces caused by exercise.

Center of mass mechanics of chimpanzee bipedal walking.

Center of mass (CoM) oscillations were documented for 81 bipedal walking strides of three chimpanzees. Full-stride ground reaction forces were recorded as well as kinematic data to synchronize force to gait events and to determine speed. Despite being a bent-hip, bent-knee (BHBK) gait, chimpanzee walking uses pendulum-like motion with vertical oscillations of the CoM that are similar in pattern and relative magnitude to those of humans. Maximum height is achieved during single support and minimum height during double support. The mediolateral oscillations of the CoM are more pronounced relative to stature than in human walking when compared at the same Froude speed. Despite the pendular nature of chimpanzee bipedalism, energy recoveries from exchanges of kinetic and potential energies are low on average and highly variable. This variability is probably related to the poor phasic coordination of energy fluctuations in these facultatively bipedal animals. The work on the CoM per unit mass and distance (mechanical cost of transport) is higher than that in humans, but lower than that in bipedally walking monkeys and gibbons. The pronounced side sway is not passive, but constitutes 10% of the total work of lifting and accelerating the CoM. CoM oscillations of bipedally walking chimpanzees are distinctly different from those of BHBK gait of humans with a flat trajectory, but this is often described as "chimpanzee-like" walking. Human BHBK gait is a poor model for chimpanzee bipedal walking and offers limited insights for reconstructing early hominin gait evolution.

A three-dimensional musculoskeletal model of the chimpanzee (Pan troglodytes) pelvis and hind limb.

Musculoskeletal models have become important tools for studying a range of muscle-driven movements. However, most work has been in modern humans, with few applications in other species. Chimpanzees are facultative bipeds and our closest living relatives, and have provided numerous important insights into our own evolution. A chimpanzee musculoskeletal model would allow integration across a wide range of laboratory-based experimental data, providing new insights into the determinants of their locomotor performance capabilities, as well as the origins and evolution of human bipedalism. Here, we described a detailed three-dimensional (3D) musculoskeletal model of the chimpanzee pelvis and hind limb. The model includes geometric representations of bones and joints, as well as 35 muscle-tendon units that were represented using 44 Hill-type muscle models. Muscle architecture data, such as muscle masses, fascicle lengths and pennation angles, were drawn from literature sources. The model permits calculation of 3D muscle moment arms, muscle-tendon lengths and isometric muscle forces over a wide range of joint positions. Muscle-tendon moment arms predicted by the model were generally in good agreement with tendon-excursion estimates from cadaveric specimens. Sensitivity analyses provided information on the parameters that model predictions are most and least sensitive to, which offers important context for interpreting future results obtained with the model. Comparisons with a similar human musculoskeletal model indicate that chimpanzees are better suited for force production over a larger range of joint positions than humans. This study represents an important step in understanding the integrated function of the neuromusculoskeletal systems in chimpanzee locomotion.

Ground reaction forces and center of mass mechanics of bipedal capuchin monkeys: implications for the evolution of human bipedalism.

Tufted capuchin monkeys are known to use both quadrupedalism and bipedalism in their natural environments. Although previous studies have investigated limb kinematics and metabolic costs, their ground reaction forces (GRFs) and center of mass (CoM) mechanics during two and four-legged locomotion are unknown. Here, we determine the hind limb GRFs and CoM energy, work, and power during bipedalism and quadrupedalism over a range of speeds and gaits to investigate the effect of differential limb number on locomotor performance. Our results indicate that capuchin monkeys use a "grounded run" during bipedalism (0.83-1.43 ms(-1)) and primarily ambling and galloping gaits during quadrupedalism (0.91-6.0 ms(-1)). CoM energy recoveries are quite low during bipedalism (2-17%), and in general higher during quadrupedalism (4-72%). Consistent with this, hind limb vertical GRFs as well as CoM work, power, and collisional losses are higher in bipedalism than quadrupedalism. The positive CoM work is 2.04 ± 0.40 Jkg(-1) m(-1) (bipedalism) and 0.70 ± 0.29 Jkg(-1) m(-1) (quadrupedalism), which is within the range of published values for two and four-legged terrestrial animals. The results of this study confirm that facultative bipedalism in capuchins and other nonhuman primates need not be restricted to a pendulum-like walking gait, but rather can include running, albeit without an aerial phase. Based on these results and similar studies of other facultative bipeds, we suggest that important transitions in the evolution of hominin locomotor performance were the emergences of an obligate, pendulum-like walking gait and a bouncy running gait that included a whole-body aerial phase.

