Dynamic arm swinging in human walking

Humans tend to swing their arms when they walk, a curious behaviour since the arms play no obvious role in bipedal gait. It might be costly to use muscles to swing the arms, and it is unclear whether potential benefits elsewhere in the body would justify such costs. To examine these costs and benefits, we developed a passive dynamic walking model with free-swinging arms. Even with no torques driving the arms or legs, the model produced walking gaits with arm swinging similar to humans. Passive gaits with arm phasing opposite to normal were also found, but these induced a much greater reaction moment from the ground, which could require muscular effort in humans. We therefore hypothesized that the reduction of this moment may explain the physiological benefit of arm swinging. Experimental measurements of humans (n = 10) showed that normal arm swinging required minimal shoulder torque, while volitionally holding the arms still required 12 per cent more metabolic energy. Among measures of gait mechanics, vertical ground reaction moment was most affected by arm swinging and increased by 63 per cent without it. Walking with opposite-to-normal arm phasing required minimal shoulder effort but magnified the ground reaction moment, causing metabolic rate to increase by 26 per cent. Passive dynamics appear to make arm swinging easy, while indirect benefits from reduced vertical moments make it worthwhile overall.     

A unified theory for the energy cost of legged locomotion

Small animals are remarkably efficient climbers but comparatively poor runners, a well-established phenomenon in locomotor energetics that drives size-related differences in locomotor ecology yet remains poorly understood. Here, I derive the energy cost of legged locomotion from two complementary components of muscle metabolism, Activation–Relaxation and Cross-bridge cycling. A mathematical model incorporating these costs explains observed patterns of locomotor cost both within and between species, across a broad range of animals (insects to ungulates), for a wide range of substrate slopes including level running and vertical climbing. This ARC model unifies work- and force-based models for locomotor cost and integrates whole-organism locomotor cost with cellular muscle physiology, creating a predictive framework for investigating evolutionary and ecological pressures shaping limb design and ranging behaviour.     

Gait‐specific metabolic costs and preferred speeds in ring‐tailed lemurs (Lemur catta), with implications for the scaling of locomotor costs

Metabolic costs of resting and locomotion have been used to gain novel insights into the behavioral ecology and evolution of a wide range of primates; however, most previous studies have not considered gait‐specific effects. Here, metabolic costs of ring‐tailed lemurs (Lemur catta) walking, cantering and galloping are used to test for gait‐specific effects and a potential correspondence between costs and preferred speeds. Metabolic costs, including the net cost of locomotion (COL) and net cost of transport (COT), change as a curvilinear function of walking speed and (at least provisionally) as a linear function of cantering and galloping speeds. The baseline quantity used to calculate net costs had a significant effect on the magnitude of speed‐specific estimates of COL and COT, especially for walking. This is because non‐locomotor metabolism constitutes a substantial fraction (41–61%, on average) of gross metabolic rate at slow speeds. The slope‐based estimate of the COT was 5.26 J kg^?1 m^?1 for all gaits and speeds, while the gait‐specific estimates differed between walking (0.5 m s^?1: 6.69 J kg^?1 m^?1) and cantering/galloping (2.0 m s^?1: 5.61 J kg^?1 m^?1). During laboratory‐based overground locomotion, ring‐tailed lemurs preferred to walk at ～0.5 m s^?1 and canter/gallop at ～2.0 m s^?1, with the preferred walking speed corresponding well to the COT minima. Compared with birds and other mammals, ring‐tailed lemurs are relatively economical in walking, cantering, and galloping. These results support the view that energetic optima are an important movement criterion for locomotion in ring‐tailed lemurs, and other terrestrial animals. Am J Phys Anthropol, 2012. © 2012 Wiley Periodicals, Inc.     

