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.     

Origin of bipedalism

Human and chimpanzee locomotor behaviors are described and compared using field patterns derived from measurements of the motions at the joints. Field patterns of human and ape bipedalism are so different that it is doubted whether the nonhuman type could ever have been a precursor of the human type. Chimpanzee quadrupedal vertical climbing and human bipedalism are, on the other hand, similar and a particular variety of this kind of climbing probably was the precursor of human bipedalism. Animals adapted to this variation would have had some brachiation-like morphological traits in their pectoral limbs and some hominid-like morphological traits in their pelvic limbs, traits anticipating the human condition. The australopithecines possessed these traits and must have been adapted to arboreal quadrupedal vertical climbing, having the capacity, at the same time, to perform facultative terrestrial bipedalism, moving on the ground in a manner visually identical to that of humans.     

Do highly trained monkeys walk like humans? A kinematic study of bipedal locomotion in bipedally trained Japanese macaques

In this study, we examined the kinematics of bipedal walking in macaque monkeys that have been highly trained to stand and walk bipedally, and compared them to the kinematics of bipedal walking in ordinary macaques. The results revealed that the trained macaques walked with longer and less frequent strides than ordinary subjects. In addition, they appear to have used inverted pendulum mechanics during bipedal walking, which resulted in an efficient exchange of potential and kinetic energy. These gait characteristics resulted from the relatively more extended hindlimb joints of the trained macaques. By contrast, the body of the ordinary macaques translated downward during the single-limb stance phase due to more flexed hindlimb joints. This resulted in almost in-phase fluctuations of potential and kinetic energy, which indicated that energy transformation was less efficient in the ordinary macaques. The findings provide two insights into the early stage of the evolution of human bipedalism. First, the finding that training considerably improved bipedal walking a posteriori may explain why the very first bipeds that might not yet have been morphologically adapted to bipedal walking continued to walk bipedally. The evolutionary transition from quadrupedalism to bipedalism might not be as difficult as has been envisioned. In addition, the finding that macaques, which are phylogenetically distant from humans and in which bipedal walking is unlike human walking, could develop humanlike gait characteristics with training, provides strong support for the commonly held but unproven idea that the characteristics of the human gait are advantageous to human bipedalism.     

Bipedal tool use strengthens chimpanzee hand preferences

The degree to which non-human primate behavior is lateralized, at either individual or population levels, remains controversial. We investigated the relationship between hand preference and posture during tool use in chimpanzees (Pan troglodytes) during bipedal tool use. We experimentally induced tool use in a supported bipedal posture, an unsupported bipedal posture, and a seated posture. Neither bipedal tool use nor these supported conditions have been previously evaluated in apes. The hypotheses tested were 1) bipedal posture will increase the strength of hand preference, and 2) a bipedal stance, without the use of one hand for support, will elicit a right hand preference. Results supported the first, but not the second hypothesis: bipedalism induced the subjects to become more lateralized, but not in any particular direction. Instead, it appears that subtle pre-existing lateral biases, to either the right or left, were emphasized with increasing postural demands. This result has interesting implications for theories of the evolution of tool use and bipedalism, as the combination of bipedalism and tool use may have helped drive extreme lateralization in modern humans, but cannot alone account for the preponderance of right-handedness.     

Trabecular bone ontogeny in the human proximal femur

Ontogenetic changes in the human femur associated with the acquisition of bipedal locomotion, especially the development of the bicondylar angle, have been well documented. The purpose of this study is to quantify changes in the three-dimensional structure of trabecular bone in the human proximal femur in relation to changing functional and external loading patterns with age. High-resolution X-ray computed tomography scan data were collected for 15 juvenile femoral specimens ranging in age from prenatal to approximately nine years of age. Serial slices were collected for the entire proximal femur of each individual with voxel resolutions ranging from 0.017 to 0.046 mm depending on the size of the specimen. Spherical volumes of interest were defined within the proximal femur, and the bone volume fraction, trabecular thickness, trabecular number, and fabric anisotropy were calculated in three dimensions. Bone volume fraction, trabecular number, and degree of anisotropy decrease between the age of 6 months and 12 months, with the lowest values for these parameters occurring in individuals near 12 months of age. By age 2-3 years, the bone volume, thickness, and degree of anisotropy increase slightly, and regions in the femoral neck become more anisotropic corresponding to the thickening of the inferior cortical bone of the neck. These results suggest that trabecular structure in the proximal femur reflects the shift in external loading patterns associated with the initiation of unassisted walking in infants.     

