Substrate alters forelimb to hindlimb peak force ratios in primates

It is often claimed that the walking gaits of primates are unusual because, unlike most other mammals, primates appear to have higher vertical peak ground reaction forces on their hindlimbs than on their forelimbs. Many researchers have argued that this pattern of ground reaction force distribution is part of a general adaptation to arboreal locomotion. This argument is frequently used to support models of primate locomotor evolution. Unfortunately, little is known about the force distribution patterns of primates walking on arboreal supports, nor do we completely understand the mechanisms that regulate weight distribution in primates. We collected vertical peak force data for seven species of primates walking quadrupedally on instrumented terrestrial and arboreal supports. Our results show that, when walking on arboreal vs. terrestrial substrates, primates generally have lower vertical peak forces on both limbs but the difference is most extreme for the forelimb. We found that force reduction occurs primarily by decreasing forelimb and, to a lesser extent, hindlimb stiffness. As a result, on arboreal supports, primates experience significantly greater functional differentiation of the forelimb and hindlimb than on the ground. These data support long-standing theories that arboreal locomotion was a critical factor in the differentiation of the forelimbs and hindlimbs in primates. This change in functional role of the forelimb may have played a critical role in the origin of primates and facilitated the evolution of more specialized locomotor behaviors.     

Quadrupedal locomotion in squirrel monkeys (Cebidae: Saimiri sciureus): a cineradiographic study of limb kinematics and related substrate reaction forces

Quadrupedal locomotion of squirrel monkeys on small-diameter support was analyzed kinematically and kinetically to specify the timing between limb movements and substrate reaction forces. Limb kinematics was studied cineradiographically, and substrate reaction forces were synchronously recorded. Squirrel monkeys resemble most other quadrupedal primates in that they utilize a diagonal sequence/diagonal couplets gait when walking on small branches. This gait pattern and the ratio between limb length and trunk length influence limb kinematics. Proximal pivots of forelimbs and hindlimbs are on the same horizontal plane, thus giving both limbs the same functional length. However, the hindlimbs are anatomically longer than the forelimbs. Therefore, hindlimb joints must be more strongly flexed during the step cycle compared to the forelimb joints. Because the timing of ipsilateral limb movements prevents an increasing amount of forelimb retraction, the forelimb must be more protracted during the initial stance phase. At this posture, gravity acts with long moment arms at proximal forelimb joints. Squirrel monkeys support most of their weight on their hindlimbs. The timing of limb movements and substrate reaction forces shows that the shift of support to the hindlimbs is mainly done to reduce the compressive load on the forelimb. The hypothesis of the posterior weight shift as a dynamic strategy to reduce load on forelimbs, proposed by Reynolds ([1985]) Am. J. Phys. Anthropol. 67:335-349; [1985] Am. J. Phys. Anthropol. 67:351-362), is supported by the high correlation of ratios between forelimb and hindlimb peak vertical forces and the range of motion of shoulder joint and scapula in primates.     

Locomotor mechanics of the slender loris (Loris tardigradus)

The quadrupedal walking gaits of most primates can be distinguished from those of most other mammals by the presence of diagonal-sequence (DS) footfall patterns and higher peak vertical forces on the hindlimbs compared to the forelimbs. The walking gait of the woolly opossum (Caluromys philander), a highly arboreal marsupial, is also characterized by diagonal-sequence footfalls and relatively low peak forelimb forces. Among primates, three species--Callithrix, Nycticebus, and Loris--have been reported to frequently use lateral-sequence (LS) gaits and experience relatively higher peak vertical forces on the forelimbs. These patterns among primates and other mammals suggest a strong association between footfall patterns and force distribution on the limbs. However, current data for lorises are limited and the frequency of DS vs. LS walking gaits in Loris is still ambiguous. To test the hypothesis that patterns of footfalls and force distribution on the limbs are functionally linked, kinematic and kinetic data were collected simultaneously for three adult slender lorises (Loris tardigradus) walking on a 1.25 cm horizontal pole. All subjects in this study consistently used diagonal-sequence walking gaits and always had higher peak vertical forces on their forelimbs relative to their hindlimbs. These results call into question the hypothesis that a functional link exists between the presence of diagonal-sequence walking gaits and relatively higher peak vertical forces on the hindlimbs. In addition, this study tested models that explain patterns of force distribution based on limb protraction angle or limb compliance. None of the Loris subjects examined showed kinematic patterns that would support current models proposing that weight distribution can be adjusted by actively shifting weight posteriorly or by changing limb stiffness. These data reveal the complexity of adaptations to arboreal locomotion in primates and indicate that diagonal-sequence walking gaits and relatively low forelimb forces could have evolved independently.     

