Energetic costs of bipedal and quadrupedal walking in Japanese macaques

Differential leg and trunk operation during skipping without and with hurdles in bipedal Japanese macaque

When locomoting bipedally at higher speeds, macaques preferred unilateral skipping (galloping). The same skipping pattern was maintained while hurdling across two low obstacles at the distance of a stride within our experimental track. The present study investigated leg and trunk joint rotations and leg joint moments, with the aim of clarifying the differential leg and trunk operation during skipping in bipedal macaques. Especially at the hip, the range of joint rotation and extension at lift off was larger in the leading than in the trailing leg. The flexing knee absorbed energy and the extending ankle generated work during each step. The trunk showed only minor deviations from symmetry. Hurdling amplified the differences and notably resulted in a quasi-elastic use of the leading knee and in an asymmetric operation of the trunk.

Skipping without and with hurdles in bipedal macaque: global mechanics

Macaques trained to perform bipedally used running gaits across a wide range of speeds. At higher speeds they preferred unilateral skipping (galloping). The same asymmetric stepping pattern was used while hurdling across two low obstacles placed at the distance of a stride within our experimental track. In bipedal macaques during skipping, we expected a differential use of the trailing and leading legs. The present study investigated global properties of the effective and virtual leg, the location of the virtual pivot point (VPP), and the energetics of the center of mass (CoM), with the aim of clarifying the differential leg operation during skipping in bipedal macaques. When skipping, macaques displayed minor double support and aerial phases during one stride. Asymmetric leg use was indicated by differences in leg kinematics. Axial damping and tangential leg work did not influence the indifferent peak ground reaction forces and impulses, but resulted in a lift of the CoM during contact of the leading leg. The aerial phase was largely due to the use of the double support. Hurdling amplified the differential leg operation. Here, higher ground reaction forces combined with increased double support provided the vertical impulse to overcome the hurdles. Following CoM dynamics during a stride, skipping and hurdling represented bouncing gaits. The elevation of the VPP of bipedal macaques resembled that of human walking and running in the trailing and leading phases, respectively. Because of anatomical restrictions, macaque unilateral skipping differs from that of humans, and may represent an intermediate gait between grounded and aerial running.

Biomechanics and the origins of human bipedal walking: The last 50 years

Motion analysis, as applied to evolutionary biomechanics, has experienced its own evolution over the last 50 years. Here we review how an ever-increasing fossil record, together with continuing advancements in biomechanics techniques, have shaped our understanding of the origin of upright bipedal walking. The original, and long-established hypothesis held by Lamarck (1809), Darwin (1859) and Keith (1934), amongst others, maintained that bipedality originated in an arboreal context. However, the first field studies of gorilla and chimpanzees from the 1960's, highlighted their so-called 'knucklewalking' quadrupedalism, leading scientists to assume, semi-automatically, that knucklewalking must have been the precursor to bipedality. It would not be until the discovery of skeletons of early human relatives Australopithecus afarensis and Australopithecus prometheus, and the inclusion of methods of analysis from computer science, biomechanics, sports science and medicine, that the knucklewalking hypothesis would be most robustly challenged. Their short, but human-like lower limbs and human-like hand indicated that knucklewalking was not part of our ancestral locomotor repertoire. Rather, most current research in evolutionary biomechanics agrees it was a combination of climbing and bipedalism, both in an arboreal context, which facilitated upright, terrestrial, bipedal walking over short distances.

Non-human primate models and systems for gait and neurophysiological analysis

Brain-computer interfaces (BCIs) have garnered extensive interest and become a groundbreaking technology to restore movement, tactile sense, and communication in patients. Prior to their use in human subjects, clinical BCIs require rigorous validation and verification (V&V). Non-human primates (NHPs) are often considered the ultimate and widely used animal model for neuroscience studies, including BCIs V&V, due to their proximity to humans. This literature review summarizes 94 NHP gait analysis studies until 1 June, 2022, including seven BCI-oriented studies. Due to technological limitations, most of these studies used wired neural recordings to access electrophysiological data. However, wireless neural recording systems for NHPs enabled neuroscience research in humans, and many on NHP locomotion, while posing numerous technical challenges, such as signal quality, data throughout, working distance, size, and power constraint, that have yet to be overcome. Besides neurological data, motion capture (MoCap) systems are usually required in BCI and gait studies to capture locomotion kinematics. However, current studies have exclusively relied on image processing-based MoCap systems, which have insufficient accuracy (error: ≥4° and 9 mm). While the role of the motor cortex during locomotion is still unclear and worth further exploration, future BCI and gait studies require simultaneous, high-speed, accurate neurophysiological, and movement measures. Therefore, the infrared MoCap system which has high accuracy and speed, together with a high spatiotemporal resolution neural recording system, may expand the scope and improve the quality of the motor and neurophysiological analysis in NHPs.

Effects of exercise habituation and aging on the intersegmental coordination of lower limbs during walking with sinusoidal speed change

Background

The time courses of the joint elevation angles of the thigh, shank, and foot in one stride during walking can be well approximated by a “plane” in a triaxial space. This intersegmental coordination (IC) of the lower limb elevation angles is referred to as the planar covariation law. We examined the effects of exercise habituation and aging on the thickness of the IC plane of the lower limbs under sinusoidal speed changing conditions.

