Inertial properties of hominoid limb segments

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.

Exploring the effects of skeletal architecture and muscle properties on bipedal standing in the common chimpanzee (Pan troglodytes) from the perspective of biomechanics

Introduction: It is well known that the common chimpanzee, as both the closest living relative to humans and a facultative bipedal, has the capability of bipedal standing but cannot do so fully upright. Accordingly, they have been of exceeding significance in elucidating the evolution of human bipedalism. There are many reasons why the common chimpanzee can only stand with its hips-knees bent, such as the distally oriented long ischial tubercle and the almost absent lumbar lordosis. However, it is unknown how the relative positions of their shoulder-hip-knee-ankle joints are coordinated. Similarly, the distribution of the biomechanical characteristics of the lower-limb muscles and the factors that affect the erectness of standing as well as the muscle fatigue of the lower limbs remain a mystery. The answers are bound to light up the evolutional mechanism of hominin bipedality, but these conundrums have not been shed much light upon, because few studies have comprehensively explored the effects of skeletal architecture and muscle properties on bipedal standing in common chimpanzees. Methods: Thus, we first built a musculoskeletal model comprising the head-arms-trunk (HAT), thighs, shanks, and feet segments of the common chimpanzee, and then, the mechanical relationships of the Hill-type muscle-tendon units (MTUs) in bipedal standing were deduced. Thereafter, the equilibrium constraints were established, and a constrained optimization problem was formulated where the optimization objective was defined. Finally, thousands of simulations of bipedal standing experiments were performed to determine the optimal posture and its corresponding MTU parameters including muscle lengths, muscle activation, and muscle forces. Moreover, to quantify the relationship between each pair of the parameters from all the experimental simulation outcomes, the Pearson correlation analysis was employed. Results: Our results demonstrate that in the pursuit of the optimal bipedal standing posture, the common chimpanzee cannot simultaneously achieve maximum erectness and minimum muscle fatigue of the lower limbs. For uni-articular MTUs, the relationship between muscle activation, relative muscle lengths, together with relative muscle forces, and the corresponding joint angle is generally negatively correlated for extensors and positively correlated for flexors. For bi-articular MTUs, the relationship between muscle activation, coupled with relative muscle forces, and the corresponding joint angles does not show the same pattern as in the uni-articular MTUs. Discussion: The results of this study bridge the gap between skeletal architecture, along with muscle properties, and biomechanical performance of the common chimpanzee during bipedal standing, which enhances existing biomechanical theories and advances the comprehension of bipedal evolution in humans.

Muscle architecture and muscle fibre type composition in the forelimb of two African mole-rat species, Bathyergus suillus and Heterocephalus glaber

The scratch-digging Cape dune mole-rat (Bathyergus suillus), and the chisel-toothed digging naked mole-rat (Heterocephalus glaber) are African mole-rats that differ in their digging strategy. The aim of this study was to determine if these behavioural differences are reflected in the muscle architecture and fibre-type composition of the forelimb muscles. Muscle architecture parameters of 39 forelimb muscles in both species were compared. Furthermore, muscle fibre type composition of 21 forelimb muscles were analysed using multiple staining protocols. In B. suillus, muscles involved with the power stroke of digging (limb retractors and scapula elevators), showed higher muscle mass percentage, force output and shortening capacity compared to those in H. glaber. Additionally, significantly higher percentages of glycolytic fibres were observed in the scapular elevators and digital flexors of B. suillus compared to H. glaber, suggesting that the forelimb muscles involved in digging in B. suillus provide fast, powerful motions for effective burrowing. In contrast, the m. sternohyoideus a head and neck flexor, had significantly more oxidative fibres in H. glaber compared to B. suillus. In addition, significantly greater physiological cross-sectional area and fascicle length values were seen in the neck flexor, m. sternocleidomastoideus, in H. glaber compared to B. suillus, which indicates a possible adaptation for chisel-tooth digging. While functional demands may play a significant role in muscle morphology, the phylogenetic differences between the two species may play an additional role which needs further study.

Segmental morphometrics of bonobos (Pan paniscus): are they really different from chimpanzees (Pan troglodytes)?