Hip extensor EMG and forelimb/hind limb weight support asymmetry in primate quadrupeds

Higher weight support on the hind limb than forelimb is among the distinctive characteristics of primate quadrupeds. Although often assumed to be due to a more posteriorly positioned whole body center of mass, there are little data to support such a difference. Reynolds (1985. Am J Phys Anthropol 67:335-349) notes that the distribution of forces on the limbs can also be influenced by average limb posture, but suggests that this effect is too small to account for the asymmetry in weight support observed in primates. Instead, he proposes that high hind limb forces are brought about by an active process of shifting weight off the forelimbs and onto the hind limbs through use of hind limb retractors. In this study, we use video records of walking animals to explore the degree to which average limb posture in primates and other quadrupedal mammals deviates from vertical, and use electromyography to test Reynolds' model of hind limb retractor activity and posterior weight shift. The limb posture results indicate that primate forelimbs oscillate about a vertical or slightly retracted axis, and though the hind limbs are slightly protracted, the magnitude of deviation from vertical is too small to have a major effect on weight support distribution. The electromyographic results reveal higher levels of hip extensor activity in antipronograde primates that bear a higher proportion of weight on their hind limbs. This lends support to Reynolds' suggestion that some primates use muscles to actively shift weight onto hind limbs to relieve stresses on forelimbs less well structured for weight support.     

Stresses on the limbs of quadrupedal primates

Data is presented from eight primates on the ground reaction forces on the limbs during locomotion. These subjects supported from 30 to 45% of their body weight on their forelimbs. Other quadrupedal mammals support 55-60% of their body weight on their forelimbs. The increase of peak vertical force with speed varies greatly between the subjects. The variation in weight supported by the forelimbs and the peak forces on the forelimbs is proposed to correlate with variation in locomotor adaptations. It is suggested that the occurrence of bipedalism in primates represents the extreme expression of the tendency in primates to reduce the compressive forces on their forelimbs.     

Seasonal variation in body mass and locomotor kinetics of the fat-tailed dwarf lemur (Cheirogaleus medius)

The fat-tailed dwarf lemur (Cheirogaleus medius) is unusual among primates in storing large amounts of fat subcutaneously prior to hibernating during the winter months. In doing so, it increases its body mass by more than 50%, with a substantial weight gain in the tail. This seasonal increase in mass provides a unique natural experiment to examine how changes in body mass affect substrate reaction forces during locomotion. As body mass increases, it is expected that the limbs of the fat-tailed dwarf lemur will be subjected to greater peak vertical substrate reaction forces during quadrupedal walking. However, whether or not these peak substrate reaction forces will increase proportionally across forelimbs and hindlimbs as body mass increases is unknown. Substrate reaction forces were collected on four adult C. medius walking quadrupedally on a 28-mm pole attached to a force platform. Peak vertical substrate reaction forces (Vpk) (N) were analyzed and compared for a cross-sectional sample of different body masses (180-300 g). Forelimb and hindlimb Vpk were positively correlated with body mass, with hindlimb Vpk always higher than forelimb Vpk. However, the rate at which Vpk increased relative to body mass was higher for the hindlimb than the forelimb. This disproportion in weight distribution between the forelimbs and hindlimbs as body mass increases appears to be linked to the accumulation of fat in the tail. It is likely that storing fat in the tail region may shift the center of mass more caudally, from a more cranial position when the tail is thinner. Such a caudal shift of the center of mass-either morphological or dynamic-is believed to have played an important role in the functional differentiation of the limbs and the evolution of locomotor modes of several tetrapod groups, including dinosaurs and primates.     

