Predicting the energy cost of terrestrial locomotion: a test of the LiMb model in humans and quadrupeds

Exploring How the Arms Can Help the Legs in Facilitating Gait Rehabilitation

Inspired by the ideas from the fields of gait rehabilitation, neuroscience, and locomotion biomechanics and energetics, a body of work is reviewed that has led to propose a conceptual framework for novel "self-assistive" walking devices that could further promote walking recovery from incomplete spinal cord injuries. The underlying rationale is based on a neural coupling mechanism that governs the coordinated movements of the arms and legs during walking, and that the excitability of these neural pathways can be exploited by actively engaging the arms during locomotor training. Self-assistive treadmill walking rehabilitation devices are envisioned as an approach that would allow an individual to actively use their arms to help the legs during walking. It is hoped that the conceptual framework inspires the design and use of self-assistive walking devices that are tailored to assist individuals with an incomplete spinal cord injury to regain their functional walking ability.

The dual function of prokinesis in the feeding and locomotor systems of parrots

Prokinesis, a mode of avian cranial kinesis involving motion between the neurocranium and upper beak, has long been investigated in biomechanical analyses of avian feeding and drinking. However, the modern avian beak is also used for non-feeding functions. Here, we investigate the dual function of prokinesis in the feeding and locomotor systems of the rosy-faced lovebird (Agapornis roseicollis). Lovebirds and other parrots utilize their beak both during feeding and as a third limb during vertical climbing. Thus, we experimentally measured both force-generating potential and movement of the rosy-faced lovebird mandible and maxilla (via prokinetic flexion of the craniofacial hinge) during tripedal climbing and mandibular/maxillary adduction. We found that whereas the maxilla is primarily responsible for generating force during locomotion, the mandible is primarily responsible for generating force during forceful jaw adduction, hinting at a remarkable capacity to alter prokinetic function with differing neuromuscular control. The ability of the prokinetic apparatus to perform functions with competing optimality criteria via modulation of motor control illustrates the functional plasticity of the avian cranial kinesis and sheds new light on the adaptive significance of cranial mobility.

Impact of advanced footwear technology on elite men's in the evolution of road race performance

Advanced footwear technology (AFT) changed footwear design concepts by using a curved carbon fibre plate in combination with new, more compliant and resilient foams. The aim of this study was (1) to examine the individual effects of AFT on the evolution of the main road events and (2) to re-assess the impact of AFT on the world's top-100 performance in men's 10k, half-marathon and marathon events. Data from the top-100 men's 10k, half-marathon and marathon performances were collected between 2015 and 2019. The shoes used by the athletes were identified in 93.1% of the cases by publicly available photographs. Runners wearing AFT had an average performance of 1671 ± 22.28 s compared to 1685 ± 18.97 s of runners not using AFT in 10k (0.83%) (p < 0.001), 3589 ± 29.79 s compared to 3607 ± 30.49 s in half-marathon (0.50%) (p < 0.001) and 7563 ± 86.10 s compared to 7637 ± 72.51 s in the marathon (0.97%) (p < 0.001). Runners wearing AFTs were faster by ~1% in the main road events compared to non-users. Individual analysis showed that ~25% of the runners did not benefit from the use of this type of footwear. The results of this study suggest that AFT has a clear positive impact on running performance in main road events.

Muscle endurance: Is bipedalism the cause?

One may ask if the transition to bipedalism from the condition of quadrupedalism, which occurred about 7 million years ago, has been the cause or consequence of a series of fundamental physiological muscular aspects including the cost of locomotion, a crucial determinant of endurance, which has been found to be significantly lower in humans than in apes. This issue strictly links to unsolved issues regarding the significance of several muscular structural and functional adaptations, classically attributed to bipedalism and its associated locomotions, which we cannot simply attribute to the acquisition of the upright position and which may directly or indirectly contribute to the observed changes in muscle energetics that make the modern human an exceptional endurance walker and runner compared to quadrupedals.

Tag-based estimates of bottlenose dolphin swimming behavior and energetics

Current estimates of marine mammal hydrodynamic forces tend to be made using camera-based kinematic data for a limited number of fluke strokes during a prescribed swimming task. In contrast, biologging tag data yield kinematic measurements from thousands of strokes, enabling new insights into swimming behavior and mechanics. However, there have been limited tag-based estimates of mechanical work and power. In this work, we investigated bottlenose dolphin (Tursiops truncatus) swimming behavior using tag-measured kinematics and a hydrodynamic model to estimate propulsive power, work and cost of transport. Movement data were collected from six animals during prescribed straight-line swimming trials to investigate swimming mechanics over a range of sustained speeds (1.9-6.1 m s-1). Propulsive power ranged from 66 W to 3.8 kW over 282 total trials. During the lap trials, the dolphins swam at depths that mitigated wave drag, reducing overall drag throughout these mid- to high-speed tasks. Data were also collected from four individuals during undirected daytime (08:30-18:00 h) swimming to examine how self-selected movement strategies are used to modulate energetic efficiency and effort. Overall, self-selected swimming speeds (individual means ranging from 1.0 to 1.96 m s-1) tended to minimize cost of transport, and were on the lower range of animal-preferred speeds reported in literature. The results indicate that these dolphins moderate propulsive effort and efficiency through a combination of speed and depth regulation. This work provides new insights into dolphin swimming behavior in both prescribed tasks and self-selected swimming, and presents a path forward for continuous estimates of mechanical work and power from wild animals.

Locomotor energetics in the Indonesian blue-tongued skink (Tiliqua gigas) with implications for the cost of belly-dragging in early tetrapods

During the last decade, biomechanical and kinematic studies have suggested that a belly-dragging gait may have represented a critical locomotor stage during tetrapod evolution. This form of locomotion is hypothesized to facilitate animals to move on land with relatively weaker pectoral muscles. The Indonesian blue-tongued skink (Tiliqua gigas) is known for its belly-dragging locomotion and is thought to employ many of the same spatiotemporal gait characteristics of stem tetrapods. Conversely, the savannah monitor (Varanus exanthematicus) employs a raised quadrupedal gait. Thus, differences in the energetic efficiency of locomotion between these taxa may elucidate the role of energetic optimization in driving gait shifts in early tetrapods. Five Tiliqua and four Varanus were custom-fitted for 3D printed helmets that, combined with a Field Metabolic System, were used to collect open-flow respirometry data including O₂ consumption, CO₂ production, water vapor pressure, barometric pressure, room temperature, and airflow rates. Energetic data were collected for each species at rest, and when walking at three different speeds. Energetic consumption in each taxon increased at greater speeds. On a per-stride basis, energetic costs appear similar between taxa. However, significant differences were observed interspecifically in terms of net cost of transport. Overall, energy expenditure was ~20% higher in Tiliqua at equivalent speeds, suggesting that belly-dragging does impart a tangible energetic cost during quadrupedal locomotion. This cost, coupled with the other practical constraints of belly-dragging (e.g., restricting top-end speed and reducing maneuverability in complex terrains) may have contributed to the adoption of upright quadrupedal walking throughout tetrapod locomotor evolution.