The inertial properties of body segments reflect performance and locomotor habits in primates. While Pan paniscus is generally described as more gracile, lighter in body mass, and as having relatively longer and heavier hindlimbs than Pan troglodytes, both species exhibit very similar patterns of (quadrupedal and bipedal) kinematics, but show slightly different locomotor repertoires. We used a geometric model to estimate the inertial properties for all body segments (i.e. head, trunk, upper and lower arms, hand, thigh, shank and foot) using external length and diameter measurements of 12 anaesthetized bonobos (eight adults and four immatures). We also calculated whole limb inertial properties. When we compared absolute and relative segment morphometric and inertial variables between bonobos and chimpanzees, we found that adult bonobos are significantly lighter than adult chimpanzees. The bonobo is also shorter in head length, upper and lower arm lengths, and foot length, and is generally lighter in most absolute segment mass values (except head and hand). In contrast, the bonobo has a longer trunk. When scaled relative to body mass, most differences disappear between the two species. Only the longer trunk and the shorter head of the bonobo remain apparent, as well as the lighter thigh compared with the chimpanzee. We found similar values of natural pendular periods of the limbs in both species, despite differences in absolute limb lengths, masses, mass centres (for the hindlimb) and moments of inertia. While our data contradict the commonly accepted view that bonobos have relatively longer and heavier hindlimbs than chimpanzees, they are consistent with the observed similarities in the quadrupedal and bipedal kinematics between these species. The morphological differences between both species are more subtle than those previously described from postcranial osteological materials.

Segmental morphometrics of the olive baboon (Papio anubis): a longitudinal study from birth to adulthood

The linear dimensions and inertial characteristics of the body are important in locomotion and they change considerably during the ontogeny of animals, including humans. This longitudinal and ontogenetic study has produced the largest dataset to date of segmental morphometrics in a Catarrhini species, the olive baboon. The objectives of the study were to quantify the changes in body linear and inertial dimensions and to explore their (theoretical) mechanical significance for locomotion. We took full-body measurements of captive individuals at regular intervals. Altogether, 14 females and 16 males were followed over a 7-year period, i.e. from infancy to adulthood. Our results show that individual patterns of growth are very consistent and follow the general growth pattern previously described in olive baboons. Furthermore, we obtained similar growth curve structures for segment lengths and masses, although the respective time scales were slightly different. The most significant changes in body morphometrics occurred during the first 2 years of life and concerned the distal parts of the body. Females and males were similar in size and shape at birth. The rate and duration of growth produced substantial size-related differences throughout ontogeny, while body shapes remained very similar between the sexes. We also observed significant age-related variations in limb composition, with a proximal shift of the centre of mass within the limbs, mainly due to changes in mass distribution and in the length of distal segments. Finally, we observed what we hypothesize to be 'early biomechanical optimization' of the limbs for quadrupedal walking. This is due to a high degree of convergence between the limbs' natural pendular periods in infants, which may facilitate the onset of quadrupedal walking. Furthermore, the mechanical significance of the morphological changes observed in growing baboons may be related to changing functional demands with the onset of autonomous (quadrupedal) locomotion. From a wider perspective, these data provide unique insights into questions surrounding both the processes of locomotor development in primates and how these processes might evolve.

The hominins: a very conservative tribe? Last common ancestors, plasticity and ecomorphology in Hominidae. Or, What's in a name?

In the early 20th century the dominant paradigm for the ecological context of the origins of human bipedalism was arboreal suspension. In the 1960s, however, with recognition of the close genetic relationship of humans, chimpanzees and bonobos, and with the first field studies of mountain gorillas and common chimpanzees, it was assumed that locomotion similar to that of common chimpanzees and mountain gorillas, which appeared to be dominated by terrestrial knuckle-walking, must have given rise to human bipedality. This paradigm has been popular, if not universally dominant, until very recently. However, evidence that neither the knuckle-walking or vertical climbing of these apes is mechanically similar to human bipedalism, as well as the hand-assisted bipedality and orthograde clambering of orang-utans, has cast doubt on this paradigm. It now appears that the dominance of terrestrial knuckle-walking in mountain gorillas is an artefact seen only in the extremes of their range, and that both mountain and lowland gorillas have a generalized orthogrady similar to that seen in orang-utans. These data, together with evidence for continued arboreal competence in humans, mesh well with an increasing weight of fossil evidence suggesting that a mix of orang-utan and gorilla-like arboreal locomotion and upright terrestrial bipedalism characterized most australopiths. The late split date of the panins, corresponding to dates for separation of Homo and Australopithecus, leads to the speculation that competition with chimpanzees, as appears to exist today with gorillas, may have driven ecological changes in hominins and perhaps cladogenesis. However, selection for ecological plasticity and morphological conservatism is a core characteristic of Hominidae as a whole, including Hominini.