Methods

Seventeen sedentary young (SY), 16 active young (AY), and 16 active elderly (AE) adults walked on a treadmill in accordance with a sinusoidal speed changing protocol at 120, 60, and 30 s periods with an amplitude of ± 0.56 m·s⁻¹. Motion of the lower limbs from the sagittal direction was recorded to calculate the elevation angles of the lower limbs. When the best-fit IC plane was determined, the smallest standard deviation of the IC plane was considered as the anteroposterior gait variability of the lower limbs. The coefficient of variance of the step width was also quantified to evaluate the lateral step variability (CV_SW).

Results

The standard deviation of the IC plane was significantly greater in the order of SY, AY, and AE, regardless of the sinusoidal wave periods of the changing speed. The CV_SW was not significantly different among the three groups.

Conclusions

Exercise habituation influences anteroposterior gait variability of the lower limbs, but not lateral step variability, even in young adults. Given these, gait adaptability for sinusoidal speed changes does not always decline with aging.

Trial registration

UMIN000031456 (R000035911; registered February 23, 2018).

Supplementary Information

The online version contains supplementary material available at 10.1186/s40101-022-00298-w.

Forest terrains influence walking kinematics among indigenous Tsimane of the Bolivian Amazon

Laboratory-based studies indicate that a major evolutionary advantage of bipedalism is enabling humans to walk with relatively low energy expenditure. However, such studies typically record subjects walking on even surfaces or treadmills that do not represent the irregular terrains our species encounters in natural environments. To date, few studies have quantified walking kinematics on natural terrains. Here we used high-speed video to record marker-based kinematics of 21 individuals from a Tsimane forager-horticulturalist community in the Bolivian Amazon walking on three different terrains: a dirt field, a forest trail and an unbroken forest transect. Compared with the field, in the unbroken forest participants contacted the ground with more protracted legs and flatter foot postures, had more inclined trunks, more flexed hips and knees, and raised their feet higher during leg swing. In contrast, kinematics were generally similar between trail and field walking. These results provide preliminary support for the idea that irregular natural surfaces like those in forests cause humans to alter their walking kinematics, such that travel in these environments could be more energetically expensive than would be assumed from laboratory-based data. These findings have important implications for the evolutionary energetics of human foraging in environments with challenging terrains.

Development of bipedal walking in olive baboons, Papio anubis: A kinematic analysis

OBJECTIVE

Although extant nonhuman primates are not habitual bipeds, they are able to walk bipedally from an early age. In humans, children improve their walking skills through developmental processes and learning experience. In nonhuman primates, infants do not routinely experience bipedalism and their musculoskeletal system gradually specializes for other locomotor modes. The aim of this study is to explore the development of occasional bipedal walking in olive baboon and to test whether the postural adjustments change with age.

MATERIALS AND METHODS

We collected kinematics and spatiotemporal parameters of bipedal gait in an ontogenetic sample of 24 baboons. Data were collected at the primatology station of the CNRS (France) and a total of 47 bipedal strides were extracted for the present analysis.

RESULTS

Adults and adolescents walk bipedally in the same way, and the average kinematic pattern is similar across the age-classes. Infants walk bipedally with longer duty factor, they present larger movement amplitude of the thigh and the amplitude of the knee joint decreases with speed. In contrast, older baboons increase the amplitude of the knee and ankle joints with speed.

DISCUSSION

In a non-adapted biped, the postural adjustments of bipedal walking vary with age. In infant baboons, the balance requirements are likely to be higher and these are solved by adopting a "blocking strategy". In older baboons, the postural adjustments are focused on the lower limb and the movements increase with speed. These results may echo, in some respects, the developmental sequence of the intersegmental coordination described in the ontogeny of human locomotion.

Wading through water: effects of water depth and speed on the drag and kinematics of walking Chilean flamingos, Phoenicopterus chilensis

Wading behaviours, in which an animal walks while partially submerged in water, are present in a variety of taxa including amphibians, reptiles, mammals and birds. Despite the ubiquity of wading behaviours, few data are available to evaluate how animals adjust their locomotion to accommodate changes in water depth. Because drag from water might impose additional locomotor costs, wading animals might be expected to raise their feet above the water up to a certain point until such behaviours lead to awkward steps and are abandoned. To test for such mechanisms, we measured drag on models of the limbs of Chilean flamingos (Phoenicopterus chilensis) and measured their limb and body kinematics as they walked and waded through increasing depths of water in a zoo enclosure. Substantial drag was incurred by models of both open- and closed-toed feet, suggesting that flamingos could avoid some locomotor costs by stepping over water, rather than through it, during wading. Step height was highest while wading through intermediate water depths and while wading at a faster speed. Stride length increased with increasing water depth and velocity, and the limb joints generally flexed more while moving through intermediate water depths. However, movements of the head and neck were not strongly correlated with water depth or velocity. Our results show a wide range of kinematic changes that occur to allow wading birds to walk through different water depths, and have implications for better understanding the locomotor strategies employed by semi-aquatic species.

Simulating the evolution of bipedalism and the absence of static bipedal hexapods

In nature, very few animals locomote on two legs. Static bipedalism can be found in four limbed and five limbed animals like dogs, cats, birds, monkeys and kangaroos, but it cannot be seen in hexapods or other multi-limbed animals. In this paper, we present a simulation with a novel perspective on the evolution of static bipedalism, with a virtual creature evolving its body and controllers, and we apply an evolutionary algorithm to explore the locomotion transition from octapods to bipods. We find that the presence of four limbs in the evolutionary trajectory of the creature scaffolds a parametric jump that enables bipedalism, and shows that hexapods, without undergoing such transformation, struggle to evolve into bipeds. An analysis of the transitional parameters points to the role of a shorter femur length in helping maintain the stability of the body, and the tibia length is responsible for improving the forward speed.