Stride frequency or length? A phylogenetic approach to understand how animals regulate locomotor speed

Speed regulation in animals involves stride frequency and stride length. While the relationship between these variables has been well documented, it remains unresolved whether animals primarily modify stride frequency or stride length to increase speed. In this study, we explored the interrelationships between these three variables across a sample of 103 tetrapods and assessed whether speed regulation strategy is influenced by mechanical, allometric, phylogenetic or ecological factors. We observed that crouched terrestrial species tend to regulate speed through stride frequency. Such a strategy is energetically costly, but results in greater locomotor maneuverability and greater stability. In contrast, regulating speed through stride length is closely tied to larger arboreal animals with relatively extended limbs. Such movements reduce substrate oscillations on thin arboreal supports and/or helps to reduce swing phase costs. The slope of speed on frequency is lower in small crouched animals than in large-bodied erect species. As a result, substantially more rapid limb movements are matched with only small speed increases in crouched, small-bodied animals. Furthermore, the slope of speed on stride length was inversely proportional to body mass. As such, small changes in stride length can result in relatively rapid speed increases for small-bodied species. These results are somewhat counterintuitive, in that larger species, which have longer limbs and take longer strides, do not appear to gain as much speed increase out of lengthening their stride. Conversely, smaller species that cycle their limbs rapidly do not gain as much speed out of increasing stride frequency as do larger species.

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.

The Influence of Snow Properties on Speed and Gait Choice in the Svalbard Rock Ptarmigan (Lagopus muta hyperborea)

Substrate supportiveness is linked to the metabolic cost of locomotion, as it influences the depth to which the foot of a moving animal will sink. As track depth increases, animals typically reduce their speed to minimize any potential energetic imbalance. Here, we examine how self-selected speed in the Svalbard rock ptarmigan is affected by snow supportiveness and subsequent footprint depth measured using thin-blade penetrometry and 3D photogrammetry, respectively. Our findings indicate that snow supportiveness and footprint depth are poor predictors of speed (r 2 = 0.149) and stride length (r 2 = 0.106). The ptarmigan in our study rarely sunk to depths beyond the intertarsal joint, regardless of the speed, suggesting that at this relatively shallow depth any increased cost is manageable. 3D reconstructions also indicate that the ptarmigan may exploit the compressive nature of snow to generate thrust during stance, as a trend toward greater foot rotations in deeper footprints was found. It remains unclear whether the Svalbard ptarmigan are deliberately avoiding unsupportive snowy substrates. However, if they do, these results would be consistent with the idea that animals should choose routes that minimize energy costs of locomotion.

Recent Improvements in Marathon Run Times Are Likely Technological, Not Physiological

Every women’s and men’s world records from 5 km to the marathon has been broken since the introduction of carbon fibre plate (CFP) shoes in 2016. This step-wise increase in performance coincides with recent advancements in shoe technology that increase the elastic properties of the shoe thereby reducing the energy cost of running. The latest CFP shoes are acknowledged to increase running economy by more than 4%, corresponding to a greater than 2% improvement in performance/run time. The recently modified rules governing competition shoes for elite athletes, announced by World Athletics, that includes sole thickness must not exceed 40 mm and must not contain more than one rigid embedded plate, appear contrary to the true essence and credibility of sport as access to this performance-defining technology becomes the primary differentiator of sporting performance in elite athletes. This is a particular problem in sports such as athletics where the primary sponsor of the athlete is very often a footwear manufacturing company. The postponement of the 2020 Summer Olympics provides a unique opportunity for reflection by the world of sport and time to commission an independent review to evaluate the impact of technology on the integrity of sporting competition. A potential solution to solve this issue can involve the reduction of the stack height of a shoe to 20 mm. This simple and practical solution would prevent shoe technology from having too large an impact on the energy cost of running and, therefore, determining the performance outcome.

Mechanics of heel-strike plantigrady in African apes

The initiation of a walking step with a heel strike is a defining characteristic of humans and great apes but is rarely found in other mammals. Despite the considerable importance of heel strike to an understanding of human locomotor evolution, no one has explicitly tested the fundamental mechanical question of why great apes use a heel strike. In this report, we test two hypotheses (1) that heel strike is a function of hip protraction and/or knee extension and (2) that short-legged apes with a midfoot that dorsiflexes at heel lift and long digits for whom digitigrady is not an option use heel-strike plantigrady. This strategy increases hip translation while potentially moderating the cost of redirecting the center of mass ('collisional costs') during stance via rollover along the full foot from the heel to toes. We quantified hind limb kinematics and relative hip translation in ten species of primates, including lemurs, terrestrial and arboreal monkeys, chimpanzees, and gorillas. Chimpanzees and gorillas walked with relatively extended knees but only with moderately protracted hips or hind limbs, partially rejecting the first hypothesis. Nonetheless, chimpanzees attained relative hip translations comparable with those of digitigrade primates. Heel-strike plantigrady may be a natural result of a need for increased hip translations when forelimbs are relatively long and digitigrady is morphologically restricted. In addition, foot rollover from the heel to toe in large, short-legged apes may reduce energetic costs of redirecting the center of mass at the step-to-step transition as it appears to do in humans. Heel strike appears to have been an important mechanism for increasing hip translation, and possibly reducing energetic costs, in early hominins and was fundamental to the evolution of the modern human foot and human bipedalism.

