Passive bipedal walking with phasic muscle contraction

The existence of self-organizing walking patterns is often considered the result of a mechanical system interacting with the environment and a (neural) oscillating unit. The pattern generators might be thought of as an indispensable component for the existence of limit cycle behavior. This paper shows that this is not a necessity for the existence of a self-organizing bipedal walking pattern. Stable walking cycles emerge from a simple passive bipedal structure, with an energy source inevitably present to sustain the oscillation. In this work the energy source is chosen to be phasic muscle contraction. A two-dimensional model is composed of two legs and a hip mass, symbolizing the trunk. The stance leg stiffness is generated by two muscles. The hip stiffness is generated by four muscles. Muscle activation is caused by two reflex-like trigger signals, without feedback control. Human equivalent model parameters such as geometry and mass distribution were assumed. With return map analysis, the model is analyzed on periodic behavior. Stable walking cycles were found and could be manipulated during walking by varying the muscle or reflex parameters, forcing the oscillation to converge to a new attractor. Received: 5 November 1998 / Accepted in revised form: 26 March 1999     

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

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

Peer interaction in infant chimpanzees

The peer interactions of 6 infant chimpanzees (Pan troglodytes) ranging in age from 18 to 50 months were observed in a seminatural context. The infants and their mothers lived as members of a captive social group at the Yerkes Regional Primate Research Center. An analysis of contact initiations between infants indicated that the most preferred peer interactant was the youngest and the least preferred was the oldest infant. Infants also initiated more interactions with the offspring of adults that had the closest relationships with both themselves and their mothers. These results indicate that a number of factors may influence the peer affiliations of infant chimpanzees, including the age of the infant and the mother's social relationships.     

Dynamic modelling for implementation of a right turn in bipedal walking

This paper deals with the development of a conceptual model for the control of a multilink biped during a turning maneuver. The skeletal model is a seven link biped for which the equations of motion are derived. A set of lower limb muscles are idealized by simple force actuators with no co-contraction of agonist-antagonist muscle pairs. A nonlinear control scheme is proposed to guide the model along the desired trajectory and to control ground reaction forces. The input to the system is a desired set of trajectories as functions of time and the patterns of desired ground reaction forces in a turn. One set of such inputs are inferred from the existing literature. With this input, the nonlinear control strategy allows computation of muscular forces needed for the turning maneuver.     

Electromyography of back muscles during quadrupedal and bipedal walking in primates

Despite the extensive electromyographic research that has addressed limb muscle function during primate quadrupedalism, the role of the back muscles in this locomotor behavior has remained undocumented. We report here the results of an electromyographic (EMG) analysis of three intrinsic back muscles (multifidus, longissimus, and iliocostalis) in the baboon (Papio anubis), chimpanzee (Pan troglodytes), and orangutan (Pongo pygmaeus) during quadrupedal walking. The recruitment patterns of these three back muscles are compared to those reported for the same muscles during nonprimate quadrupedalism. In addition, the function of the back muscles during quadrupedalism and bipedalism in the two hominoids is compared. Results indicate that the back muscles restrict trunk movements during quadrupedalism by contracting with the touchdown of one or both feet, with more consistent activity associated with touchdown of the contralateral foot. Moreover, despite reported differences in their gait preferences and forelimb muscle EMG patterns, primates and nonprimate mammals recruit their back muscles in an essentially similar fashion during quadrupedal walking. These quadrupedal EMG patterns also resemble those reported for chimpanzees, gibbons and humans (but not orangutans) walking bipedally. The fundamental similarity in back muscle function across species and locomotor behaviors is consistent with other data pointing to conservatism in the evolution of the neural control of tetrapod limb movement, but does not preclude the suggestion (based on forelimb muscle EMG and spinal lesion studies) that some aspects of primate neural circuitry are unique. © 1994 Wiley-Liss, Inc.     

Insufficient visual information leads to spontaneous bipedal walking in Japanese monkeys

Japanese monkeys walked spontaneously on their hind limbs, when their vision was impaired either by narrowing the visual field or by reducing the incoming light. These variables were manipulated via goggles with translucent pipes and neutral density filters. The bipedal locomotion was observed more frequently as the impairment of the incoming visual information increased. It is very likely that facultative bipeds walk on their hind limbs when they feel the need to “free” their forelimbs to grope their way.     

Dynamics of primate bipedal walking as viewed from the force of foot

Bipedal walking of the six species of anthropoid primates including man were examined by means of the force plate technique. Though each species has a particular pattern of bipedal walking, we can classify two types of patterns in these primates as far as the foot force is concerned. The first type includes the man, chimpanzee, and spider monkey and the second type contains the Japanese monkey, hamadryas baboon, and gibbon. It was emphasized that the similarity of man to the chimpanzee and spider monkey in bipedal walking has some evolutionary significance.     