Forward dynamic simulation of Japanese macaque bipedal locomotion demonstrates better energetic economy in a virtualised plantigrade posture

A plantigrade foot with a large robust calcaneus is regarded as a distinctive morphological feature of the human foot; it is presumably the result of adaptation for habitual bipedal locomotion. The foot of the Japanese macaque, on the other hand, does not have such a feature, which hampers it from making foot-ground contact at the heel during bipedal locomotion. Understanding how this morphological difference functionally affects the generation of bipedal locomotion is crucial for elucidating the evolution of human bipedalism. In this study, we constructed a forward dynamic simulation of bipedal locomotion in the Japanese macaque based on a neuromusculoskeletal model to evaluate how virtual manipulation of the foot structure from digitigrade to plantigrade affects the kinematics, dynamics, and energetics of bipedal locomotion in a nonhuman primate whose musculoskeletal anatomy is not adapted to bipedalism. The normal bipedal locomotion generated was in good agreement with that of actual Japanese macaques. If, as in human walking, the foot morphology was altered to allow heel contact, the vertical ground reaction force profile became double-peaked and the cost of transport decreased. These results suggest that evolutionary changes in the foot structure were important for the acquisition of human-like efficient bipedal locomotion.

Thermodynamics of Animal Locomotion

Muscles are biological actuators extensively studied in the frame of Hill's classic empirical model as isolated biomechanical entities, which hardly applies to a living organism subjected to physiological and environmental constraints. Here we elucidate the overarching principle of a living muscle action for locomotion, considering it from the thermodynamic viewpoint as an assembly of actuators (muscle units) connected in parallel, operating via chemical-to-mechanical energy conversion under mixed (potential and flux) boundary conditions. Introducing the energy cost of effort as the generalization of the well-known oxygen cost of transport in the frame of our compact locally linear nonequilibrium thermodynamics model, we analyze oxygen consumption measurement data from a documented experiment on energy cost management and optimization by horses moving at three different gaits. Horses adapt to a particular gait by mobilizing a nearly constant number of muscle units minimizing waste production per unit distance covered; this number significantly changes during transition between gaits. The mechanical function of the animal is therefore determined both by its own thermodynamic characteristics and by the metabolic operating point of the locomotor system.

Research methodology for in vivo measurements of resting energy expenditure, daily body temperature, metabolic heat and non-viral tissue-specific gene therapy in baboons

A large number of studies have shown that the baboon is one of the most commonly used non-human primate (NHP) research model for the study of immunometabolic complex traits such as type 2 diabetes (T2D), insulin resistance (IR), adipose tissue dysfunction (ATD), dyslipidemia, obesity (OB) and cardiovascular disease (CVD). This paper reports on innovative technologies and advanced research strategies for energetics and translational medicine with this NHP model. This includes the following: measuring resting energy expenditure (REE) with the mobile indirect calorimeter Breezing®; monitoring daily body temperature using subcutaneously implanted data loggers; quantifying metabolic heat with veterinary infrared thermography (IRT) imaging, and non-viral non-invasive, tissue-specific ultrasound-targeted microbubble destruction (UTMD) gene-based therapy. These methods are of broad utility; for example, they may facilitate the engineering of ectopic overexpression of brown adipose tissue (BAT) mUCP-1 via UTMD-gene therapy into baboon SKM to achieve weight loss, hypophagia and immunometabolic improvement. These methods will be valuable to basic and translational research, and human clinical trials, in the areas of metabolism, cardiovascular health, and immunometabolic and infectious diseases.

Response of the Axial Skeleton to Bipedal Loading Behaviors in an Experimental Animal Model

Many derived aspects of modern human axial skeletal morphology reflect our reliance on obligate bipedal locomotion. Insight into the adaptive significance of features, particularly in the spine, has been gained through experimental studies that induce bipedal standing or walking in quadrupedal mammals. Using an experimental animal model (Rattus norvegicus), the present study builds on earlier work by incorporating additional metrics of the cranium, employing quantitative methods established in the paleoanthropological literature, and exploring how variation in mechanical loading regimes impacts axial anatomy. Rats were assigned to one of five experimental groups, including "fully loaded bipedal walking," "partially loaded bipedal walking," "standing bipedally," "quadrupedal walking," and "no exercise control," and engaged in the behavior over 12-weeks. From μCT data obtained at the beginning and end of the experiment, we measured foramen magnum position and orientation, lumbar vertebral body wedging, cranial surface area of the lumbar and first sacral vertebral bodies, and sacral mediolateral width. Results demonstrate that bipedal rodents generally have more anteriorly positioned foramina magna, more dorsally wedged lumbar vertebrae, greater articular surface areas of lumbar and first sacral vertebral bodies, and sacra that exhibit greater mediolateral widths, compared to quadrupedal rodents. We further document variation among bipedal loading behavior groups (e.g., bipedal standing vs. walking). Our experimental animal model reveals how loading behaviors and adaptations may be specifically linked, and implicates a potential role for developmental plasticity in the evolutionary acquisition of bipedal adaptations in the hominin lineage. Anat Rec, 2018. © 2018 American Association for Anatomy.