Connecting the legs with a spring improves human running economy

Human running is inefficient. For every 10 calories burned, less than 1 is needed to maintain a constant forward velocity - the remaining energy is, in a sense, wasted. The majority of this wasted energy is expended to support the bodyweight and redirect the center of mass during the stance phase of gait. An order of magnitude less energy is expended to brake and accelerate the swinging leg. Accordingly, most devices designed to increase running efficiency have targeted the costlier stance phase of gait. An alternative approach is seen in nature: spring-like tissues in some animals and humans are believed to assist leg swing. While it has been assumed that such a spring simply offloads the muscles that swing the legs, thus saving energy, this mechanism has not been experimentally investigated. Here, we show that a spring, or 'exotendon', connecting the legs of a human reduces the energy required for running by 6.4±2.8%, and does so through a complex mechanism that produces savings beyond those associated with leg swing. The exotendon applies assistive forces to the swinging legs, increasing the energy optimal stride frequency. Runners then adopt this frequency, taking faster and shorter strides, and reduce the joint mechanical work to redirect their center of mass. Our study shows how a simple spring improves running economy through a complex interaction between the changing dynamics of the body and the adaptive strategies of the runner, highlighting the importance of considering each when designing systems that couple human and machine.

Terrestrial locomotion energy costs vary considerably between species: no evidence that this is explained by rate of leg force production or ecology

Inter-specifically, relative energy costs of terrestrial transport vary several-fold. Many pair-wise differences of locomotor costs between similarly-sized species are considerable, and are yet to be explained by morphology or gait kinematics. Foot contact time, a proxy for rate of force production, is a strong predictor of locomotor energy costs across species of different size and might predict variability between similarly sized species. We tested for a relationship between foot contact time and metabolic rate during locomotion from published data. We investigated the phylogenetic correlation between energy expenditure rate and foot contact time, conditioned on fixed effects of mass and speed. Foot contact time does not explain variance in rate of energy expenditure during locomotion, once speed and body size are accounted for. Thus, perhaps surprisingly, inter-specific differences in the mass-independent net cost of terrestrial transport (NCOT) are not explained by rates of force production. We also tested for relationships between locomotor energy costs and eco-physiological variables. NCOT did not relate to any of the tested eco-physiological variables; we thus conclude either that interspecific differences in transport cost have no influence on macroecological and macrophysiological patterns, or that NCOT is a poor indicator of animal energy expenditure beyond the treadmill.

A suspensory way of life: Integrating locomotion, postures, limb movements, and forces in two-toed sloths Choloepus didactylus (Megalonychidae, Folivora, Pilosa)

Over the last decade, we have learned much about the anatomy, evolutionary history, and biomechanics of the extant sloths. However, most of this study has involved studying sloths in controlled conditions, and few studies have explored how these animals are behaving in a naturalistic setting. In this study, we integrate positional activities in naturalistic conditions with kinematic and kinetic observations collected on a simulated runway to best capture the biomechanical behavior of Linnaeus's two-toed sloths. We confirm that the dominant positional behaviors consist of hanging below the support using a combination of forelimbs and hindlimbs, and walking quadrupedally below the branches. The majority of these behaviors occur on horizontal substrates that are approximately 5-10 cm in diameter. The kinematics of suspensory walking observed both in the naturalistic settings and on simulated arboreal runways are dominated by movement of the proximal limb elements, while distal limb elements tend to show little excursion. Joint kinematics are similar between the naturalistic setting and the simulated runway, but movements of the shoulder and hip tend to be exaggerated while moving in simulated conditions. Kinetic patterns of the two-toed sloth can be explained almost entirely by considering them as an inverted linked strut. However, medially directed forces toward the substrate were more frequent than expected in the forelimb, which may help sloths maintain a better "grip" on the substrate. This study serves as a model of how to gain a comprehensive understanding of the functional-adaptive profile of a particular species.

Long term consistency and location specificity of equine gluteus medius muscle activity during locomotion on the treadmill

Background

The equine m. gluteus medius (GM) is the largest muscle of the horse, its main movement function is the extension of the hip joint. The objective of the present study was to measure equine GM activity in three adjacent locations on GM during walk and trot on a treadmill, in order to document potential differences. Fourteen Haflinger mares were measured using surface electromyography and kinematic markers to identify the motion cycles on three occasions over 16 weeks. The electrodes were placed on left and right gluteus medius muscle over the middle of its widest part and 5 cm lateral and medial of it. For data processing, electrical activity was normalised to its maximum value and timing was normalised to the motion cycle. A Gaussian distribution approach was used to determine up to 10 modes of focussed activity, and results were analysed separately for stance and swing phase of the ipsilateral hindlimb.

Results

Fair reliability was found for mean mode values (Cronbach’s alpha = 0.66) and good reliability was found for mean mode locations (Cronbach’s alpha = 0.71) over the three data collection days. The magnitude of muscle activity identified as mean mode value was much larger at trot than at walk, and mean mode value was significantly different between stance phases of walk and trot for all electrode positions (p < 0.01). The pattern of muscle activity identified as mean mode location was significantly different for walk and trot at all electrode positions, both during stance and swing phases (p < 0.001). This indicates the different timing pattern between the gaits. Results of the three electrode positions on the same muscle during each gait were not significantly different when comparing the same measurement.

Conclusions

The middle of the equine GM does not show any indication of functional differentiation during walk and trot on a treadmill; this might be due to lack of segmentation as such, or due to lack of need for segmented use for these very basic main tasks of the muscle. The reliability of the sEMG measurements over several weeks was fair to good, an indication for the robustness of the methodology.

Collision-based energetic comparison of rolling and hopping over obstacles

Locomotion of machines and robots operating in rough terrain is strongly influenced by the mechanics of the ground-machine interactions. A rolling wheel in terrain with obstacles is subject to collisional energy losses, which is governed by mechanics comparable to hopping or walking locomotion. Here we investigate the energetic cost associated with overcoming an obstacle for rolling and hopping locomotion, using a simple mechanics model. The model considers collision-based interactions with the ground and the obstacle, without frictional losses, and we quantify, analyse, and compare the sources of energetic costs for three locomotion strategies. Our results show that the energetic advantages of the locomotion strategies are uniquely defined given the moment of inertia and the Froude number associated with the system. We find that hopping outperforms rolling at larger Froude numbers and vice versa. The analysis is further extended for a comparative study with animals. By applying size and inertial properties through an allometric scaling law of hopping and trotting animals to our models, we found that the conditions at which hopping becomes energetically advantageous to rolling roughly corresponds to animals' preferred gait transition speeds. The energetic collision losses as predicted by the model are largely verified experimentally.