A simple model of bipedal walking predicts the preferred speed-step length relationship

We used a simple model of passive dynamic walking, with the addition of active powering on level ground, to study the preferred relationship between speed and step length in humans. We tested several hypothetical metabolic costs, with one component proportional to the mechanical work associated with pushing off with the stance leg at toe-off, and another component associated with several possible costs of forcing oscillations of the swing leg. For this second component, a cost based on the amount of force needed to oscillate the leg divided by the time duration of that force predicts the preferred speed-step length relationship much better than other costs, such as the amount of mechanical work done in swinging the leg. The cost of force/time models the need to recruit fast muscle fibers for large forces at short durations. The actual mechanical work performed by muscles on the swing leg appears to be of relatively less importance, although it appears to be minimized by the use of short bursts of muscle activity in near-isometric conditions. The combined minimization of toe-off mechanical work and force divided by time predicts the preferred speed-step length relationship.     

Humans exploit the biomechanics of bipedal gait during visually guided walking over complex terrain

How do humans achieve such remarkable energetic efficiency when walking over complex terrain such as a rocky trail? Recent research in biomechanics suggests that the efficiency of human walking over flat, obstacle-free terrain derives from the ability to exploit the physical dynamics of our bodies. In this study, we investigated whether this principle also applies to visually guided walking over complex terrain. We found that when humans can see the immediate foreground as little as two step lengths ahead, they are able to choose footholds that allow them to exploit their biomechanical structure as efficiently as they can with unlimited visual information. We conclude that when humans walk over complex terrain, they use visual information from two step lengths ahead to choose footholds that allow them to approximate the energetic efficiency of walking in flat, obstacle-free environments.     

Stresses exerted in the hindlimb muscles of common chimpanzees (Pan troglodytes) during bipedal locomotion

Recent studies have indicated that chimpanzee bipedality is mechanically inefficient and dynamically unlike that of humans, thus undermining the chimpanzee analogy for mechanical aspects of the early evolution of hominid bipedalism. This paper continues this theme by measuring the forces and stresses engendered by the muscles during bipedal locomotion, for an untrained chimpanzee and for data from chimpanzees which have been encouraged to walk bipedally, presented in the literature. Peak stresses in the triceps surae were lower for the untrained chimpanzee than for the trained subjects because during the late stance phase, when peak ankle moments occur, the centre of pressure of the ground reaction force on the foot of the untrained chimpanzee stayed close to the ankle joint. In contrast, for the trained subjects it moved closer to the toes, as in human bipedalism. Quadriceps and hip extensor stresses are approximately 30% larger for the untrained chimpanzee than for the trained subjects, because the trained chimpanzees walked with a more erect posture. These results may reflect the way in which muscles can develop in response to training, since research on humans has shown that muscle physiological cross-sectional area increases as a result of exercise, resulting in smaller stresses for a given muscle force. During a slow walk, untrained chimpanzees were found to exert far greater muscle stresses than humans do when running at moderate speed, particularly in the muscles that extend the hip, because of the bent-hip, bent-knee posture.     

Diagonal walking in captive infant vervet monkeys

The frequency of use of diagonal walking by three motorically immature vervet monkeys (Cercopithecus aethiops) is discussed. The subjects were housed at The Balcones Research Center (Austin, Texas) and ranged from 15–105 days of age. Mixed longitudinal data taken from film indicated that lateral walknig was the first gait employed by the infant vervets studied, but that diagonal walking completely replaced lateral walking by 81 days of age. These data were compared with those of captive infant Macaca mulatta, and feral infant Papio anubis. General agreement was found regarding onset and frequency of use of diagonal walking in the three genera.     

Peer separation in infant chimpanzees,a pilot study

Seven nursery reared chimpanzees (Pan troglodytes), 7.5 to 10.5 months of age, were studied to determine the effects of a short period of separation from their peers on behavior and heart rate. The chimpanzees were separated from their mothers at birth, and reared in the nursery in a group living environment. The experiment encompassed a 13-day period, including 4 days of normative baseline, 5 days during which three of the infants were separated and housed in isolation while the other four controls remained together, and 4 days in which all of the animals were reunited. Six quantified behavioral observations and five heart rate measurements were obtained daily. Following separation, the isolated infants were behaviorally agitated, and exhibited increases in vocalizations, rocking and self-clasp behaviors, as well as changes in facial expression including cry face, whimper face and pout face. Time spent locomoting decreased in all seven animals during the separation period. Agitated behavior in the separated and isolated infants alternated with stationary withdrawn behavior. Individual differences were prominent. Heart rate was notable by the generally poorly developed circadian rhythmicity throughout the 13 day period; significant HR changes did not appear otherwise associated with separation. Day three of separation appeared to represent a point of transition with stereotyped motor behaviors developing in the three isolated infants and in one control infant.     