Bipedal gait versatility in the Japanese macaque (Macaca fuscata)

It was previously believed that, among primates, only humans run bipedally. However, there is now growing evidence that at least some non-human primates can not only run bipedally but can also generate a running gait with an aerial phase. Japanese macaques trained for bipedal performances have been known to exhibit remarkable bipedal locomotion capabilities, but no aerial-phase running has previously been reported. In the present study, we investigated whether Japanese macaques could run with an aerial phase by collecting bipedal gait sequences from three macaques on a level surface at self-selected speeds (n = 188). During our experiments, body kinematics and ground reaction forces were recorded by a motion-capture system and two force plates installed within a wooden walkway. Our results demonstrated that macaques were able to utilize a variety of bipedal gaits including grounded running, skipping, and even running with an aerial phase. The self-selected bipedal locomotion speed of the macaques was fast, with Froude speed ranging from 0.4 to 1.3. However, based on congruity, no single trial that could be categorized as a pendulum-like walking gait was observed. The parameters describing the temporal, kinematic, and dynamic characteristics of macaque bipedal running gaits follow the patterns previously documented for other non-human primates and terrestrial birds that use running gaits, but are different from those of humans and from birds' walking gaits. The present study confirmed that when a Japanese macaque engages in bipedal locomotion, even without an aerial phase, it generally utilizes a spring-like running mechanism because the animals have a limited ability to stiffen their legs. That limitation is due to anatomical restrictions determined by the morphology and structure of the macaque musculoskeletal system. The general adoption of grounded running in macaques and other non-human primates, along with its absence in human bipedal locomotion, suggests that abandonment of compliant gait was a critical transition in the evolution of human obligatory bipedalism.

Neuromechanical interactions between the limbs during human locomotion: an evolutionary perspective with translation to rehabilitation

During bipedal locomotor activities, humans use elements of quadrupedal neuronal limb control. Evolutionary constraints can help inform the historical ancestry for preservation of these core control elements support transfer of the huge body of quadrupedal non-human animal literature to human rehabilitation. In particular, this has translational applications for neurological rehabilitation after neurotrauma where interlimb coordination is lost or compromised. The present state of the field supports including arm activity in addition to leg activity as a component of gait retraining after neurotrauma.

Vertical bipedal locomotion in wild bearded capuchin monkeys (Sapajus libidinosus)

When carrying objects, nonhuman primates often show bipedal locomotion. Studies of primate bipedality, however, in both nature and captivity, have concentrated on locomotion on horizontal substrates, either terrestrially or arboreally. No observational or experimental study seems to have looked at non-horizontal bipedality, yet we show here that it occurs often in nature in Sapajus libidinosus, the bearded capuchin monkey. The context is transport of small food items from source to site of consumption, in which the monkeys usually carry handfuls of maize kernels over several meters' distance, both on the ground and in the trees. Most impressively, over a fifth of such bouts are done vertically, when the tree trunk is fully upright. Such vertical bipedality, with or without transport, apparently has not been reported before.

The energy costs of wading in water

Studies measuring the energy costs of wading in water have been limited to higher walking speeds in straight lines, in deep water. However, much foraging in water, by both humans and other primates, is conducted in the shallows and at low speeds of locomotion that include elements of turning, as befits searching for cryptic or hidden foods within a patch. The present study brings together data on the rate of oxygen consumption during wading by humans from previous studies, and augments these with new data for wading in shallower depths, with slower and more tortuous walking, to obtain a better understanding both of the absolute costs of wading in typical scenarios of aquatic foraging and of how the cost of wading varies as a function of water depth and speed of locomotion. Previous and present data indicate that, at low speeds, wading has a similar energetic cost to walking on land, particularly at lower water depths, and only at higher speeds is the cost of wading noticeably more expensive than when water is absent. This is probably explained by the relatively small volume of water that must be displaced during locomotion in shallow waters coupled with the compensating support to the limbs that the water affords. The support to the limbs/body provided by water is discussed further, in the context of bipedal locomotion by non-human primates during wading.

Exploring diagonal gait using a forward dynamic three-dimensional chimpanzee simulation

Primates are unusual among terrestrial quadrupedal mammals in that at walking speeds they prefer diagonal rather than lateral gaits. A number of reasons have been proposed for this preference in relation to the arboreal ancestry of modern primates: stability, energetic cost, neural control, skeletal loading, and limb interference avoiding. However, this is a difficult question to explore experimentally since most primates only occasionally use anything other than diagonal gaits. An alternative approach is to produce biologically realistic computer simulations of primate gait that enable the constraints of biomechanical loading and the energetics of different modes of locomotion to be explored. In this paper we describe such a model for the chimpanzee Pan troglodytes. The simulation is able to produce spontaneous quadrupedal locomotion, and the footfall sequences generated are split between lateral and diagonal footfall sequences with no obvious energetic benefit associated with either option. However, out of 10 successful simulation runs, 5 were lateral sequence/lateral couplet gaits indicating a preference for a specific lateral footfall sequence with a relatively tightly constrained phase difference between the fore- and hindlimbs. This suggests that the choice of diagonal walking gaits in chimpanzees is not a simple mechanical phenomenon and that diagonal walking gaits in primates are selected for by multiple factors.