Comparison of spatiotemporal gait characteristics between vertical climbing and horizontal walking in primates

During quadrupedal walking, most primates utilize diagonal sequence diagonal couplet gaits, large limb excursions, and hindlimb-biased limb-loading. These gait characteristics are thought to be basal to the Order, but the selective pressure underlying these gait changes remains unknown. Some researchers have examined these characteristics during vertical climbing and propose that primate quadrupedal gait characteristics may have arisen due to the mechanical challenges of moving on vertical supports. Unfortunately, these studies are usually limited in scope and do not account for varying strategies based on body size or phylogeny. Here, we test the hypothesis that the spatiotemporal gait characteristics that are used during horizontal walking in primates are also present during vertical climbing irrespective of body size and phylogeny. We examined footfall patterns, diagonality, speed, and stride length in eight species of primates across a range of body masses. We found that during vertical climbing primates slow down, keep more limbs in contact with the substrate at any one time, and increase the frequency of lateral sequence gaits compared to horizontal walking. Taken together these characteristics are assumed to increase stability during locomotion. Phylogenetic relatedness and body size differences have little influence on locomotor patterns observed across species. These data reject the idea that the suite of spatiotemporal gait features observed in primates during horizontal walking are in some way evolutionarily linked to selective pressures associated with mechanical requirements of vertical climbing. These results also highlight the importance of behavioral flexibility for negotiating the challenges of locomotion in an arboreal environment.

Pelvic Breadth and Locomotor Kinematics in Human Evolution

A broad pelvis is characteristic of most, if not all, pre-modern hominins. In at least some early australopithecines, most notably the female Australopithecus afarensis specimen known as "Lucy," it is very broad and coupled with very short lower limbs. In 1991, Rak suggested that Lucy's pelvic anatomy improved locomotor efficiency by increasing stride length through rotation of the wide pelvis in the axial plane. Compared to lengthening strides by increasing flexion and extension at the hips, this mechanism could avoid potentially costly excessive vertical oscillations of the body's center of mass (COM). Here, we test this hypothesis. We examined 3D kinematics of walking at various speeds in 26 adult subjects to address the following questions: Do individuals with wider pelves take longer strides, and do they use a smaller degree of hip flexion and extension? Is pelvic rotation greater in individuals with shorter legs, and those with narrower pelves? Our results support Rak's hypothesis. Subjects with wider pelves do take longer strides for a given velocity, and for a given stride length they flex and extend their hips less, suggesting a smoother pathway of the COM. Individuals with shorter legs do use more pelvic rotation when walking, but pelvic breadth was not related to pelvic rotation. These results suggest that a broad pelvis could benefit any bipedal hominin, but especially a short-legged australopithecine such as Lucy, by improving locomotor efficiency, particularly when carrying an infant or traveling in a foraging group with individuals of varying sizes. Anat Rec, 300:739-751, 2017. © 2017 Wiley Periodicals, Inc.

Energetic cost of walking in fossil hominins

OBJECTIVE

Many biomechanical studies consistently show that a broader pelvis increases the reaction forces and bending moments across the femoral shaft, increasing the energetic costs of unloaded locomotion. However, a biomechanical model does not provide the real amount of metabolic energy expended in walking. The aim of this study is to test the influence of pelvis breadth on locomotion cost and to evaluate the locomotion efficiency of extinct Pleistocene hominins.

MATERIAL AND METHODS

The current study measures in vivo the influence of pelvis width on the caloric cost of locomotion, integrating anthropometry, body composition and indirect calorimetry protocols in a sample of 46 subjects of both sexes.

RESULTS

We show that a broader false pelvis is substantially more efficient for locomotion than a narrower one and that the influence of false pelvis width on the energetic cost is similar to the influence of leg length. Two models integrating body mass, femur length and bi-iliac breadth are used to estimate the net and gross energetic costs of locomotion in a number of extinct hominins. The results presented here show that the locomotion of Homo was not energetically more efficient than that of Australopithecus and that the locomotion of extinct Homo species was not less efficient than that of modern Homo sapiens.

DISCUSSION

The changes in the anatomy of the pelvis and lower limb observed with the appearance of Homo ergaster probably did not fully offset the increased expenditure resulting from a larger body mass. Moreover, the narrow pelvis in modern humans does not contribute to greater efficiency of locomotion.

The role of plantigrady and heel-strike in the mechanics and energetics of human walking with implications for the evolution of the human foot

Human bipedal locomotion is characterized by a habitual heel-strike (HS) plantigrade gait, yet the significance of walking foot-posture is not well understood. To date, researchers have not fully investigated the costs of non-heel-strike (NHS) walking. Therefore, we examined walking speed, walk-to-run transition speed, estimated locomotor costs (lower limb muscle volume activated during walking), impact transient (rapid increase in ground force at touchdown) and effective limb length (ELL) in subjects (n=14) who walked at self-selected speeds using HS and NHS gaits. HS walking increases ELL compared with NHS walking since the center of pressure translates anteriorly from heel touchdown to toe-off. NHS gaits led to decreased absolute walking speeds (P=0.012) and walk-to-run transition speeds (P=0.0025), and increased estimated locomotor energy costs (P<0.0001) compared with HS gaits. These differences lost significance after using the dynamic similarity hypothesis to account for the effects of foot landing posture on ELL. Thus, reduced locomotor costs and increased maximum walking speeds in HS gaits are linked to the increased ELL compared with NHS gaits. However, HS walking significantly increases impact transient values at all speeds (P<0.0001). These trade-offs may be key to understanding the functional benefits of HS walking. Given the current debate over the locomotor mechanics of early hominins and the range of foot landing postures used by nonhuman apes, we suggest the consistent use of HS gaits provides key locomotor advantages to striding bipeds and may have appeared early in hominin evolution.