Pendular mechanics and the kinematics and energetics of brachiating locomotion

Because brachiating locomotion is characterized by a pattern of swinging movements, brachiation has often been analogized to pendular motion, and aspects of the mechanics of pendular systems have been used to provide insight into both energetic and structural design aspects of this locomotor mode. However, there are several limitations to this approach. First, the motions of brachiating animals only approximate pendular motion, and therefore the energetics of these two systems are only roughly comparable. Second, the kinematic similarity between brachiation and pendular motion will be maximal at only one velocity, and the correspondence will be even less at greater or lesser speeds. Third, all forms of terrestrial locomotion that involve the use of limbs incorporate elements of pendular systems, and therefore brachiation is not unusual in this respect. Finally, it has been suggested that the mechanics of pendular motion will constrain the maximum attainable body size of brachiating animals and that this mechanical situation explains the lack of brachiating primates of greater than 30-kg body size; the present analysis provides evidence that the constraints on body size are far less strict than previously indicated and that extrinsic factors such as the geometry of the forest environment are more likely to dictate maximum body size for brachiators.     

The kangaroo's tail propels and powers pentapedal locomotion

When moving slowly, kangaroos plant their tail on the ground in sequence with their front and hind legs. To determine the tail''s role in this ‘pentapedal’ gait, we measured the forces the tail exerts on the ground and calculated the mechanical power it generates. We found that the tail is responsible for as much propulsive force as the front and hind legs combined. It also generates almost exclusively positive mechanical power, performing as much mass-specific mechanical work as does a human leg during walking at the same speed. Kangaroos use their muscular tail to support, propel and power their pentapedal gait just like a leg.     

Energetically costly behaviour and the evolution of resting metabolic rate in insects

1. A general hypothesis is presented to explain interspecific differences in size-independent resting metabolic rate. This hypothesis is based on a presumed trade-off between a low resting metabolism and adaptations of metabolism during activity.
2. With such a trade-off, selection to reduce resting metabolism is less intense in active species than in species where resting metabolism constitutes a large proportion of the daily metabolic costs. Those animals that spend more energy on activity should therefore have a higher resting metabolic rate than animals that spend less energy on activity.
3. A literature review reveals that flying insects have higher resting metabolic rates than species that use energetically less demanding types of locomotion.
4. Insects producing acoustic advertisement signals can be shown to have higher mass-independent resting metabolic rates than closely related species without this energetically demanding behaviour.
5. Literature data on vertebrate resting metabolic rates are also consistent with the presented hypothesis: the more energy animals spend on activity, the higher the mass-independent resting metabolic rate.     

Energetic costs of bipedal and quadrupedal walking in Japanese macaques

We investigated the energetic costs of quadrupedal and bipedal walking in two Japanese macaques. The subjects were engaged in traditional bipedal performance for years, and are extremely adept bipeds. The experiment was conducted in an airtight chamber with a gas analyzer. The subjects walked quadrupedally and bipedally at fixed velocities (<5 km/hr) on a treadmill in the chamber for 2.5-6 min. We estimated energy consumption from carbon dioxide (CO2) production. While walking bipedally, energetic expenditure increased by 30% relative to quadrupedalism in one subject, and by 20% in another younger subject. Energetic costs increased linearly with velocity in quadrupedalism and bipedalism, with bipedal/quadrupedal ratios remaining almost constant. Our experiments were relatively short in duration, and thus the observed locomotor costs may include presteady-state high values. However, there was no difference in experimental duration between bipedal and quadrupedal trials. Thus, the issue of steady state cannot cancel the difference in energetic costs. Furthermore, we observed that switching of locomotor mode (quadrupedalism to bipedalism) during a session resulted in a significant increase of CO2 production. Taylor and Rowntree ([1973] Science 179:186-187) noted that the energetic costs for bipedal and quadrupedal walking were the same in chimpanzees and capuchin monkeys. Although the reason for this inconsistency is not clear, species-specific differences should be considered regarding bipedal locomotor energetics among nonhuman primates. Extra costs for bipedalism may not be great in these macaques. Indeed, it is known that suspensory locomotion in Ateles consumes 1.3-1.4 times as much energy relative to quadrupedal progression. This excess ratio surpasses the bipedal/quadrupedal energetic ratios in these macaques.     