Functional implications of variation in lumbar vertebral count among hominins

As early as the 1970s, Robinson defined lumbar vertebrae according to their zygapophyseal orientation. He identified six lumbar elements in fossil Sts 14 Australopithecus africanus, one more than is commonly present in modern humans. It is now generally inferred that the modal number of lumbar vertebrae for australopiths and early Homo was six, from which the mode of five in later Homo is derived. The two central questions this study investigates are (1) to what extent do differences in human lumbar vertebral count affect lordotic shape and lumbar function, and (2) what does lumbar number variation imply about lumbar spine function in early hominins? To address these questions, I first outline a biomechanical model of lumbar number effect on lordotic function. I then identify relevant morphological differences in the human modal and extra-modal variants, which I use to test the model. These tests permit evaluation of the human L6 variant as a model for reconstructing early hominin modal number and spine function. Application of the biomechanical model in reconstructing australopith/early Homo lumbar spines highlights shared principles of Euler column strength and sagittal spine flexibility among early and modern hominins. Within modern humans, the extra-modal L6 variant has an extended series of three cranially positioned kyphotic vertebrae and strongly oblique zygapophyseal facets at the last lumbar level. Although they share the same radius and length of lumbar curvature, the L6 variant differs functionally from the L5 mode in its expanded range of sagittal flexion/extension and enhanced resistance to shear. Given the modal number of six lumbar vertebrae in australopiths and early Homo, lumbar spine mobility and strength would have been key properties of vertebral function in early bipeds whose upper and lower body segments were coupled by close approximation of the thorax and iliac crests.     

New postcranial fossils of Australopithecus afarensis from Hadar, Ethiopia (1990-2007)

Renewed fieldwork at Hadar, Ethiopia, from 1990 to 2007, by a team based at the Institute of Human Origins, Arizona State University, resulted in the recovery of 49 new postcranial fossils attributed to Australopithecus afarensis. These fossils include elements from both the upper and lower limbs as well as the axial skeleton, and increase the sample size of previously known elements for A. afarensis. The expanded Hadar sample provides evidence of multiple new individuals that are intermediate in size between the smallest and largest individuals previously documented, and so support the hypothesis that a single dimorphic species is represented. Consideration of the functional anatomy of the new fossils supports the hypothesis that no functional or behavioral differences need to be invoked to explain the morphological variation between large and small A. afarensis individuals. Several specimens provide important new data about this species, including new vertebrae supporting the hypothesis that A. afarensis may have had a more human-like thoracic form than previously appreciated, with an invaginated thoracic vertebral column. A distal pollical phalanx confirms the presence of a human-like flexor pollicis longus muscle in A. afarensis. The new fossils include the first complete fourth metatarsal known for A. afarensis. This specimen exhibits the dorsoplantarly expanded base, axial torsion and domed head typical of humans, revealing the presence of human-like permanent longitudinal and transverse arches and extension of the metatarsophalangeal joints as in human-like heel-off during gait. The new Hadar postcranial fossils provide a more complete picture of postcranial functional anatomy, and individual and temporal variation within this sample. They provide the basis for further in-depth analyses of the behavioral and evolutionary significance of A. afarensis anatomy, and greater insight into the biology and evolution of these early hominins.     

A hominine hip bone, KNM-ER 3228, from East Lake Turkana, Kenya

A male hominine partial hip bone, KNM -ER 3228, from East Lake Turkana , Kenya is described. In most of its features this specimen resembles modern human male hip bones. This is especially true for functional features related to weight transfer from the trunk to the pelvis and within the pelvis, and to the effective action of musculature arising from the pelvis during the performance of the modern human type of bipedalism . KNM -ER 3228 is very similar to the Olduvai Hominid 28 and the Arago XLIV hip bones, both attributed to Homo erectus .     

Mechanics of increased support of weight by the hindlimbs in primates

Quadrupedal primates support most of their weight on their hindlimbs during locomotion. Neither the position of their center of gravity nor the average position of their foot contacts is substantially different from that of other quadrupeds supporting most of their weight on their forelimbs. Arguments are presented to support the theory that high levels of hindlimb retractor activity will produce this shift of support to the hindlimbs. If this muscular activity is appropriately timed, it will generate only low horizontal accelerations, which can be offset by small changes in the average position of the limbs. Estimates of muscular force are derived from force plate and kinematic data, which indicate that primates in fact do exhibit the postulated pattern of muscular activity. It is suggested that this shift occurs to reduce the compressive forces on the forelimbs.     

Metatarsal robusticity in bipedal rats

All metatarsals have a significantly greater robusticity in the male than in the female rat. The robusticity formula of the rat's foot is 1 > 5 > 2 > 3 > 4. In bipedal rats that formula remains unchanged, but the robusticity of the metatarsals is increased especially in females. The tripod arrangement of the human foot with its particular robustness of the marginal metatarsals 1 and 5 and a strong calcaneum has been related to upright posture. The similar robusticity pattern in the rat's marginal metatarsals 1 and 5 raises the question of whether that part of the formula might not represent a more general plantigrade pattern.