Metabolic cost of human hopping

To interpret the movement strategies employed in locomotion, it is necessary to understand the source of metabolic cost. Muscles must consume metabolic energy to do work, but also must consume energy to generate force. The energy lost during steady locomotion and, hence, the amount of mechanical work muscles need to perform to replace it can be reduced and, in theory, even eliminated by elastically storing and returning some portion of this energy via the tendons. However, even if muscles do not need to perform any mechanical work, they still must generate sufficient force to tension tendons and support body weight. This study shows that the metabolic cost per hop of human hopping can largely be explained by the cost of producing force over the duration of a hop. Metabolic cost determined via oxygen consumption is compared with theoretical predictions made using a number of different cost functions that include terms for average muscle work, force, force rate and impulse (time integral of muscle force). Muscle impulse alone predicts metabolic cost per hop as well as more complex functions that include terms for muscle work, force and force rate, and explains a large portion (92%) of the variation in metabolic cost per hop. This is equivalent to 1/effective mechanical advantage, explaining a large portion (66%) of the variation in metabolic cost per time per unit body weight. This result contrasts with studies that suggest that muscle force rate or muscle force rate per time determines the metabolic cost per time of force production in other bouncing gaits such as running.

Arboreal Locomotion in Eurasian Harvest Mice Micromys Minutus (Rodentia: Muridae): The Gaits of Small Mammals

Body size imposes significant constraints on arboreal locomotion. Despite the wealth of research in larger arboreal mammals, there is a lack of data on arboreal gaits of small mammals. In this context, the present study explores arboreal locomotion in one of the smallest rodents, the Eurasian harvest mice Micromys minutus (∼10 g). We examined gait metrics (i.e., diagonality, duty factor [DF], DF index, velocity, stride length, and stride frequency) of six adult male mice on simulated arboreal substrates of different sizes (2, 5, 10, and 25 mm) and inclinations (0⁰ and 45⁰ ). Micromys minutus employed slow, lateral sequence symmetrical gaits on the smaller substrates, which shifted to progressively faster symmetrical gaits of higher diagonality on larger substrates. Both ascents and descents were associated with a higher diagonality, and ascents with a higher DF index compared to horizontal locomotion, underscoring the role of the grasping hind feet. Velocity increase was brought about primarily by an increase in stride frequency, a pattern often encountered in other small mammals, with a secondary and significant contribution of stride length. These findings indicate that, except for velocity and the way it is regulated, there are no significant differences in gait metrics between larger and smaller arboreal mammals. Moreover, the locomotor adaptations of Eurasian harvest mice represent behavioral mechanisms that promote stable, safe, and continuous navigation along slender substrates and ultimately contribute to the successful exploitation of the arboreal milieu.

Gait changes in a line of mice artificially selected for longer limbs

In legged terrestrial locomotion, the duration of stance phase, i.e., when limbs are in contact with the substrate, is positively correlated with limb length, and negatively correlated with the metabolic cost of transport. These relationships are well documented at the interspecific level, across a broad range of body sizes and travel speeds. However, such relationships are harder to evaluate within species (i.e., where natural selection operates), largely for practical reasons, including low population variance in limb length, and the presence of confounding factors such as body mass, or training. Here, we compared spatiotemporal kinematics of gait in Longshanks, a long-legged mouse line created through artificial selection, and in random-bred, mass-matched Control mice raised under identical conditions. We used a gait treadmill to test the hypothesis that Longshanks have longer stance phases and stride lengths, and decreased stride frequencies in both fore- and hind limbs, compared with Controls. Our results indicate that gait differs significantly between the two groups. Specifically, and as hypothesized, stance duration and stride length are 8–10% greater in Longshanks, while stride frequency is 8% lower than in Controls. However, there was no difference in the touch-down timing and sequence of the paws between the two lines. Taken together, these data suggest that, for a given speed, Longshanks mice take significantly fewer, longer steps to cover the same distance or running time compared to Controls, with important implications for other measures of variation among individuals in whole-organism performance, such as the metabolic cost of transport.

The apparently contradictory energetics of hopping and running: the counter-intuitive effect of constraints resolves the paradox

Metabolic rate appears to increase with the rate of force application for running. Leg function during ground contact is similar in hopping and running, so one might expect that this relationship would hold for hopping as well. Surprisingly, metabolic rate appeared to decrease with increasing force rate for hopping. However, this paradox is the result of comparing different cross-sections of the metabolic cost landscapes for hopping and running. The apparent relationship between metabolic rate and force rate observed in treadmill running is likely not a fundamental characteristic of muscle physiology, but a result of runners responding to speed constraints, i.e. runners selecting step frequencies that minimize metabolic cost per distance for a series of treadmill-specified speeds. Evaluating hopping metabolic rate over a narrow range of hop frequencies similar to that selected by treadmill runners yields energy use trends similar to those of running.

Physiological, aerodynamic and geometric constraints of flapping account for bird gaits, and bounding and flap-gliding flight strategies

Aerodynamically economical flight is steady and level. The high-amplitude flapping and bounding flight style of many small birds departs considerably from any aerodynamic or purely mechanical optimum. Further, many large birds adopt a flap-glide flight style in cruising flight which is not consistent with purely aerodynamic economy. Here, an account is made for such strategies by noting a well-described, general, physiological cost parameter of muscle: the cost of activation. Small birds, with brief downstrokes, experience disproportionately high costs due to muscle activation for power during contraction as opposed to work. Bounding flight may be an adaptation to modulate mean aerodynamic force production in response to (1) physiological pressure to extend the duration of downstroke to reduce power demands during contraction; (2) the prevention of a low-speed downstroke due to the geometric constraints of producing thrust; (3) an aerodynamic cost to flapping with very low lift coefficients. In contrast, flap-gliding birds, which tend to be larger, adopt a strategy that reduces the physiological cost of work due both to activation and contraction efficiency. Flap-gliding allows, despite constraints to modulation of aerodynamic force lever-arm, (1) adoption of moderately large wing-stroke amplitudes to achieve suitable muscle strains, thereby reducing the activation costs for work; (2) reasonably quick downstrokes, enabling muscle contraction at efficient velocities, while being (3) prevented from very slow weight-supporting upstrokes due to the cost of performing 'negative' muscle work.