Planar covariation of limb elevation angles during bipedal walking in the Japanese macaque

We investigated the planar covariation of lower limb segment elevation angles during bipedal walking in macaques to elucidate the mechanisms underlying the origin and evolution of the planar law in human walking. Two Japanese macaques and four adult humans walking on a treadmill were recorded, and the time course of the elevation angles at the thigh, shank and foot segments relative to the vertical axis were calculated. Our analyses indicated that the planar law also applies to macaque bipedal walking. However, planarity was much lower in macaques, and orientations of the plane differed between the two species because of differences in the foot elevation angle. The human foot is rigidly structured to form a longitudinal arch, whereas the macaque's foot is more flexible and bends at the midtarsal region in the stance phase. This difference in midfoot flexibility between the two species studied was the main source of the difference in the planar law. Thus, the evolution of a stable midfoot in early hominins may have preceded the acquisition of the strong planar intersegmental coordination and possibly facilitated the subsequent emergence of habitual bipedal walking in the human lineage.

A non-human primate model of bipedal locomotion under restrained condition allowing gait studies and single unit brain recordings

For decades, several animal models of locomotion have allowed a better understanding of the basic physiological mechanisms of gait. However, unlike most of the mammals, the Order Primates is characterized by fundamental changes in locomotor behaviour. In particular, some primates use a specific pattern of locomotion and are able to naturally walk bipedally due possibly to a specific supra-spinal control of locomotion. These features must be taken into account when one considers to study the intrinsic properties of human gait. Thus, an experimental model of bipedal locomotion allowing precise and reproducible analysis of gait in non-human primate is still lacking. This study describes a non-human primate model of bipedal locomotion under restrained condition. We undertook a kinematic and biomechanic study in three Macaca fascicularis trained to walk bipedally on a treadmill. One of the primate was evaluated in complete head fixation. Gait visual analysis and electromyographic recordings provided pertinent description of the gait pattern. Step frequencies, step lengths, cycle and stance phase durations were correlated with Froude number (dimensionless velocity), whereas swing phase durations remained non-correlated. Gait patterns observed in our model were similar to those obtained in freely bipedal Macaca fuscata and to a lesser extend to Humans. Gait pattern was not modified by head fixation thereby allowing us to perform precise and repetitive micro electrode recordings of deep cerebral structures. Thus, the present model could provide a pertinent pre-clinical tool to study gait parameters and their neuronal control but also could be helpful to validate new therapeutics interventions.

Features of hand-foot crawling behavior in human adults

Interlimb coordination of crawling kinematics in humans shares features with other primates and nonprimate quadrupeds, and it has been suggested that this is due to a similar organization of the locomotor pattern generators (CPGs). To extend the previous findings and to further explore the neural control of bipedal vs. quadrupedal locomotion, we used a crawling paradigm in which healthy adults crawled on their hands and feet at different speeds and at different surface inclinations (13°, 27°, and 35°). Ground reaction forces, limb kinematics, and electromyographic (EMG) activity from 26 upper and lower limb muscles on the right side of the body were collected. The EMG activity was mapped onto the spinal cord in approximate rostrocaudal locations of the motoneuron pools to characterize the general features of cervical and lumbosacral spinal cord activation. The spatiotemporal pattern of spinal cord activity significantly differed between quadrupedal and bipedal gaits. In addition, participants exhibited a large range of kinematic coordination styles (diagonal vs. lateral patterns), which is in contrast to the stereotypical kinematics of upright bipedal walking, suggesting flexible coupling of cervical and lumbosacral pattern generators. Results showed strikingly dissimilar directional horizontal forces for the arms and legs, considerably retracted average leg orientation, and substantially smaller sacral vs. lumbar motoneuron activity compared with quadrupedal gait in animals. A gradual transition to a more vertical body orientation (increasing the inclination of the treadmill) led to the appearance of more prominent sacral activity (related to activation of ankle plantar flexors), typical of bipedal walking. The findings highlight the reorganization and adaptation of CPG networks involved in the control of quadrupedal human locomotion and a high specialization of the musculoskeletal apparatus to specific gaits.

The energetic cost of walking: a comparison of predictive methods

BACKGROUND

The energy that animals devote to locomotion has been of intense interest to biologists for decades and two basic methodologies have emerged to predict locomotor energy expenditure: those based on metabolic and those based on mechanical energy. Metabolic energy approaches share the perspective that prediction of locomotor energy expenditure should be based on statistically significant proxies of metabolic function, while mechanical energy approaches, which derive from many different perspectives, focus on quantifying the energy of movement. Some controversy exists as to which mechanical perspective is "best", but from first principles all mechanical methods should be equivalent if the inputs to the simulation are of similar quality. Our goals in this paper are 1) to establish the degree to which the various methods of calculating mechanical energy are correlated, and 2) to investigate to what degree the prediction methods explain the variation in energy expenditure.

METHODOLOGY/PRINCIPAL FINDINGS

We use modern humans as the model organism in this experiment because their data are readily attainable, but the methodology is appropriate for use in other species. Volumetric oxygen consumption and kinematic and kinetic data were collected on 8 adults while walking at their self-selected slow, normal and fast velocities. Using hierarchical statistical modeling via ordinary least squares and maximum likelihood techniques, the predictive ability of several metabolic and mechanical approaches were assessed. We found that all approaches are correlated and that the mechanical approaches explain similar amounts of the variation in metabolic energy expenditure. Most methods predict the variation within an individual well, but are poor at accounting for variation between individuals.

CONCLUSION

Our results indicate that the choice of predictive method is dependent on the question(s) of interest and the data available for use as inputs. Although we used modern humans as our model organism, these results can be extended to other species.