Posture, gait and the ecological relevance of locomotor costs and energy-saving mechanisms in tetrapods

A reanalysis of locomotor data from functional, energetic, mechanical and ecological perspectives reveals that limb posture has major effects on limb biomechanics, energy-saving mechanisms and the costs of locomotion. Regressions of data coded by posture (crouched vs. erect) reveal nonlinear patterns in metabolic cost, limb muscle mass, effective mechanical advantage, and stride characteristics. In small crouched animals energy savings from spring and pendular mechanisms are inconsequential and thus the metabolic cost of locomotion is driven by muscle activation costs. Stride frequency appears to be the principal functional parameter related to the decreasing cost of locomotion in crouched animals. By contrast, the shift to erect limb postures invoked a series of correlated effects on the metabolic cost of locomotion: effective mechanical advantage increases, relative muscle masses decrease, metapodial limb segments elongate dramatically (as limbs shift from digitigrade to unguligrade designs) and biological springs increase in size and effectiveness. Each of these factors leads to decreases in the metabolic cost of locomotion in erect forms resulting from real and increasing contributions of pendular savings and spring savings. Comparisons of the relative costs and ecological relevance of different gaits reveal that running is cheaper than walking in smaller animals up to the size of dogs but running is more expensive than walking in horses. Animals do not necessarily use their cheapest gaits for their predominant locomotor activity. Therefore, locomotor costs are driven more by ecological relevance than by the need to optimize locomotor economy.     

The metabolic costs of 'bent-hip, bent-knee' walking in humans

The costs of different modes of bipedalism are a key issue in reconstructing the likely gait of early human ancestors such as Australopithecus afarensis. Some workers, on the basis of morphological differences between the locomotor skeleton of A. afarensis and modern humans, have proposed that this hominid would have walked in a 'bent-hip, bent-knee' (BHBK) posture like that seen in the voluntary bipedalism of untrained chimpanzees. Computer modelling studies using inverse dynamics indicate that on the basis of segment proportions AL-288-1 should have been capable of mechanically effective upright walking, but in contrast predicted that BHBK walking would have been highly ineffective. The measure most pertinent to natural selection, however, is more likely to be the complete, physiological, or metabolic energy cost. We cannot measure this parameter in a fossil. This paper presents the most complete investigation yet of the metabolic and thermoregulatory costs of BHBK walking in humans. Data show that metabolic costs including the basal metabolic rate (BMR) increase by around 50% while the energy costs of locomotion and blood lactate production nearly double, heat load is increased, and core temperature does not return to normal within 20 minutes rest. Net effects imply that a resting period of 150% activity time would be necessary to prevent physiologically intolerable heat load. Preliminary data for children suggest that scaling effects would not significantly reduce relative costs for hominids of AL-288-1's size. Data from recent studies using forwards dynamic modelling confirm that similar total (including BMR) and locomotor metabolic costs would have applied to BHBK walking by AL-288-1. We explore some of the ecological consequences of our findings.     

Infant carrying: the role of increased locomotory costs in early tool development

Among the costs of reproduction, carrying one's infant incurs one of the greatest drains on maternal energy, simply because of the added mass alone. Because of the dearth of archaeological evidence, however, how early bipeds dealt with the additional cost of having to carry infants who were less able to support their body weight against gravity is not particularly well understood. This article presents evidence on the caloric drain of carrying an infant in one's arms versus having a tool with which to sling the infant and carry her passively. The burden of carrying an infant in one's arms is on average 16% greater than having a tool to support the baby's mass and seems to have the potential to be a greater energetic burden even than lactation. In addition, carrying a baby in one's arms shortens and quickens the stride. An anthropometric trait that seems to offset some of the increased cost of carrying a baby in the arms is a wider bi-trochanteric width.     