Children and adults minimise activated muscle volume by selecting gait parameters that balance gross mechanical power and work demands

Terrestrial locomotion on legs is energetically expensive. Compared with cycling, or with locomotion in swimming or flying animals, walking and running are highly uneconomical. Legged gaits that minimise mechanical work have previously been identified and broadly match walking and running at appropriate speeds. Furthermore, the ‘cost of muscle force’ approaches are effective in relating locomotion kinetics to metabolic cost. However, few accounts have been made for why animals deviate from either work-minimising or muscle-force-minimising strategies. Also, there is no current mechanistic account for the scaling of locomotion kinetics with animal size and speed. Here, we report measurements of ground reaction forces in walking children and adult humans, and their stance durations during running. We find that many aspects of gait kinetics and kinematics scale with speed and size in a manner that is consistent with minimising muscle activation required for the more demanding between mechanical work and power: spreading the duration of muscle action reduces activation requirements for power, at the cost of greater work demands. Mechanical work is relatively more demanding for larger bipeds – adult humans – accounting for their symmetrical M-shaped vertical force traces in walking, and relatively brief stance durations in running compared with smaller bipeds – children. The gaits of small children, and the greater deviation of their mechanics from work-minimising strategies, may be understood as appropriate for their scale, not merely as immature, incompletely developed and energetically sub-optimal versions of adult gaits.

Highlighted Article: The gross mechanics of walking and running children and adults support a new model for the costs dominating level terrestrial locomotion – muscle activation for mechanical work or power.

The muscle-mechanical compromise framework: Implications for the scaling of gait and posture

Many aspects of animal and human gait and posture cannot be predicted from purely mechanical work minimization or entirely based on optimizing muscle efficiency. Here, the Muscle-Mechanical Compromise Framework is introduced as a conceptual paradigm for considering the interactions and compromises between these two objectives. Current assumptions in implementing the Framework are presented. Implications of the compromise are discussed and related to the scaling of running mechanics and animal posture.

Morphology and motion: hindlimb proportions and swing phase kinematics in terrestrially locomoting charadriiform birds

Differing limb proportions in terms of length and mass, as well as differences in mass being concentrated proximally or distally, influence the limb's moment of inertia (MOI), which represents its resistance to being swung. Limb morphology—including limb segment proportions—thus likely has direct relevance for the metabolic cost of swinging the limb during locomotion. However, it remains largely unexplored how differences in limb proportions influence limb kinematics during swing phase. To test whether differences in limb proportions are associated with differences in swing phase kinematics, we collected hindlimb kinematic data from three species of charadriiform birds differing widely in their hindlimb proportions: lapwings, oystercatchers, and avocets. Using these three species, we tested for differences in maximum joint flexion, maximum joint extension, and range of motion (RoM), in addition to differences in maximum segment angular velocity and excursion. We found that the taxa with greater limb MOI—oystercatchers and avocets—flex their limbs more than lapwings. However, we found no consistent differences in joint extension and RoM among species. Likewise, we found no consistent differences in limb segment angular velocity and excursion, indicating that differences in limb inertia in these three avian species do not necessarily underlie the rate or extent of limb segment movements. The observed increased limb flexion among these taxa with distally heavy limbs resulted in reduced MOI of the limb when compared to a neutral pose. A trade-off between exerting force to actively flex the limb and potential savings by a reduction of MOI is skewed towards reducing the limb's MOI due to MOI being in part a function of the radius of gyration squared. Increased limb flexion likely is a means to lower the cost of swinging the limbs.

Human locomotion and heat loss: an evolutionary perspective

Humans are unique in many respects including being furless, striding bipeds that excel at walking and running long distances in hot conditions. This review summarizes what we do and do not know about the evolution of these characteristics, and how they are related. Although many details remain poorly known, the first hominins (species more closely related to humans than to chimpanzees) apparently diverged from the chimpanzee lineage because of selection for bipedal walking, probably because it improved their ability to forage efficiently. However, because bipedal hominins are necessarily slow runners, early hominins in open habitats likely benefited from improved abilities to dump heat in order to forage safely during times of peak heat when predators were unable to hunt them. Endurance running capabilities evolved later, probably as adaptations for scavenging and then hunting. If so, then there would have been strong selection for heat-loss mechanisms, especially sweating, to persistence hunt, in which hunters combine endurance running and tracking to drive their prey into hyperthermia. As modern humans dispersed into a wide range of habitats over the last few hundred thousand years, recent selection has helped populations cope better with a broader range of locomotor and thermoregulatory challenges, but all humans remain essentially adapted for long distance locomotion rather than speed, and to dump rather than retain heat.

The human iliotibial band is specialized for elastic energy storage compared with the chimp fascia lata

This study examines whether the human iliotibial band (ITB) is specialized for elastic energy storage relative to the chimpanzee fascia lata (FL). To quantify the energy storage potential of these structures, we created computer models of human and chimpanzee lower limbs based on detailed anatomical dissections. We characterized the geometry and force-length properties of the FL, tensor fascia lata (TFL) and gluteus maximus (GMax) in four chimpanzee cadavers based on measurements of muscle architecture and moment arms about the hip and knee. We used the chimp model to estimate the forces and corresponding strains in the chimp FL during bipedal walking, and compared these data with analogous estimates from a model of the human ITB, accounting for differences in body mass and lower extremity posture. We estimate that the human ITB stores 15- to 20-times more elastic energy per unit body mass and stride than the chimp FL during bipedal walking. Because chimps walk with persistent hip flexion, the TFL and portions of GMax that insert on the FL undergo smaller excursions (origin to insertion) than muscles that insert on the human ITB. Also, because a smaller fraction of GMax inserts on the chimp FL than on the human ITB, and thus its mass-normalized physiological cross-sectional area is about three times less in chimps, the chimp FL probably transmits smaller muscle forces. These data provide new evidence that the human ITB is anatomically derived compared with the chimp FL and potentially contributes to locomotor economy during bipedal locomotion.

Intraspecific scaling of the minimum metabolic cost of transport in leghorn chickens (Gallus gallus domesticus): links with limb kinematics, morphometrics and posture

The minimum metabolic cost of transport (CoT_min; J kg⁻¹ m⁻¹) scales negatively with increasing body mass (∝M_b^−1/3) across species from a wide range of taxa associated with marked differences in body plan. At the intraspecific level, or between closely related species, however, CoT_min does not always scale with M_b. Similarity in physiology, dynamics of movement, skeletal geometry and posture between closely related individuals is thought to be responsible for this phenomenon, despite the fact that energetic, kinematic and morphometric data are rarely collected together. We examined the relationship between these integrated components of locomotion in leghorn chickens (Gallus gallus domesticus) selectively bred for large and bantam (miniature) varieties. Interspecific allometry predicts a CoT_min ∼16% greater in bantams compared with the larger variety. However, despite 38% and 23% differences in M_b and leg length, respectively, the two varieties shared an identical walking CoT_min, independent of speed and equal to the allometric prediction derived from interspecific data for the larger variety. Furthermore, the two varieties moved with dynamic similarity and shared geometrically similar appendicular and axial skeletons. Hip height, however, did not scale geometrically and the smaller variety had more erect limbs, contrary to interspecific scaling trends. The lower than predicted CoT_min in bantams for their M_b was associated with both the more erect posture and a lower cost per stride (J kg⁻¹ stride⁻¹). Therefore, our findings are consistent with the notion that a more erect limb is associated with a lower CoT_min and with the previous assumption that similarity in skeletal shape, inherently linked to walking dynamics, is associated with similarity in CoT_min.