Origins of primate locomotion: gait mechanics of the woolly opossum

The locomotion of primates differs from that of other mammals in three fundamental ways. During quadrupedal walking, primates use diagonal sequence gaits, protract their arms more at forelimb touchdown, and experience lower vertical substrate reaction forces on their forelimbs relative to their hindlimbs. It is widely held that the unusual walking gaits of primates represent a basal adaptation for movement on thin, flexible branches and reflect a major change in the functional role of the forelimb. However, little data on nonprimate arboreal mammals exist to test this notion. To that end, we examined the gait mechanics of the woolly opossum (Caluromys philander), a marsupial convergent with small-bodied prosimians in ecology, behavior, and morphology. Data on the footfall sequence, relative arm protraction, and peak vertical substrate reaction forces were obtained from videotapes and force records for three adult woolly opossums walking quadrupedally on a wooden runway and a thin pole. For all steps recorded on both substrates, woolly opossums always used diagonal sequence walking gaits, protracted their arms beyond 90 degrees relative to horizontal body axis, and experienced peak vertical substrate reaction forces on forelimbs that were significantly lower than on hindlimbs. The woolly opossum is the first nonprimate mammal to show locomotor mechanics that are identical to those of primates. This case of convergence between primates and a committed fine-branch, arboreal marsupial strongly implies that the earliest primates evolved gait specializations for fine-branch locomotion, which reflect important changes in forelimb function.     

Forelimb and hindlimb forces in walking and galloping primates

One trait that distinguishes the walking gaits of most primates from those of most mammalian nonprimates is the distribution of weight between the forelimbs and hindlimbs. Nonprimate mammals generally experience higher vertical peak substrate reaction forces on the forelimb than on the hindlimb. Primates, in contrast, generally experience higher vertical peak substrate reaction forces on the hindlimb than on the forelimb. It is currently unclear whether this unusual pattern of force distribution characterizes other primate gaits as well. The available kinetic data for galloping primates are limited and present an ambiguous picture about peak-force distribution among the limbs. The present study investigates whether the pattern of forelimb-to-hindlimb force distribution seen during walking in primates is also displayed during galloping. Six species of primates were video-recorded during walking and galloping across a runway or horizontal pole instrumented with a force-plate. The results show that while the force differences between forelimb and hindlimb are not significantly different from zero during galloping, the pattern of force distribution is generally the same during walking and galloping for most primate species. These patterns and statistical results are similar to data collected during walking on the ground. The pattern of limb differentiation exhibited by primates during walking and galloping stands in contrast to the pattern seen in most nonprimate mammals, in which forelimb forces are significantly higher. The data reported here and by Demes et al. ([1994] J. Hum. Evol. 26:353-374) suggest that a relative reduction of forelimb vertical peak forces is part of an overall difference in locomotor mechanics between most primates and most nonprimate mammals during both walking and galloping.     

Understanding hind limb weight support in chimpanzees with implications for the evolution of primate locomotion

Most quadrupedal mammals support a larger amount of body weight on their forelimbs compared with their hind limbs during locomotion, whereas most primates support more of their body weight on their hind limbs. Increased hind limb weight support is generally interpreted as an adaptation that reduces stress on primates' highly mobile forelimb joints. Thus, increased hind limb weight support was likely vital for the evolution of primate arboreality. Despite its evolutionary importance, the mechanism used by primates to achieve this important kinetic pattern remains unclear. Here, we examine weight support patterns in a sample of chimpanzees (Pan troglodytes) to test the hypothesis that limb position, combined with whole body center of mass position (COM), explains increased hind limb weight support in this taxon. Chimpanzees have a COM midway between their shoulders and hips and walk with a relatively protracted hind limb and a relatively vertical forelimb, averaged over a step. Thus, the limb kinematics of chimpanzees brings their feet closer to the COM than their hands, generating greater hind limb weight support. Comparative data suggest that these same factors likely explain weight support patterns for a broader sample of primates. It remains unclear whether primates use these limb kinematics to increase hind limb weight support, or whether they are byproducts of other gait characteristics. The latter hypothesis raises the intriguing possibility that primate weight support patterns actually evolved as byproducts of other traits, or spandrels, rather than as adaptations to increase forelimb mobility. Am J Phys Anthropol, 2009. © 2008 Wiley‐Liss, Inc.     

Hindlimb dominance during primate high-speed locomotion

Quadrupedal locomotion was mechanically studied for four species of primates, the chimpanzee, the rhesus macaque, the tufted capuchin, and the ring-tailed lemur, from low to high speeds of about two to ten times the anterior trunk length per second. A wide variety of locomotor patterns was observed during the high-speed locomotion of these primates. Positive correlations were observed between the peak magnitude of foot force components and speed. The differentiation of the foot force between the forelimb and the hindlimb did not largely change with a change of speed for each species. The vertical component and the accelerating component for the rhesus macaque were relatively large in the forelimb from low- to high-speed locomotion. The rhesus macaque, which habitually locomotes on the ground, differed in the quadrupedal locomotion from the other relatively arboreal primates, for which the hindlimb was clearly dominant in their dynamic force-producing distribution between the forelimbs and the hindlimbs. The previously reported locomotor difference, which was indicated among primates from the foot force pattern between the forelimb and the hindlimb during walking, also applied to high-speed locomotion.     