Locomotor strategies in obese and non-obese children

Objective: The constant strain in obese children may increase the risks of articular problems in adulthood. In the short term, obesity in children could lead to modifications of the gait pattern. The purpose of this study was to compare biomechanical parameters between obese and non‐obese children during self‐paced walking. Research Methods and Procedures: Gait analysis was performed on 10 non‐obese and 10 obese (body weight > 95th percentile) children between 8 and 13 years of age. Subjects were asked to walk at their own pace on a 10‐m walkway with two embedded AMTI force plates (Advanced Mechanical Technology, Watertown, MA) sampling at 960 Hz. Kinematics were captured with eight VICON optoelectronic cameras (Oxford Metrics Limited, Oxford, United Kingdom) recording at 60 Hz. Results: Obese children modified their hip motor pattern by shifting from extensor to flexor moment earlier in the gait cycle. This led obese children to significantly decrease the mechanical work done by the hip extensors during weight acceptance and significantly increase the mechanical work done by the hip flexors compared with non‐obese children. The ratio of power‐absorption‐by‐hip‐flexors to power‐generation‐by‐hip‐flexors was also significantly increased in the obese group compared with non‐obese children. Finally, there was a significant decrease in the single support duration in the obese group compared with non‐obese. Discussion: The kinetics analyzed showed that obese children could take advantage of a passive hip strategy to achieve forward progression during walking. However, considering that they are mechanically less efficient to transfer energy, walking at a natural cadence should be an appropriate exercise to reduce weight in obese children.     

Energetic costs of activity by lizards in the field

1. The available data related to the activity energetics of lizards in the field were collated with respect to three indices of activity energetics: the ecological cost of transport (ECT) expressed as a percentage of the total energy, the proportion of total energy used in all forms of activity (%AR), and the sustained metabolic scope (SusMS), defined as the ratio of the total energy expenditure to the total resting metabolism.
2. The ECT values of lizards ranged from 3 to 36% with five of 11 species having values >20%. The percentage AR ranged from 23 to 80% for lizards during active seasons, with most species having values > 50%. The SusMS ranged from 1·1 to 5·1.
3. Values of ECT are higher for lizards than for mammals, in part because the costs of maintenance metabolism are higher in mammals.
4. The percentage AR and SusMS values of mammals are higher than those of lizards.
5. It follows from the previous two points that the proportion of the total daily energy that is expended in non-locomotory activities is disproportionately higher in mammals compared with lizards.
6. The energy expended in locomotion is a significant portion of the energy budget of lizards. This is generally true for all seasons in which there is activity.     

Biomechanics and foraging profitability: an approach to assessing trophic needs and impacts of diving ducks

A biomechanical model of underwater locomotion is described, and data required by the model presented for 3 species of diving duck (Aythya spp.). Based on field observations of behavior and foods consumed, the model is used to estimate energy costs of foraging and minimum food intake rates of canvasbacks (Aythya valisineria) in two habitats in North Carolina. Increased water depth from 0.5 m in Lake Mattamuskeet to 1.5 m in Pamlico Sound increased the net cost of time spent foraging at the bottom by 43%. Biomechanical calculations are combined with data on intake rates at different food densities (Takekawa, 1987) to determine minimum food densities for profitable foraging in Lake Mattamuskeet. Field observations of behavior are used to adjust minimum intake per dive for unsuccessful dives spent locating food patches. Density and dispersion of plant tuber foods in Lake Mattamuskeet, before and after the fall staging period, suggest that the fraction of habitat with tuber densities above a profitability threshold is more critical to canvasbacks than average tuber density. Such factors are important in relating bird energy requirements and benthic sampling data to carrying capacity and total area of usable habitat. The proportion of foods that can be fed upon profitably also determines the fraction of food organisms subject to depletion as components of trophic pathways.     