Summary: Chickens with differing body size and posture but similar skeletal shape show no difference in the cost of transport.

Effects of lower limb length and body proportions on the energy cost of overground walking in older persons

Background. Although walking has been extensively investigated in its biomechanical and physiological aspects, little is known on whether lower limb length and body proportions affect the energy cost of overground walking in older persons. Methods. We enrolled 50 men and 12 women aged 65 years and over, mean 69.1 ± SD 5.4, who at the end of their cardiac rehabilitation program performed the six-minute walk test while wearing a portable device for direct calorimetry and who walked a distance comparable to that of nondisabled community-dwelling older persons. Results. In the multivariable regression model (F = 12.75, P < 0.001, adjusted R² = 0.278) the energy cost of overground walking, expressed as the net energy expenditure, in kg⁻¹ sec⁻¹, needed to provide own body mass with 1 joule kinetic energy, was inversely related to lower limb length and directly related to lower limb length to height ratio (β ± SE(β) = −3.72∗10⁻³ ± 0.74∗10⁻³, P < 0.001, and 6.61∗10⁻³ ± 2.14∗10⁻³, P = 0.003, resp.). Ancillary analyses also showed that, altogether, 1 cm increase in lower limb length reduced the energy cost of overground walking by 2.57% (95%CI 2.35–2.79). Conclusions. Lower limb length and body proportions actually affect the energy cost of overground walking in older persons.

Partitioning the metabolic cost of human running: a task-by-task approach

Compared with other species, humans can be very tractable and thus an ideal "model system" for investigating the metabolic cost of locomotion. Here, we review the biomechanical basis for the metabolic cost of running. Running has been historically modeled as a simple spring-mass system whereby the leg acts as a linear spring, storing, and returning elastic potential energy during stance. However, if running can be modeled as a simple spring-mass system with the underlying assumption of perfect elastic energy storage and return, why does running incur a metabolic cost at all? In 1980, Taylor et al. proposed the "cost of generating force" hypothesis, which was based on the idea that elastic structures allow the muscles to transform metabolic energy into force, and not necessarily mechanical work. In 1990, Kram and Taylor then provided a more explicit and quantitative explanation by demonstrating that the rate of metabolic energy consumption is proportional to body weight and inversely proportional to the time of foot-ground contact for a variety of animals ranging in size and running speed. With a focus on humans, Kram and his colleagues then adopted a task-by-task approach and initially found that the metabolic cost of running could be "individually" partitioned into body weight support (74%), propulsion (37%), and leg-swing (20%). Summing all these biomechanical tasks leads to a paradoxical overestimation of 131%. To further elucidate the possible interactions between these tasks, later studies quantified the reductions in metabolic cost in response to synergistic combinations of body weight support, aiding horizontal forces, and leg-swing-assist forces. This synergistic approach revealed that the interactive nature of body weight support and forward propulsion comprises ∼80% of the net metabolic cost of running. The task of leg-swing at most comprises ∼7% of the net metabolic cost of running and is independent of body weight support and forward propulsion. In our recent experiments, we have continued to refine this task-by-task approach, demonstrating that maintaining lateral balance comprises only 2% of the net metabolic cost of running. In contrast, arm-swing reduces the cost by ∼3%, indicating a net metabolic benefit. Thus, by considering the synergistic nature of body weight support and forward propulsion, as well as the tasks of leg-swing and lateral balance, we can account for 89% of the net metabolic cost of human running.

Influence of lower limb configuration on walking cost in Late Pleistocene humans

It has been proposed that Neandertals had about 30% higher gross cost of transport than anatomically modern humans (AMH) and that such difference implies higher daily energy demands and reduced foraging ranges in Neandertals. Thus, reduced walking economy could be among the factors contributing to the Neandertals' loss in competition with their anatomically modern successors. Previously, Neandertal walking cost had been estimated from just two parameters and based upon a pooled-sex sample. In the present study, we estimate sex-specific walking cost of Neandertals using a model accounting for body mass, lower limb length, lower limb proportions, and other features of lower limb configuration. Our results suggest that Neandertals needed more energy to walk a given distance than did AMH but the difference was less than half of that previously estimated in males and even far less pronounced in females. In contrast, comparison of the estimated walking cost adjusted to body mass indicates that Neandertals spent less energy per kilogram of body mass than AMH thanks to their lower limb configuration, males having 1-5% lower and females 1-3% lower mass-specific net cost of transport than AMH of the same sex. The primary cause of high cost of transport in Neandertal males is thus their great body mass, possibly a consequence of adaptation to cold, which was not fully offset by their cost-moderating lower limb configuration. The estimated differences in absolute energy spent for locomotion between Neandertal and AMH males would account for about 1% of previously estimated daily energy expenditure of Neandertal or AMH males.

Scale effects between body size and limb design in quadrupedal mammals

Recently the metabolic cost of swinging the limbs has been found to be much greater than previously thought, raising the possibility that limb rotational inertia influences the energetics of locomotion. Larger mammals have a lower mass-specific cost of transport than smaller mammals. The scaling of the mass-specific cost of transport is partly explained by decreasing stride frequency with increasing body size; however, it is unknown if limb rotational inertia also influences the mass-specific cost of transport. Limb length and inertial properties--limb mass, center of mass (COM) position, moment of inertia, radius of gyration, and natural frequency--were measured in 44 species of terrestrial mammals, spanning eight taxonomic orders. Limb length increases disproportionately with body mass via positive allometry (length ∝ body mass(0.40)); the positive allometry of limb length may help explain the scaling of the metabolic cost of transport. When scaled against body mass, forelimb inertial properties, apart from mass, scale with positive allometry. Fore- and hindlimb mass scale according to geometric similarity (limb mass ∝ body mass(1.0)), as do the remaining hindlimb inertial properties. The positive allometry of limb length is largely the result of absolute differences in limb inertial properties between mammalian subgroups. Though likely detrimental to locomotor costs in large mammals, scale effects in limb inertial properties appear to be concomitant with scale effects in sensorimotor control and locomotor ability in terrestrial mammals. Across mammals, the forelimb's potential for angular acceleration scales according to geometric similarity, whereas the hindlimb's potential for angular acceleration scales with positive allometry.