Bipedality from locomotor autonomy to adulthood in captive olive baboon (Papio anubis): Cross-sectional follow-up and first insight into the impact of body mass distribution

OBJECTIVE

Despite that the biomechanics of standing and walking bipedally has been extensively studied in nonhuman primates, the morphological features that may constrain, or facilitate, the control of balance and thus of the spontaneous occurrence of bipedal behavior are poorly known. We aim to test the relationship between body mass distribution and bipedal behavior using a nonhuman primate species, the olive baboon, Papio anubis, raised in captivity.

MATERIALS AND METHODS

We collected quantitative data on the frequency and duration of bipedalism together with morphometrics on a sample of 22 individuals. We used ontogenetic changes as a natural experiment that provides insights into the impact of morphology. Specifically we focus on 1) quantifying how body mass distribution changes from infancy to adulthood in baboons; and 2) whether the different patterns of mass distribution influence the behavioral variables, i.e., a) the frequency and b) the duration of bouts of bipedal behavior realized in different activity contexts.

RESULTS

With regard to assisted bipedal behaviors, the duration and frequency of bouts of standing, contrary to walking, are significantly related to age. With regard to unassisted bipedal behaviors, no correlation to age is observed; the bout duration of walking is strongly correlated to body mass and mass distribution, contrary to the frequency of walking as well as the bout duration and frequency of bipedal standing.

DISCUSSION

Our results suggest a close relationship between the pattern of mass distribution and the mechanism of balance control in the spontaneous bipedal walking of baboons. The mechanical effects of the pattern of mass distribution on the ability to perform bipedally in extant nonhuman primates are discussed in the context of the evolution toward habitual bipedalism.

Energetic benefits and adaptations in mammalian limbs: Scale effects and selective pressures

Differences in limb size and shape are fundamental to mammalian morphological diversity; however, their relevance to locomotor costs has long been subject to debate. In particular, it remains unknown if scale effects in whole limb morphology could partially underlie decreasing mass-specific locomotor costs with increasing limb length. Whole fore- and hindlimb inertial properties reflecting limb size and shape-moment of inertia (MOI), mass, mass distribution, and natural frequency-were regressed against limb length for 44 species of quadrupedal mammals. Limb mass, MOI, and center of mass position are negatively allometric, having a strong potential for lowering mass-specific locomotor costs in large terrestrial mammals. Negative allometry of limb MOI results in a 40% reduction in MOI relative to isometry's prediction for our largest sampled taxa. However, fitting regression residuals to adaptive diversification models reveals that codiversification of limb mass, limb length, and body mass likely results from selection for differing locomotor modes of running, climbing, digging, and swimming. The observed allometric scaling does not result from selection for energetically beneficial whole limb morphology with increasing size. Instead, our data suggest that it is a consequence of differing morphological adaptations and body size distributions among quadrupedal mammals, highlighting the role of differing limb functions in mammalian evolution.

The extraordinary athletic performance of leaping gibbons

The distance that animals leap depends on their take-off angle and velocity. The velocity is generated solely by mechanical work during the push-off phase of standing-start leaps. Gibbons are capable of exceptional leaping performance, crossing gaps in the forest canopy exceeding 10 m, yet possess none of the adaptations possessed by specialist leapers synonymous with maximizing mechanical work. To understand this impressive performance, we recorded leaps of the gibbons exceeding 3.7 m. Gibbons perform more mass-specific work (35.4 J kg(-1)) than reported for any other species to date, accelerating to 8.3 ms(-1) in a single movement and redefining our estimates of work performance by animals. This energy (enough for a 3.5 m vertical leap) is 60 per cent higher than that achieved by galagos, which are renowned for their remarkable leaping performance. The gibbons' unusual morphology facilitates a division of labour among the hind limbs, forelimbs and trunk, resulting in modest power requirements compared with more specialized leapers.