Inefficient use of inverted pendulum mechanism during quadrupedal walking in the Japanese macaque

In animal walking, the gravitational potential and kinetic energy of the center of mass (COM) fluctuates out-of-phase to reduce the energetic cost of locomotion via an inverted pendulum mechanism, and, in canine quadrupedal walking, up to 70% of the mechanical energy can be recovered. However, the rate of energy recovery for quadrupedal walking in primates has been reported to be comparatively lower. The present study analyzed fluctuations in the potential and kinetic energy of the COM during quadrupedal walking in the Japanese macaque to clarify the mechanisms underlying this inefficient utilization of the inverted pendulum mechanism in primates. Monkeys walked on a wooden walkway at a self-selected speed, and ground reaction forces were measured, using a force platform, to calculate patterns of mechanical energy fluctuation and rates of energy recovery. Our results demonstrated that rates of energy recovery for quadrupedal walking in Japanese macaques were approximately 30–50%, much smaller than those reported for dogs. Comparisons of the patterns of mechanical energy fluctuation suggested that the potential and kinetic energies oscillated relatively more in-phase, and amplitudes did not attain near equality during quadrupedal walking in Japanese macaques, possibly because of greater weight support (reaction force) of the hindlimbs and more protracted forelimbs at touchdown in the Japanese macaque, two of the three commonly accepted locomotor characteristics distinguishing primates from non-primate mammals.     

Enthesis bilateral asymmetry in humans and African apes

Entheses (skeletal muscle and tendon attachment sites) have often been used to infer handedness and activity variability among human populations. However, the specific roles that intensity vs. frequency of muscle contractions play in modifying entheses are not well understood and the assumption that entheses reflect muscle activity levels has been challenged. This study explores the effect of habitual muscular activity on enthesis morphology in humans and African apes by investigating bilateral asymmetry in the forelimbs and hindlimbs of these taxa. Humans have generally more developed entheses in the lower limb while African apes have generally more developed entheses in the forelimbs. All species studied have more asymmetric forelimbs than hindlimbs except humans that show more asymmetrical expression of bony spurs in the lower limbs than in the upper limbs. When comparing species, humans are always more asymmetric in ethesis development than apes for both the forelimbs and hindlimbs, which reflects the relatively greater asymmetry in limb use in humans and the more symmetric use in apes. Enthesis development may reflect cross-symmetry patterns in humans and, more subtly, a moderate handedness in apes during manipulative activities. This study suggests that enthesis morphology provides information on muscle activity levels, with greater development of entheses associated with more habitual or powerful muscle use. The general similarity of ape and human responses to muscle activity suggests that muscle activity influenced enthesis development in Plio-Pleistocene hominins and that interpretation of muscle markings in these fossils can provide data for functional inferences in these extinct species.     

Limb joint surface areas and their ratios in Malagasy lemurs and other mammals

Surface areas of humeral and femoral heads scale largely as a function of body size. However, differences in the relative sizes of these articular surfaces are correlated with differential joint mobility and force transmission through fore- and hindlimbs. They can therefore assist interpretation of the positional behavior of extinct species. In this paper, we document variation in ratios of humeral head surface area to femoral head surface area among extant primates and other mammals. We then examine a group of extinct primates: the subfossil lemurs of Madagascar. Many Malagasy le murs, including some giant extinct species with very long forelimbs and short hindlimbs, have relatively small humeral heads and large femoral heads. We explore the adaptive implications of this pattern. © 1995 Wiley-Liss, Inc.     