A new look at the Dynamic Similarity Hypothesis: the importance of swing phase

The Dynamic Similarity Hypothesis (DSH) suggests that when animals of different size walk at similar Froude numbers (equal ratios of inertial and gravitational forces) they will use similar size-corrected gaits. This application of similarity theory to animal biomechanics has contributed to fundamental insights in the mechanics and evolution of a diverse set of locomotor systems. However, despite its popularity, many mammals fail to walk with dynamically similar stride lengths, a key element of gait that determines spontaneous speed and energy costs. Here, we show that the applicability of the DSH is dependent on the inertial forces examined. In general, the inertial forces are thought to be the centripetal force of the inverted pendulum model of stance phase, determined by the length of the limb. If instead we model inertial forces as the centripetal force of the limb acting as a suspended pendulum during swing phase (determined by limb center of mass position), the DSH for stride length variation is fully supported. Thus, the DSH shows that inter-specific differences in spatial kinematics are tied to the evolution of limb mass distribution patterns. Selection may act on morphology to produce a given stride length, or alternatively, stride length may be a "spandrel" of selection acting on limb mass distribution.

Walking, running, and resting under time, distance, and average speed constraints: optimality of walk-run-rest mixtures

On a treadmill, humans switch from walking to running beyond a characteristic transition speed. Here, we study human choice between walking and running in a more ecological (non-treadmill) setting. We asked subjects to travel a given distance overground in a given allowed time duration. During this task, the subjects carried, and could look at, a stopwatch that counted down to zero. As expected, if the total time available were large, humans walk the whole distance. If the time available were small, humans mostly run. For an intermediate total time, humans often use a mixture of walking at a slow speed and running at a higher speed. With analytical and computational optimization, we show that using a walk-run mixture at intermediate speeds and a walk-rest mixture at the lowest average speeds is predicted by metabolic energy minimization, even with costs for transients-a consequence of non-convex energy curves. Thus, sometimes, steady locomotion may not be energy optimal, and not preferred, even in the absence of fatigue. Assuming similar non-convex energy curves, we conjecture that similar walk-run mixtures may be energetically beneficial to children following a parent and animals on long leashes. Humans and other animals might also benefit energetically from alternating between moving forward and standing still on a slow and sufficiently long treadmill.

Symmetrical gaits and center of mass mechanics in small-bodied, primitive mammals

Widely accepted relationships between gaits (footfall patterns) and center of mass mechanics have been formulated from observations for cursorial mammals. However, sparse data on smaller or more generalized forms suggest a fundamentally different relationship. This study explores locomotor dynamics in one eutherian and five metatherian (marsupials) mammals-all small-bodied (<2 kg) with generalized body plans that utilize symmetrical gaits. Across our sample, trials conforming to vaulting mechanics occurred least frequently (<10% of all trials) while bouncing mechanics was obtained most commonly (60%); the remaining trials represented mixed mechanics. Contrary to the common situation in large mammals, there was no evidence for discrete gait switching within symmetrical gaits as speed increased. This was in part due to the common practice of grounded running. The adaptive advantage of different patterns of center-of-mass motion and their putative energy savings remain questionable in small-bodied mammals.

The human foot and heel-sole-toe walking strategy: a mechanism enabling an inverted pendular gait with low isometric muscle force?

Mechanically, the most economical gait for slow bipedal locomotion requires walking as an ‘inverted pendulum’, with: I, an impulsive, energy-dissipating leg compression at the beginning of stance; II, a stiff-limbed vault; and III, an impulsive, powering push-off at the end of stance. The characteristic ‘M’-shaped vertical ground reaction forces of walking in humans reflect this impulse–vault–impulse strategy. Humans achieve this gait by dissipating energy during the heel-to-sole transition in early stance, approximately stiff-limbed, flat-footed vaulting over midstance and ankle plantarflexion (powering the toes down) in late stance. Here, we show that the ‘M’-shaped walking ground reaction force profile does not require the plantigrade human foot or heel–sole–toe stance; it is maintained in tip–toe and high-heel walking as well as in ostriches. However, the unusual, stiff, human foot structure—with ground-contacting heel behind ankle and toes in front—enables both mechanically economical inverted pendular walking and physiologically economical muscle loading, by producing extreme changes in mechanical advantage between muscles and ground reaction forces. With a human foot, and heel–sole–toe strategy during stance, the shin muscles that dissipate energy, or calf muscles that power the push-off, need not be loaded at all—largely avoiding the ‘cost of muscle force’—during the passive vaulting phase.

The effects of distal limb segment shortening on locomotor efficiency in sloped terrain: implications for Neandertal locomotor behavior

Past studies of human locomotor efficiency focused on movement over flat surfaces and concluded that Neandertals were less efficient than modern humans due to a truncated limb morphology, which may have developed to aid thermoregulation in cold climates. However, it is not clear whether this potential locomotor disadvantage would also exist in nonflat terrain. This issue takes on added importance since Neandertals likely spent a significant proportion of their locomotor schedule on sloped, mountainous terrains in the Eurasian landscape. Here a model is developed that determines the relationship between lower limb segment lengths, terrain slope, excursion angle at the hip, and step length. The model is applied to Neandertal and modern human lower limb reconstructions. In addition, for a further independent test that also allows more climateterrain cross comparisons, the same model is applied to bovids living in different terrains and climates. Results indicate that: (1) Neandertals, despite exhibiting shorter lower limbs, would have been able to use similar stride frequencies per speed as longer-limbed modern humans on sloped terrain, due to their lower crural indices; and (2) shortened distal limb segments are characteristic of bovids that inhabit more rugged terrains, regardless of climate. These results suggest that the shortened distal lower limb segments of Neandertals were not a locomotor disadvantage within more rugged environments.

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.