The effect of substrate compliance on the biomechanics of gibbon leaps

The storage and recovery of elastic strain energy in the musculoskeletal systems of locomoting animals has been extensively studied, yet the external environment represents a second potentially useful energy store that has often been neglected. Recent studies have highlighted the ability of orangutans to usefully recover energy from swaying trees to minimise the cost of gap crossing. Although mechanically similar mechanisms have been hypothesised for wild leaping primates, to date no such energy recovery mechanisms have been demonstrated biomechanically in leapers. We used a setup consisting of a forceplate and two high-speed video cameras to conduct a biomechanical analysis of captive gibbons leaping from stiff and compliant poles. We found that the gibbons minimised pole deflection by using different leaping strategies. Two leap types were used: slower orthograde leaps and more rapid pronograde leaps. The slower leaps used a wider hip joint excursion to negate the downward movement of the pole, using more impulse to power the leap, but with no increase in work done on the centre of mass. Greater hip excursion also minimised the effective leap distance during orthograde leaps. The more rapid leaps conversely applied peak force earlier in stance where the pole was effectively stiffer, minimising deflection and potential energy loss. Neither leap type appeared to usefully recover energy from the pole to increase leap performance, but the gibbons demonstrated an ability to best adapt their leap biomechanics to counter the negative effects of the compliant pole.

Arboreality, terrestriality and bipedalism

The full publication of Ardipithecus ramidus has particular importance for the origins of hominin bipedality, and strengthens the growing case for an arboreal origin. Palaeontological techniques however inevitably concentrate on details of fragmentary postcranial bones and can benefit from a whole-animal perspective. This can be provided by field studies of locomotor behaviour, which provide a real-world perspective of adaptive context, against which conclusions drawn from palaeontology and comparative osteology may be assessed and honed. Increasingly sophisticated dynamic modelling techniques, validated against experimental data for living animals, offer a different perspective where evolutionary and virtual ablation experiments, impossible for living mammals, may be run in silico, and these can analyse not only the interactions and behaviour of rigid segments but increasingly the effects of compliance, which are of crucial importance in guiding the evolution of an arboreally derived lineage.

Mechanical constraints on the functional morphology of the gibbon hind limb

Gibbons utilize a number of locomotor modes in the wild, including bipedalism, leaping and, most of all, brachiation. Each locomotor mode puts specific constraints on the morphology of the animal; in some cases these may be complementary, whereas in others they may conflict. Despite several studies of the locomotor biomechanics of gibbons, very little is known about the musculoskeletal architecture of the limbs. In this study, we present quantitative anatomical data of the hind limb for four species of gibbon (Hylobates lar, H. moloch, H. pileatus and Symphalangus syndactylus). Muscle mass and fascicle lengths were obtained from all of the major hind limb muscles and the physiological cross-sectional area was calculated and scaled to remove the effect of body size. The results clearly indicate that, for all of the species studied, the major hip, knee and ankle extensors are short-fascicled and pennate. The major hip and knee flexors, however, are long-fascicled, parallel muscles with relatively small physiological cross-sectional areas. We hypothesize that the short-fascicled muscles could be coupled with a power-amplifying mechanism and are predominantly useful in leaping. The long-fascicled knee and hip flexors are adapted for a wide range of joint postures and can play a role in flexing the legs during brachiation.

Growth of segment parameters and a morphological classification for children between 15 and 36 months

This study is part of a research program that aims at a better understanding of the influence of individual morphological differences and physical growth on development of independent walking in toddlers. As morphometric and segment inertial parameters for toddlers aged between 15 and 36 months are indispensable for the mechanical analyses inherent to this purpose, parameter data were collected. The provided dataset of morphological and segment inertial parameters is a valuable tool for locomotor biomechanical modelling. Analysis of the parameter data showed that there are substantial changes of most segment inertial parameters across body length and body mass. In addition, a classification system was developed to categorize toddlers on the basis of morphometry, reflecting the segment inertial constitution of the child. A principal components analysis (PCA) was applied to define the variance in physique between the children. PCA resulted in three newly composed variables: the 'Axis of chubbiness', the 'Axis of allometric growth' and the 'Axis of relative limb length'. The three axes are plotted against each other, resulting in eight morphological classes. With this classification the morphotype of toddlers between 15 and 36 months can be specified and used for further research on their walking patterns.

Exploring the mechanical basis for acceleration: pelvic limb locomotor function during accelerations in racing greyhounds (Canis familiaris)

Animals in their natural environments are confronted with a regular need to perform rapid accelerations (for example when escaping from predators or chasing prey). Such acceleration requires net positive mechanical work to be performed on the centre of mass by skeletal muscle. Here we determined how pelvic limb joints contribute to the mechanical work and power that are required for acceleration in galloping quadrupeds. In addition, we considered what, if any, biomechanical strategies exist to enable effective acceleration to be achieved. Simultaneous kinematic and kinetic data were collected for racing greyhounds undergoing a range of low to high accelerations. From these data, joint moments and joint powers were calculated for individual hindlimb joints. In addition, the mean effective mechanical advantage (EMA) of the limb and the ;gear ratio' of each joint throughout stance were calculated. Greatest increases in joint work and power with acceleration appeared at the hip and hock joints, particularly in the lead limb. Largest increases in absolute positive joint work occurred at the hip, consistent with the hypothesis that quadrupeds power locomotion by torque about the hip. In addition, hindlimb EMA decreased substantially with increased acceleration - a potential strategy to increase stance time and thus ground impulses for a given peak force. This mechanism may also increase the mechanical advantage for applying the horizontal forces necessary for acceleration.