Forward dynamic simulation of bipedal walking in the Japanese macaque: investigation of causal relationships among limb kinematics, speed, and energetics of bipedal locomotion in a nonhuman primate

Japanese macaques that have been trained for monkey performances exhibit a remarkable ability to walk bipedally. In this study, we dynamically reconstructed bipedal walking of the Japanese macaque to investigate causal relationships among limb kinematics, speed, and energetics, with a view to understanding the mechanisms underlying the evolution of human bipedalism. We constructed a two-dimensional macaque musculoskeletal model consisting of nine rigid links and eight principal muscles. To generate locomotion, we used a trajectory-tracking control law, the reference trajectories of which were obtained experimentally. Using this framework, we evaluated the effects of changes in cycle duration and gait kinematics on locomotor efficiency. The energetic cost of locomotion was estimated based on the calculation of mechanical energy generated by muscles. Our results demonstrated that the mass-specific metabolic cost of transport decreased as speed increased in bipedal walking of the Japanese macaque. Furthermore, the cost of transport in bipedal walking was reduced when vertical displacement of the hip joint was virtually modified in the simulation to be more humanlike. Human vertical fluctuations in the body's center of mass actually contributed to energy savings via an inverted pendulum mechanism.

Locomotor energetics in primates: gait mechanics and their relationship to the energetics of vertical and horizontal locomotion

All primates regularly move within three-dimensional arboreal environments and must often climb, but little is known about the energetic costs of this critical activity. Limited previous work on the energetics of incline locomotion suggests that there may be differential selective pressures for large compared to small primates in choosing to exploit a complex arboreal environment. Necessary metabolic and gait data have never been collected to examine this possibility and biomechanical mechanisms that might explain size-based differences in the cost of arboreal movement. Energetics and kinematics were collected for five species of primate during climbing and horizontal locomotion. Subjects moved on a treadmill with a narrow vertical substrate and one with a narrow horizontal substrate at their maximum sustainable speed for 10–20 min while oxygen consumption was monitored. Data during climbing were compared to those during horizontal locomotion and across size. Results show that climbing energetic costs were similar to horizontal costs for small primates (<0.5 kg) but were nearly double for larger species. Spatio-temporal gait characteristics suggest that the relationship between the cost of locomotion and the rate of force production changes between the two locomotor modes. Thus, the main determinants of climbing costs are fundamentally different from those during horizontal locomotion. These new results combining spatiotemporal and energetic data confirm and expand on our previous argument (Hanna et al.: Science 320 (2008) 898) that similar costs of horizontal and vertical locomotion in small primates facilitated the successful occupation of a fine-branch arboreal milieu by the earliest primates.

Spinopelvic pathways to bipedality: why no hominids ever relied on a bent-hip-bent-knee gait

Until recently, the last common ancestor of African apes and humans was presumed to resemble living chimpanzees and bonobos. This was frequently extended to their locomotor pattern leading to the presumption that knuckle-walking was a likely ancestral pattern, requiring bipedality to have emerged as a modification of their bent-hip-bent-knee gait used during erect walking. Research on the development and anatomy of the vertebral column, coupled with new revelations from the fossil record (in particular, Ardipithecus ramidus), now demonstrate that these presumptions have been in error. Reassessment of the potential pathway to early hominid bipedality now reveals an entirely novel sequence of likely morphological events leading to the emergence of upright walking.

The evolution of the upright posture and gait--a review and a new synthesis

During the last century, approximately 30 hypotheses have been constructed to explain the evolution of the human upright posture and locomotion. The most important and recent ones are discussed here. Meanwhile, it has been established that all main hypotheses published until the last decade of the past century are outdated, at least with respect to some of their main ideas: Firstly, they were focused on only one cause for the evolution of bipedality, whereas the evolutionary process was much more complex. Secondly, they were all placed into a savannah scenario. During the 1990s, the fossil record allowed the reconstruction of emerging bipedalism more precisely in a forested habitat (e.g., as reported by Clarke and Tobias (Science 269:521-524, 1995) and WoldeGabriel et al. (Nature 412:175-178, 2001)). Moreover, the fossil remains revealed increasing evidence that this part of human evolution took place in a more humid environment than previously assumed. The Amphibian Generalist Theory, presented first in the year 2000, suggests that bipedalism began in a wooded habitat. The forests were not far from a shore, where our early ancestor, along with its arboreal habits, walked and waded in shallow water finding rich food with little investment. In contrast to all other theories, wading behaviour not only triggers an upright posture, but also forces the individual to maintain this position and to walk bipedally. So far, this is the only scenario suitable to overcome the considerable anatomical and functional threshold from quadrupedalism to bipedalism. This is consistent with paleoanthropological findings and with functional anatomy as well as with energetic calculations, and not least, with evolutionary psychology. The new synthesis presented here is able to harmonise many of the hitherto competing theories.

Energy expenditure in chow-fed female non-human primates of various weights

BACKGROUND

Until now no technology has been available to study energy metabolism in monkeys. The objective of this study was to determine daily energy expenditures (EE) and respiratory quotients (RQ) in female monkeys of various body weights and ages.

METHODS

16 socially reared Bonnet Macaque female monkeys [5.5 +/- 1.4 kg body weight, modified BMI (length measurement from head to base of the tail) = 28.8 +/- 6.7 kg/crown-rump length, m2 and 11.7 +/- 4.6 years] were placed in the primate Enhanced Metabolic Testing Activity Chamber (Model 3000a, EMTAC Inc. Santa Barbara, CA) for 22-hour measurements of EE (kcal/kg) and RQ (VCO2/VO2). All were fed monkey chow (4.03 kcal/g) ad-libitum under a 12/12 hour light/dark cycle. Metabolic data were corrected for differences in body weight. Results were divided into day (8-hours), dark (12 hours) and morning (2-hours) periods. Data analysis was conducted utilizing SPSS (Version 13).