Evolutionary implications of the unusual walking mechanics of the common marmoset (C. jacchus)

Several features that appear to differentiate the walking gaits of most primates from those of most other mammals (the prevalence of diagonal-sequence footfalls, high degrees of humeral protraction, and low forelimb vs. hindlimb peak vertical forces) are believed to have evolved in response to requirements of locomotion on thin arboreal supports by early primates that had developed clawless grasping hands and feet. This putative relationship between anatomy, behavior, and ecology is tested here by examining gait mechanics in the common marmoset (Callithrix jacchus), a primate that has sharp claws and reduced pedal grasping, and that spends much of its time clinging on large trunks. Kinematic and kinetic data were collected on three male Callithrix jacchus as they walked across a force platform attached to the ground or to raised horizontal poles. The vast majority of all walking gaits were lateral-sequence. For all steps, the humerus was retracted (<90 degrees relative to a horizontal axis) or held in a neutral (90 degrees ) position at forelimb touchdown. Peak vertical forces on the forelimb were always higher than those on the hindlimb. These three features of the walking gaits of C. jacchus separate it from any other primate studied (including other callitrichids). The walking gaits of C. jacchus are mechanically more similar to those of small, nonprimate mammals. The results of this study support previous models that suggest that the unusual suite of features that typify the walking gaits of most primates are adaptations to the requirements of locomotion on thin arboreal supports. These data, along with data from other primates and marsupials, suggest that primate postcranial and locomotor characteristics are part of a basal adaptation for walking on thin branches.     

Mechanical and metabolic determinants of the preferred step width in human walking

We studied the selection of preferred step width in human walking by measuring mechanical and metabolic costs as a function of experimentally manipulated step width (0.00-0.45L, as a fraction of leg length L). We estimated mechanical costs from individual limb external mechanical work and metabolic costs using open circuit respirometry. The mechanical and metabolic costs both increased substantially (54 and 45%, respectively) for widths greater than the preferred value (0.15-0.45L) and with step width squared (R(2) = 0.91 and 0.83, respectively). As predicted by a three-dimensional model of walking mechanics, the increases in these costs appear to be a result of the mechanical work required for redirecting the centre of mass velocity during the transition between single stance phases (step-to-step transition costs). The metabolic cost for steps narrower than preferred (0.10-0.00L) increased by 8%, which was probably as a result of the added cost of moving the swing leg laterally in order to avoid the stance leg (lateral limb swing cost). Trade-offs between the step-to-step transition and lateral limb swing costs resulted in a minimum metabolic cost at a step width of 0.12L, which is not significantly different from foot width (0.11L) or the preferred step width (0.13L). Humans appear to prefer a step width that minimizes metabolic cost.     

Patterns of clavicular bilateral asymmetry in relation to the humerus: variation among humans

Studies of directional asymmetry in the human upper limb have extensively examined bones of the arm, forearm, and hand, but have rarely considered the clavicle. Physiologically, the clavicle is an integrated element of the upper limb, transmitting loads to the axial skeleton and supporting the distal bones. However, clavicles develop in a manner that is unique among the bones of the upper limb. Previous studies have indicated that the clavicle has a right-biased asymmetry in diaphyseal breadth, as in humeri, radii, ulnae, and metacarpals, but unlike these other elements, a left-biased length asymmetry. Few studies have assessed how clavicular asymmetry relates to these other bones of the upper limb. Bilateral directional asymmetry of the clavicle is examined in relation to the humerus in a large, geographically diverse human sample, comparing lengths and diaphyseal breadths. Dimensions were converted into percentage directional (%DA) and absolute (%AA) asymmetries. Results indicate that humans have same-side %DA bias in the clavicles and humeri, and contralateral length %DA between these elements. Diaphyseal breadths in both clavicles and humeri are more asymmetric-both in direction and amount-than lengths. Differences in diaphyseal asymmetry are shown to relate to variation in physical activities among groups, but a relationship between activity and length asymmetry is not supported. This further supports previous research, which suggests different degrees of sensitivity to loading between diaphyseal breadths and maximum lengths of long bones. Differences in lateralized behavior and the potential effects of different bone development are examined as possible influences on the patterns observed among human groups.