Estimating mass properties of dinosaurs using laser imaging and 3D computer modelling

Body mass reconstructions of extinct vertebrates are most robust when complete to near-complete skeletons allow the reconstruction of either physical or digital models. Digital models are most efficient in terms of time and cost, and provide the facility to infinitely modify model properties non-destructively, such that sensitivity analyses can be conducted to quantify the effect of the many unknown parameters involved in reconstructions of extinct animals. In this study we use laser scanning (LiDAR) and computer modelling methods to create a range of 3D mass models of five specimens of non-avian dinosaur; two near-complete specimens of Tyrannosaurus rex, the most complete specimens of Acrocanthosaurus atokensis and Strutiomimum sedens, and a near-complete skeleton of a sub-adult Edmontosaurus annectens. LiDAR scanning allows a full mounted skeleton to be imaged resulting in a detailed 3D model in which each bone retains its spatial position and articulation. This provides a high resolution skeletal framework around which the body cavity and internal organs such as lungs and air sacs can be reconstructed. This has allowed calculation of body segment masses, centres of mass and moments or inertia for each animal. However, any soft tissue reconstruction of an extinct taxon inevitably represents a best estimate model with an unknown level of accuracy. We have therefore conducted an extensive sensitivity analysis in which the volumes of body segments and respiratory organs were varied in an attempt to constrain the likely maximum plausible range of mass parameters for each animal. Our results provide wide ranges in actual mass and inertial values, emphasizing the high level of uncertainty inevitable in such reconstructions. However, our sensitivity analysis consistently places the centre of mass well below and in front of hip joint in each animal, regardless of the chosen combination of body and respiratory structure volumes. These results emphasize that future biomechanical assessments of extinct taxa should be preceded by a detailed investigation of the plausible range of mass properties, in which sensitivity analyses are used to identify a suite of possible values to be tested as inputs in analytical models.

Segmental inertial properties in dogs determined by magnetic resonance imaging

Data regarding the segmental inertial properties of the dog are currently unavailable, although such parameters are needed for dynamic analyses of canine motion. The purpose of this study was to measure the segmental inertial properties in three medium sized dogs of average build using magnetic resonance imaging. The parameters included the mass, location of centre of mass and moments of inertia for each body segment. The normalised results will serve as a preliminary foundation for various biomechanical studies in dogs, although further study is required to characterise them for specific dog breeds and to determine how they may be affected by age and gender.

Morphometrics and inertial properties in the body segments of chimpanzees (Pan troglodytes)

Inertial characteristics and dimensions of the body and body segments form an integral part of a biomechanical analysis of motion. In primate studies, however, segment inertial parameters of non-human hominoids are scarce and often obtained using varying techniques. Therefore, the principal aim of this study was to expand the existing chimpanzee inertial property data set using a non-invasive measuring technique. We also considered age- and sex-related differences within our sample. By means of a geometric model based on Crompton et al. (1996; Am J Phys Anthropol 99, 547-570) we generated inertial properties using external segment length and diameter measurements of 53 anaesthetized chimpanzees (Pan troglodytes). We report absolute inertial parameters for immature and mature subjects and for males and females separately. Proportional data were computed to allow the comparison between age classes and sex classes. In addition, we calculated whole limb inertial properties and we discuss their potential biomechanical consequences. We found no significant differences between the age classes in the proportional data except for hand and foot measures where juveniles exhibit relatively longer and heavier distal segments than adults. Furthermore, most sex-related differences can be directly attributed to the higher absolute segment masses in male chimpanzees resulting in higher moments of inertia. Additionally, males tend to have longer upper limbs than females. However, regarding proportional data we discuss the general inertial properties of the chimpanzee. The described segment inertial parameters of males and females, and of the two age classes, represent a valuable data set ready for use in a range of biomechanical locomotor models. These models offer great potential for improving our understanding of early hominin locomotor patterns.