RESULTS

Modified BMI negatively correlated with 22-hour energy expenditure in all monkeys (r = -0.80, p < 0.01). The large variability of daily energy intake (4.5 to 102.0 kcal/kg) necessitated division into two groups, non-eaters (< 13 kcal/kg) and eaters (> 23 kcal/kg). There were reductions (p < 0.05) in both 22-hour and dark period RQs in the "non-eaters" in comparison to those who were "eaters". Monkeys were also classified as "lean" (modified BMI < 25) or "obese" (modified BMI > 30). The obese group had lower EE (p < 0.05) during each time period and over the entire 22-hours (p < 0.05), in comparison to their lean counterparts.

CONCLUSION

The EMTAC proved to be a valuable tool for metabolic measurements in monkeys. The accuracy and sensitivity of the instrument allowed detection of subtle metabolic changes in relation to energy intake. Moreover, there is an association between a reduction of energy expenditure and a gain in body weight.

Brief Communication: Stand and shuffle: when does it make energetic sense?

Many reasons for the emergence of bipedalism have been proposed, including postural arguments which highlight that a sub-optimal form of bipedalism ("shuffling") might have been used by protohominids to cover short distances between resources that require bipedal standing. Bipedal shuffling may have been employed because it avoids the cost of raising the trunk from the quadrupedal orientation, which we assume is the habitual locomotor stance of protohominids. To date, these postural proposals have not been analytically assessed, a lack we rectify herein. Our model seeks to specify a threshold distance, below which bipedal shuffling uses less energy than quadrupedalism. Parameters for the model include the mechanical cost of transport, the ratio of bipedal to quadrupedal cost, and the cost associated with raising the trunk. We found that, using reasonable model parameters, open distances of approximately 9-16 m support the use of bipedal shuffling. Protohominids may have used shuffling as an energetically effective way to traverse between resource patches.

The energetics of human walking: is Froude number (Fr) useful for metabolic comparisons?

Velocity and body mass have well-known influences on the amount of metabolic energy that animals require to walk. This relationship could stem from the fact that both are key variables in calculating the mechanical energy of a system in motion. Other variables, like leg length, are also important in mechanical energy calculations and two mechanical formulations that incorporate leg length, Froude number (Fr) and the LiMb model, have been shown to correlate with human metabolic energy expenditure. Both, however, include velocity as a key variable in their calculations, so we wondered if the correlation might derive solely from their relationship with velocity rather than leg length. Using the energetic data and gait parameters from 24 human adults and 48 children, we tested several variables - velocity (V), V(2), body mass, leg length, Fr and LiMb - to see which combinations best explained the variation in oxygen consumption, a proxy of metabolic energy expenditure. An equation with V(2), body mass and leg length as covariates produced the highest R(2), explaining 88% of the variation when all subjects were combined. No significant differences in the predictive power of velocity, V(2), Fr or LiMb were detected, prompting us to conclude that neither Fr nor LiMb compensate for the effect of leg length. Leg length does influence the energetic expenditure of walking humans, but Fr and LiMb do not appear to adequately reflect that effect. The development of another method to compensate for the effect of leg length on metabolic energy consumption is essential.

Chimpanzee locomotor energetics and the origin of human bipedalism

Bipedal walking is evident in the earliest hominins [Zollikofer CPE, Ponce de Leon MS, Lieberman DE, Guy F, Pilbeam D, et al. (2005) Nature 434:755-759], but why our unique two-legged gait evolved remains unknown. Here, we analyze walking energetics and biomechanics for adult chimpanzees and humans to investigate the long-standing hypothesis that bipedalism reduced the energy cost of walking compared with our ape-like ancestors [Rodman PS, McHenry HM (1980) Am J Phys Anthropol 52:103-106]. Consistent with previous work on juvenile chimpanzees [Taylor CR, Rowntree VJ (1973) Science 179:186-187], we find that bipedal and quadrupedal walking costs are not significantly different in our sample of adult chimpanzees. However, a more detailed analysis reveals significant differences in bipedal and quadrupedal cost in most individuals, which are masked when subjects are examined as a group. Furthermore, human walking is approximately 75% less costly than both quadrupedal and bipedal walking in chimpanzees. Variation in cost between bipedal and quadrupedal walking, as well as between chimpanzees and humans, is well explained by biomechanical differences in anatomy and gait, with the decreased cost of human walking attributable to our more extended hip and a longer hindlimb. Analyses of these features in early fossil hominins, coupled with analyses of bipedal walking in chimpanzees, indicate that bipedalism in early, ape-like hominins could indeed have been less costly than quadrupedal knucklewalking.