Ontogeny of limb force distribution in squirrel monkeys (Saimiri boliviensis): insights into the mechanical bases of primate hind limb dominance

The distribution of peak vertical forces between the forelimbs and the hind limbs is one of the key traits distinguishing primate quadrupedal locomotion from that of other mammals. Whereas most mammals generate greater peak vertical forelimb forces, primates generate greater peak vertical hind limb forces. At the ultimate level, hind limb dominance in limb force distribution is typically interpreted as an adaptation to facilitate fine-branch arboreality. However, the proximate biomechanical bases for primate limb force distribution remain controversial. Three models have been previously proposed. The Center of Mass (COM) Position model attributes primates’ unique mode of limb loading to differences in the position of the whole-body COM relative to the hands and feet. The Active Weight Shift model asserts that primates actively redistribute body weight to their hind limbs by pitching the trunk up via the activation of hind limb retractor muscles. Finally, the Limb Compliance model argues that primates selectively mitigate forelimb forces by maintaining a compliant forelimb and a flat shoulder trajectory. Here, a detailed dataset of ontogenetic changes in morphology and locomotor mechanics in Bolivian squirrel monkeys (Saimiri boliviensis) was employed as a model system to evaluate each of these proposed models in turn. Over the first 10 months of life, squirrel monkeys transitioned from forelimb dominant infants to hind limb dominant juveniles, a change that was precipitated by decreases in peak vertical forelimb forces and increases in peak vertical hind limb forces. Results provided some support for all three of the models, although the COM Position and Active Weight Shift models were most strongly supported by the data. Overall, this study suggests that primates may use a variety of biomechanical strategies to achieve hind limb dominance in limb force distribution.     

Morphological conservation of limb natural pendular period in the domestic dog (Canis familiaris): Implications for locomotor energetics

For better understanding of the links between limb morphology and the metabolic cost of locomotion, we have characterized the relationships between limb length and shape and other functionally important variables in the straightened forelimbs and hindlimbs of a sample of 12 domestic dogs (Canis familiaris). Intra-animal comparisons show that forelimbs and hindlimbs are very similar (not significantly different) in natural pendular period (NPP), center-of-mass, and radius of gyration, even though they differ distinctly in mass, length, moment-of-inertia, and other limb proportions. The conservation of limb NPP, despite pronounced dissimilarity in other limb characteristics, appears to be the result of systematic differences in shape, forelimbs tending to be cylindrical and hindlimbs conical. Estimating limb NPP for other species from data in the literature on segment inertia and total limb length, we present evidence that the similarity between forelimbs and hindlimbs in NPP is generally true for mammals across a large size range. Limbs swinging with or near their natural pendular periods will maximize within-limb pendular exchange of potential and kinetic energy. As all four limbs of moderate- and large-size animals swing with the same period during walking, maximal advantage can be derived from the pendular exchange of energy only if forelimbs and hindlimbs are very similar in NPP. We hypothesize that an important constraint in the evolution of limb length and shape is the locomotor economy derived from forelimbs and hindlimbs of similar natural pendular period. J. Morphol. 234:183–196, 1997. © 1997 Wiley-Liss, Inc.     

First record of a pterosaur landing trackway

The terrestrial progression of pterosaurs, the flying reptiles of the Mesozoic Era, has been debated for over two centuries. The recent discovery of quadrupedal pterodactyloid pterosaur tracks from Late Jurassic sediments near Crayssac, France, shows that the hindlimbs moved parasagittally, as in mammals, birds and other dinosaurs, and the hypertrophied forelimbs could make tracks both close to the body wall and far outside it. Their manus tracks are unique in form, position and kinematics, which would be expected because the forelimbs were used for flight. Here, we report the first record of a pterosaur landing track, which differs substantially from typical walking trackways. The individual landed on both hind feet in parallel fashion, dragged its toes slightly as it left the track, landed again almost immediately and placed the hindfeet parallel again, then placed its forelimbs on the ground, took another short step with both hindlimbs and adjusted its forelimbs, and then began to walk off normally. The trackway shows that pterosaurs stalled to land, a reflection of their highly developed capacity for flight control and manoeuverability.     

Biomechanical analysis of vertical climbing in the spider monkey and the Japanese macaque

Climbing is one of the most important components of primate locomotor modes. We previously reported that the kinesiological characteristics of vertical climbing by the spider monkey and Japanese macaque are clearly different, based on their kinetics and kinematics. In this study, a more detailed analysis using inverse dynamics was conducted to estimate the biomechanical characteristics of vertical climbing in the spider monkey and Japanese macaque. One of the main findings was the difference in forelimb use by the two species. The results of a joint moment analysis and estimates of muscular force indicate that the spider monkey uses its forelimbs to keep the body close to the substrate, rather than to generate propulsion. The forelimb of the Japanese macaque, on the other hand, likely contributes more to propulsion. This supports the idea that "forelimb-hindlimb differentiation" is promoted in the spider monkey. The estimated muscular force also suggests that the spider monkey type of climbing could develop the hindlimb extensor muscles, which are important in bipedal posture and walking. As a result, we conclude that the spider monkey type of climbing could be functionally preadaptive for human bipedalism. This type of climbing would develop the hip and knee extensor muscles, and result in more extended lower limb joints, a more erect trunk posture, and more functionally differentiated fore- and hindlimbs, all of which are important characteristics of human bipedalism.     