Genetic variations and physical activity as determinants of limb bone morphology: an experimental approach using a mouse model.

To gain insight into past human physical activity, anthropologists often infer functional loading history from the morphology of limb bone remains. It is assumed that, during life, loading had a positive, dose-dependent effect on bone structure that can be identified despite other effects. Here, we investigate the effects of genetic background and functional loading on limb bones using mice from an artificial selection experiment for high levels of voluntary wheel running. Growing males from four replicate high runner (HR) lines and four replicate nonselected control (C) lines were either allowed or denied wheel access for 2 months. Using µCT, femoral morphology was assessed at two cortical sites (mid-diaphysis, distal metaphysis) and one trabecular site (distal metaphysis). We found that genetic differences between the linetypes (HR vs. C), between the replicate lines within linetype, and between individuals with and without the so-called "mini-muscle" phenotype (caused by a Mendelian recessive gene that halves limb muscle mass) gave rise to significant variation in nearly all morphological indices examined. Wheel access also influenced femoral morphology, although the functional response did not generally result in enhanced structure. Exercise caused moderate periosteal enlargement, but relatively greater endocortical expansion, resulting in significantly thinner cortices and reduced bone area in the metaphysis. The magnitude of the response was independent of distance run. Mid-diaphyseal bone area and area moments, and trabecular morphology, were unaffected by exercise. These results underscore the strong influence of genetics on bone structure and the complexity by which mechanical stimuli may cause alterations in it.

Three-dimensional kinematics of capuchin monkey bipedalism.

Capuchin monkeys are known to use bipedalism when transporting food items and tools. The bipedal gait of two capuchin monkeys in the laboratory was studied with three-dimensional kinematics. Capuchins progress bipedally with a bent-hip, bent-knee gait. The knee collapses into flexion during stance and the hip drops in height. The knee is also highly flexed during swing to allow the foot which is plantarflexed to clear the ground. The forefoot makes first contact at touchdown. Stride frequency is high, and stride length and limb excursion low. Hind limb retraction is limited, presumably to reduce the pitch moment of the forward-leaning trunk. Unlike human bipedalism, the bipedal gait of capuchins is not a vaulting gait, and energy recovery from pendulum-like exchanges is unlikely. It extends into speeds at which humans and other animals run, but without a human-like gait transition. In this respect it resembles avian bipedal gaits. It remains to be tested whether energy is recovered through cyclic elastic storage and release as in bipedal birds at higher speeds. Capuchin bipedalism has many features in common with the facultative bipedalism of other primates which is further evidence for restrictions on a fully upright striding gait in primates that transition to bipedalism. It differs from the facultative bipedalism of other primates in the lack of an extended double-support phase and short aerial phases at higher speeds that make it a run by kinematic definition. This demonstrates that facultative bipedalism of quadrupedal primates need not necessarily be a walking gait.

Gait dynamics of Cebus apella during quadrupedalism on different substrates.

Primates are distinguished from many mammals by emphasizing arboreal lifestyles. Primate arboreal adaptations include specializations for enhancing balance and manipulative skills. Compliant gait and diagonal sequence (DS) footfalls are hypothesized mechanisms for improving balance during arboreal quadrupedalism (AQ), while simultaneously permitting vertical peak force reductions sustained by limbs, particularly forelimbs (FLs). Capuchin monkeys (Cebus apella) are arboreally-adapted quadrupeds that use both lateral sequence (LS) and DS footfalls. As tool-users, capuchins experience selective pressures for FL manipulative capabilities, which seemingly conflict with encountering substantial locomotor stresses. We evaluate kinetic and 3-D kinematic data from 172 limb contacts of two adult males on terrestrial and arboreal substrates to address questions about C. apella gait compliancy, kinematics of LS and DS footfalls during quadrupedalism on different substrates, and whether capuchins reduce FL vertical peak forces relative to hind limb (HL) forces more than other primates that use tools or those that do not. Lower vertical peak forces during AQ are consistent with compliant gait, but mixed kinematic results obscure how the reduction occurs. Forearm adduction angle is one consistent kinematic difference between terrestrial and arboreal quadrupedalism, which may implicate frontal plane movements in gait compliancy. Major differences between DS and LS gaits were not observed in kinetic or kinematic comparisons. Capuchins exhibit low FL/HL vertical peak force ratios like several anthropoids, including tool-users (e.g., chimpanzees), and species not considered tool-users in free-ranging conditions (e.g., spider monkeys). Additional selective pressures besides simply tool use appear responsible for the relative reduction in primate forelimb forces.