Ground-reaction-force profiles of bipedal walking in bipedally trained Japanese monkeys

Ground-reaction-force (GRF) profiles of bipedal locomotion in bipedally trained Japanese macaques (performing monkeys) were analyzed in order to clarify the dynamic characteristics of their locomotion. Five trained and two ordinary monkeys participated in the experiment. They walked on a wooden walkway at a self-selected speed, and three components of the GRF vector were measured using a force platform. Our measurements reveal that trained monkeys exhibited vertical-GRF profiles that were single-peaked, similar to those of ordinary monkeys; they did not generate the double-peaked force curve that is seen in humans, despite their extensive training. However, in the trained monkeys, the peak appeared relatively earlier in the stance phase, and overall shape was more triangular than that of the more parabolic profile generated by ordinary monkeys. Comparisons of vertical fluctuation of the center of body mass calculated from the measured profiles suggest that this was larger in the trained monkeys, indicating that storage and release of potential energy actually took place in their bipedal walking. This energetic advantage seems limited, however, because efficient exchange of potential and kinetic energy during walking were not completely out of phase as in human walking. We suggest that anatomically restricted range of hip-joint motion impedes the inherently quadrupedal monkeys from generating humanlike bipedal locomotion, and that morphological rearrangement of the hip joint was an essential precondition for protohominids to acquire humanlike bipedalism.

Energy expenditure of bipedal walking is higher than that of quadrupedal walking in Japanese macaques

The authors previously compared energetic costs of bipedal and quadrupedal walking in bipedally trained macaques used for traditional Japanese monkey performances (Nakatsukasa et al. 2004 Am. J. Phys. Anthropol. 124:248-256). These macaques used inverted pendulum mechanics during bipedal walking, which resulted in an efficient exchange of potential and kinetic energy. Nonetheless, energy expenditure during bipedal walking was significantly higher than that of quadrupedal walking. In Nakatsukasa et al. (2004 Am. J. Phys. Anthropol. 124:248-256), locomotor costs were measured before subjects reached a steady state due to technical limitations. The present investigation reports sequential changes of energy consumption during 15 min of walking in two trained macaques, using carbon dioxide production as a proxy of energy consumption, as in Nakatsukasa et al. (2004 Am. J. Phys. Anthropol. 124:248-256). Although a limited number of sessions were conducted, carbon dioxide production was consistently greater during bipedal walking, with the exception of some irregularity during the first minute. Carbon dioxide production gradually decreased after 1 min, and both subjects reached a steady state within 10 min. Energy expenditure during bipedalism relative to quadrupedalism differed between the two subjects. It was considerably higher (140% of the quadrupedal walking cost) in one subject who walked with more bent-knee, bent-hip gaits. This high cost strongly suggests that ordinary macaques, who adopt further bent-knee, bent-hip gaits, consume a far greater magnitude of energy during bipedal walking.

Do highly trained monkeys walk like humans? A kinematic study of bipedal locomotion in bipedally trained Japanese macaques

In this study, we examined the kinematics of bipedal walking in macaque monkeys that have been highly trained to stand and walk bipedally, and compared them to the kinematics of bipedal walking in ordinary macaques. The results revealed that the trained macaques walked with longer and less frequent strides than ordinary subjects. In addition, they appear to have used inverted pendulum mechanics during bipedal walking, which resulted in an efficient exchange of potential and kinetic energy. These gait characteristics resulted from the relatively more extended hindlimb joints of the trained macaques. By contrast, the body of the ordinary macaques translated downward during the single-limb stance phase due to more flexed hindlimb joints. This resulted in almost in-phase fluctuations of potential and kinetic energy, which indicated that energy transformation was less efficient in the ordinary macaques. The findings provide two insights into the early stage of the evolution of human bipedalism. First, the finding that training considerably improved bipedal walking a posteriori may explain why the very first bipeds that might not yet have been morphologically adapted to bipedal walking continued to walk bipedally. The evolutionary transition from quadrupedalism to bipedalism might not be as difficult as has been envisioned. In addition, the finding that macaques, which are phylogenetically distant from humans and in which bipedal walking is unlike human walking, could develop humanlike gait characteristics with training, provides strong support for the commonly held but unproven idea that the characteristics of the human gait are advantageous to human bipedalism.

Acquisition of bipedalism: the Miocene hominoid record and modern analogues for bipedal protohominids

The well-known fossil hominoid Proconsul from the Early Miocene of Kenya was a non-specialized arboreal quadruped with strong pollicial/hallucial assisted grasping capability. It lacked most of the suspensory specializations acquired in living hominoids. Nacholapithecus, however, from the Middle Miocene of Kenya, although in part sharing with Proconsul the common primitive anatomical body design, was more specialized for orthograde climbing, 'hoisting' and bridging, with the glenoid fossae of the scapula probably being cranially orientated, the forelimbs proportionally large, and very long toes. Its tail loss suggests relatively slow movement, although tail loss may already have occurred in Proconsul. Nacholapithecus-like positional behaviour might thus have been a basis for development of more suspensory specialized positional behaviour in later hominoids. Unfortunately, after 13 Ma, there is a gap in the hominoid postcranial record in Africa until 6 Ma. Due to this gap, a scenario for later locomotor evolution prior to the divergence of Homo and Pan cannot be determined with certainty. The time gap also causes difficulties when we seek to determine polarities of morphological traits in very early hominids. Interpretation of the form-function relationships of postcranial features in incipient hominids will be difficult because it is predicted that they had incorporated bipedalism only moderately into their total positional repertoires. However, Japanese macaques, which are trained in traditional bipedal performance, may provide useful hints about bipedal adaptation in the protohominids. Kinematic analyses revealed that these macaques walked bipedally with a longer stride and lower stride frequency than used by ordinary macaques, owing to a more extended posture of the hindlimb joints. The body centre of gravity rises during the single-support phase of stance. Energetic studies of locomotion in these bipedal macaques revealed that energetic expenditure was 20-30% higher in bipedalism than in quadrupedalism, regardless of walking velocity.