Mechanics of torque generation during quadrupedal arboreal locomotion

Quadrupedal animals moving on arboreal substrates face unique challenges to maintain stability. The torque generated by the limbs around the long axis of a branch during locomotion may clarify how the animals remain stable on arboreal supports. We sought to determine what strategy gray short-tailed opossums (Monodelphis domestica) use to exert torque and avoid toppling. The opossums moved across a branch trackway about half the diameter of their bodies. Part of the trackway was instrumented to measure substrate reaction forces and torque around the long axis of the branch. Kinematic analysis was used to estimate the center of pressure of the manus and pes; from center of pressure and vertical and mediolateral forces, the torque generated by substrate reaction forces versus muscular effort could be determined. Forelimbs generated significantly greater torque than hindlimbs, which is probably explained by the greater weight-bearing role of the forelimbs. Fore- and hindlimbs generated torque in opposite directions because contralateral fore- and hindlimbs typically contacted the branch. Torque generated by muscular effort, however, was often in the same direction in both fore- and hindlimbs. The muscle-generated torque is likely the result of mediolateral movement of the center of mass caused by mediolateral undulation of the torso. These results bear an important implication for the study of arboreal locomotion: center of mass dynamics are at least as important as static positions. M. domestica is a good representative for a primitive mammal, and comparisons with arboreal specialists will shed light on how proficient arboreal locomotion evolved.     

A model for nonexercising hindlimb muscles in exercising animals

Nonexercising muscles appear to be metabolically active during exercise. Animal models for this purpose have not been established. However, we have been able to teach animals to run on their forelimbs while their hindlimbs are suspended above the treadmill with no visible limb movement. To document that indeed this mode of exercise does not provoke additional muscle activity, we have compared the levels of neural activation of the soleus and plantaris muscles using a computer analysis of the electromyographic interference pattern, recorded from bipolar fine wire electrodes implanted across each muscle. Via computer analyses of the electromyographic interference patterns the frequencies and amplitudes of motor unit action potentials were obtained. The data were sampled during 20 s of every minute of observation. Comparisons were made in four conditions: (i) resting on the treadmill while bearing weight on the hindlimbs (normal rest), (ii) running on the treadmill (15 m/min, 8% grade) on all four limbs (normal exercise), (iii) resting while the hindlimbs were suspended in a harness above the treadmill (suspended rest), and (iv) exercising with the forelimbs (15 m/min, 8% grade) while the hindlimbs were suspended above the treadmill (suspended exercise). All four experimental conditions were carried out for 90 min each and were performed by each animal. The results clearly show that muscle activities (frequencies and amplitudes), when the hindlimbs are suspended above the treadmill, at rest or during exercise, are lower than the activities in these same muscles when the animals are at rest, supporting only their body weight. Activities in the same muscles during exercise were from 300 to 2000% greater than during hindlimb suspension.(ABSTRACT TRUNCATED AT 250 WORDS)     

A radiographic study of the fetus in late pregnancy and during foaling.

The position, posture and presentation of the fetus were studied by serial radiography of the abdomen in 18 crossbred Pony mares near term and during first- and second-stage labour. In 3 mares the fetal position was assessed before and after induction of parturition with the synthetic prostaglandin, fluprostenol. In late gestation and up to the time of first-stage labour the fetus lay in ventral position with the forelimbs and poll flexed or partly flexed. At this time fetal movements were confined to flexion and extension of neck and forelimbs, but at parturition the head and limbs gradually extended the the forelimbs, head and neck rotated so that dorsal position and cranial extension were achieved. From this position, with the forelimbs and muzzle engaged in the cervical canal, delivery was quickly effected. The trunk and hindlimbs came into dorsal position during second-stage labour. The mechanics of these fetal movements and their relation to causes of dystocia are discussed.