Functional significance of genetic variation underlying limb bone diaphyseal structure.

Limb bone diaphyseal structure is frequently used to infer hominin activity levels from skeletal remains, an approach based on the well-documented ability of bone to adjust to its loading environment during life. However, diaphyseal structure is also determined in part by genetic factors. This study investigates the possibility that genetic variation underlying diaphyseal structure is influenced by the activity levels of ancestral populations and might also have functional significance in an evolutionary context. We adopted an experimental evolution approach and tested for differences in femoral diaphyseal structure in 1-week-old mice from a line that had been artificially selected (45 generations) for high voluntary wheel running and non-selected controls. As adults, selected mice are significantly more active on wheels and in home cages, and have thicker diaphyses. Structural differences at 1 week can be assumed to primarily reflect the effects of selective breeding rather than direct mechanical stimuli, given that the onset of locomotion in mice is shortly after Day 7. We hypothesized that if genetically determined diaphyseal structure reflects the activity patterns of members of a lineage, then selected animals will have relatively larger diaphyseal dimensions at 1 week compared to controls. The results provide strong support for this hypothesis and suggest that limb bone cross sections may not always only reflect the activity levels of particular fossil individuals, but also convey an evolutionary signal providing information about hominin activity in the past.

Locomotor variation and bending regimes of capuchin limb bones.

Primates are very versatile in their modes of progression, yet laboratory studies typically capture only a small segment of this variation. In vivo bone strain studies in particular have been commonly constrained to linear locomotion on flat substrates, conveying the potentially biased impression of stereotypic long bone loading patterns. We here present substrate reaction forces (SRF) and limb postures for capuchin monkeys moving on a flat substrate ("terrestrial"), on an elevated pole ("arboreal"), and performing turns. The angle between the SRF vector and longitudinal axes of the forearm or leg is taken as a proxy for the bending moment experienced by these limb segments. In both frontal and sagittal planes, SRF vectors and distal limb segments are not aligned, but form discrepant angles; that is, forces act on lever arms and exert bending moments. The positions of the SRF vectors suggest bending around oblique axes of these limb segments. Overall, the leg is exposed to greater moments than the forearm. Simulated arboreal locomotion and turns introduce variation in the discrepancy angles, thus confirming that expanding the range of locomotor behaviors studied will reveal variation in long bone loading patterns that is likely characteristic of natural locomotor repertoires. "Arboreal" locomotion, even on a linear noncompliant branch, is characterized by greater variability of force directions and discrepancy angles than "terrestrial" locomotion (significant for the forearm only), partially confirming the notion that life in trees is associated with greater variation in long bone loading. Directional changes broaden the range of external bending moments even further.

The bipedalism of the Dmanisi hominins: pigeon-toed early Homo?

In the recent description of the hominin postcranial material from Dmanisi, Georgia, Lordkipanidze and colleagues (Lordkipanidze et al. [2007] Nature 449: 305-310) claim that the Dmanisi hominins walked with more medially oriented feet than do modern humans. They draw this functional inference from two postcranial features: a wide talar neck angle and a slight medial torsion of the tibia. However, we believe that the data provided by the authors fail to support their conclusions. Talar neck angle and tibial torsion values from the Dmanisi specimens fall comfortably within the range of modern human variation. We further submit that foot orientation cannot be reliably deduced from the tibia and talus alone.

Three-dimensional kinematics and the origin of the hominin walking stride.

Humans are unique among apes and other primates in the musculoskeletal design of their lower back and pelvis. While the last common ancestor of the Pan-Homo lineages has long been thought to be 'African ape-like', including in its lower back and ilia design, recent descriptions of early hominin and Miocene ape fossils have led to the proposal that its lower back and ilia were more similar to those of some Old World monkeys, such as macaques. Here, we compared three-dimensional kinematics of the pelvis and hind/lower limbs of bipedal macaques, chimpanzees and humans walking at similar dimensionless speeds to test the effects of lower back and ilia design on gait. Our results indicate that locomotor kinematics of bipedal macaques and chimpanzees are remarkably similar, with both species exhibiting greater pelvis motion and more flexed, abducted hind limbs than humans during walking. Some differences between macaques and chimpanzees in pelvis tilt and hip abduction were noted, but they were small in magnitude; larger differences were observed in ankle flexion. Our results suggest that if Pan and Homo diverged from a common ancestor whose lower back and ilia were either 'African ape-like' or more 'Old World monkey-like', at its origin, the hominin walking stride likely involved distinct (i.e. non-human-like) pelvis motion on flexed, abducted hind limbs.