Mother-infant relations and infant development in captive chimpanzees and orang-utans

Four chimpanzee (Pan troglodytes)mother—infant dyads and four orangutan (Pongo pygmaeus)motherinfant dyads were studied for the first 11 months of the infants’ lives. For both species, ventroventral contact and nipple contact decreased over time at a similar rate, but total contact decreased earlier in the orangutans and was 50% lower than for the chimpanzees at the end of the study. Social play between the mothers and the infants did not differ in frequency between the species, but orangutans played above the ground and chimpanzees on the ground. Solitary play differed in form between the species and, like social play, reflected their differences in arboreal and terrestrial proclivities. In addition, the orangutans engaged in solitary play considerably more frequently than the chimpanzees during the second half-year of life. The developmental differences in mother-infant contact and solitary play of these apes are consistent with the differences in their speciestypical social organization. The data may reflect, therefore, early development of species differences in the social and relatively solitary natures of chimpanzees and orangutans, respectively. An erratum to this article is available at .     

The metabolic cost of walking in humans, chimpanzees, and early hominins

Bipedalism is a defining feature of the hominin lineage, but the nature and efficiency of early hominin walking remains the focus of much debate. Here, we investigate walking cost in early hominins using experimental data from humans and chimpanzees. We use gait and energetics data from humans, and from chimpanzees walking bipedally and quadrupedally, to test a new model linking locomotor anatomy and posture to walking cost. We then use this model to reconstruct locomotor cost for early, ape-like hominins and for the A.L. 288 Australopithecus afarensis specimen. Results of the model indicate that hind limb length, posture (effective mechanical advantage), and muscle fascicle length contribute nearly equally to differences in walking cost between humans and chimpanzees. Further, relatively small changes in these variables would decrease the cost of bipedalism in an early chimpanzee-like biped below that of quadrupedal apes. Estimates of walking cost in A.L. 288, over a range of hypothetical postures from crouched to fully extended, are below those of quadrupedal apes, but above those of modern humans. These results indicate that walking cost in early hominins was likely similar to or below that of their quadrupedal ape-like forebears, and that by the mid-Pliocene, hominin walking was less costly than that of other apes. This supports the hypothesis that locomotor energy economy was an important evolutionary pressure on hominin bipedalism.     

The price of play: self-organized infant mortality cycles in chimpanzees

Chimpanzees have been used extensively as a model system for laboratory research on infectious diseases. Ironically, we know next to nothing about disease dynamics in wild chimpanzee populations. Here, we analyze long-term demographic and behavioral data from two habituated chimpanzee communities in Ta? National Park, C?te d'Ivoire, where previous work has shown respiratory pathogens to be an important source of infant mortality. In this paper we trace the effect of social connectivity on infant mortality dynamics. We focus on social play which, as the primary context of contact between young chimpanzees, may serve as a key venue for pathogen transmission. Infant abundance and mortality rates at Ta? cycled regularly and in a way that was not well explained in terms of environmental forcing. Rather, infant mortality cycles appeared to self-organize in response to the ontogeny of social play. Each cycle started when the death of multiple infants in an outbreak synchronized the reproductive cycles of their mothers. A pulse of births predictably arrived about twelve months later, with social connectivity increasing over the following two years as the large birth cohort approached the peak of social play. The high social connectivity at this play peak then appeared to facilitate further outbreaks. Our results provide the first evidence that social play has a strong role in determining chimpanzee disease transmission risk and the first record of chimpanzee disease cycles similar to those seen in human children. They also lend more support to the view that infectious diseases are a major threat to the survival of remaining chimpanzee populations.     

Back muscle function during bipedal walking in chimpanzee and gibbon: implications for the evolution of human locomotion

The evolution of erect posture and locomotion continues to be a major focus of interest among paleoanthropologists and functional morphologists. To date, virtually all of our knowledge about the functional role of the back muscles in the evolution of bipedalism is based on human experimental data. In order to broaden our evolutionary perspective on the vertebral region, we have undertaken an electromyographic (EMG) analysis of three deep back muscles (multifidus, longissimus thoracis, iliocostalis lumborum) in the chimpanzee (Pan troglodytes) and gibbon (Hylobates lar) during bipedal walking. The recruitment patterns of these three muscles seen in the chimpanzee closely parallel those observed in the gibbon. The activity patterns of multifidus and longissimus are more similar to each other than either is to iliocostalis. Iliocostalis recruitment is clearly related to contact by the contralateral limb during bipedal walking in both species. It is suggested that in both the chimpanzee and gibbon, multifidus controls trunk movement primarily in the sagittal plane, iliocostalis responds to and adjusts movement in the frontal plane, while longissimus contributes to both of these functions. In many respects, the activity patterns shared by the chimpanzee and gibbon are quite consistent with recent human experimental data. This suggests a basic similarity in the mechanical constraints placed on the back during bipedalism among these three hominoids. Thus, the acquisition of habitual bipedalism in humans probably involved not so much a major change in back muscle action or function, but rather an improvement in the mechanical advantages and architecture of these muscles.