Quantitative Analysis of the Deltoid and Rotator Cuff Muscles in Humans and Great Apes

The shoulder is one of the anatomic regions differentiating orthograde primates (gibbons, orangutans, gorillas, chimpanzees, bonobos, and humans) from the rest of the pronograde primates. Orthograde primates are characterized by a dorsal position of the scapula and a more lateral orientation of the glenoid cavity. This anatomic pattern, together with adaptations in related osteological structures and muscles, serves to facilitate the elevation of the upper extremity in the scapular plane. We quantified the proportions of the muscles comprising the principal functional and stabilizing components of the glenohumeral joint —deltoid, subscapularis, supraspinatus, infraspinatus, and teres minor— in 3 species of orthograde primates: Pongo pygmaeus, Pan troglodytes, and Homo sapiens. Our objective was to determine whether quantifiable differences in these muscles relate to the functional requirements of the types of locomotion used by these 3 species: suspension/vertical climbing, knuckle-walking, and bipedalism. We observed a close similarity between the proportional mass of these muscles in Homo sapiens and Pongo pygmaeus, whereas Pan troglodytes displayed a unique anatomic pattern, particularly in the subscapularis, which may be due to differences in how the glenohumeral joint is stabilized in a great ape knuckle-walker. Our findings may help explain the high incidence of subacromial impingement syndrome in humans.     

Evolution of the hominoid vertebral column: The long and the short of it

The postcranial axial skeleton exhibits considerable morphological and functional diversity among living primates. Particularly striking are the derived features in hominoids that distinguish them from most other primates and mammals. In contrast to the primitive catarrhine morphotype, which presumably possessed an external (protruding) tail and emphasized more pronograde trunk posture, all living hominoids are characterized by the absence of an external tail and adaptations to orthograde trunk posture. Moreover, modern humans evolved unique vertebral features that satisfy the demands of balancing an upright torso over the hind limbs during habitual terrestrial bipedalism. Our ability to identify the evolutionary timing and understand the functional and phylogenetic significance of these fundamental changes in postcranial axial skeletal anatomy in the hominoid fossil record is key to reconstructing ancestral hominoid patterns and retracing the evolutionary pathways that led to living apes and modern humans. Here, we provide an overview of what is known about evolution of the hominoid vertebral column, focusing on the currently available anatomical evidence of three major transitions: tail loss and adaptations to orthograde posture and bipedal locomotion.     

An electromyographic study of the pectoralis major in Atelines and Hylobates,with special reference to the Evolution of a pars clavicularis

Among primates there is striking variation in the extent of the origin of pectoralis major from the clavicle. A significant clavicular attachment (pars clavicularis) occurs only in Alouatta, Lagothrix, Hylobates, Pan (troglodytes, paniscus and gorilla), and Homo. Interpreting this trait in nonhuman primates as an adaptation to frequent use of a mobile forelimb in climbing and suspension is contraindicated by the absence of a clavicular origin in Ateles and Pongo. We have undertaken a telemetered electromyographic study to determine any special role of the most cranial part of the pectoralis major in comparison to its caudal part, and to the deltoid, during vertical climbing, pronograde quadrupedalism, and armswinging in Ateles, Lagothrix, Alouatta, and Hylobates. The results show that the cranial pectoralis major possesses a role not shared by the caudal fibers: initiation of the recovery phase of the locomotor cycle. When ability to execute rapid or powerful recovery of the adducted forelimb is required in an animal with a shoulder joint lying on a plane cranial to that of the manubrium, the movement will be facilitated if the origin of the pectoralis major is extended onto the clavicle. Such is the case in nonhuman primates possessing this trait. The absence of a clavicular origin in Ateles and Pongo may be related to diminished selective pressures to perfect locomotor modes such as pronograde quadrupedalism, armswinging, or climbing thick vertical trunks, that demand rapid or powerful recovery of the adducted forelimb. If the arboreal ancestor of humans had evolved a clavicular origin of pectoralis major, this animal would be preadapted for certain uses of the forelimb in its bipedal descendant.     

Foramen magnum position in bipedal mammals

The anterior position of the human foramen magnum is often explained as an adaptation for maintaining balance of the head atop the cervical vertebral column during bipedalism and the assumption of orthograde trunk postures. Accordingly, the relative placement of the foramen magnum on the basicranium has been used to infer bipedal locomotion and hominin status for a number of Mio-Pliocene fossil taxa. Nonetheless, previous studies have struggled to validate the functional link between foramen magnum position and bipedal locomotion. Here, we test the hypothesis that an anteriorly positioned foramen magnum is related to bipedalism through a comparison of basicranial anatomy between bipeds and quadrupeds from three mammalian clades: marsupials, rodents and primates. Additionally, we examine whether strepsirrhine primates that habitually assume orthograde trunk postures exhibit more anteriorly positioned foramina magna compared with non-orthograde strepsirrhines. Our comparative data reveal that bipedal marsupials and rodents have foramina magna that are more anteriorly located than those of quadrupedal close relatives. The foramen magnum is also situated more anteriorly in orthograde strepsirrhines than in pronograde or antipronograde strepsirrhines. Among the primates sampled, humans exhibit the most anteriorly positioned foramina magna. The results of this analysis support the utility of foramen magnum position as an indicator of bipedal locomotion in fossil hominins.     

New characters for the functional interpretation of primate scapulae and proximal humeri

The study of muscle function in nonhuman primates through the technique of electromyography (EMG) has facilitated the identification of specific functional roles for muscles in particular behaviors. This has led to a more complete understanding of the biomechanics of certain regions of the musculoskeletal system, and should facilitate our ability to identify morphological features useful in the functional interpretation of fossil material. The current paper represents one such investigation of a new set of morphometric characters of the scapula and proximal humerus suggested by EMG analyses of shoulder muscle function. A set of new metric variables were examined on the scapulae and proximal humeri of 25 species of extant anthropoid primates, as well as on casts of scapulae and humeri of three fossil primate taxa. The variables are primarily related to the line of action and attachments of the rotator cuff muscles. The position of the scapular spine, the degree of lateral expansion of the subscapular fossa, the size and shape of the subscapularis insertion facet on the lesser tubercle, and the orientation of the infraspinatus insertion facet on the greater tubercle all appear to successfully sort the extant taxa into locomotor groups. Their appearance on the fossil specimens generally supports previous functional interpretations of each taxon's locomotor abilities based on a variety of other characters, suggesting that these traits are equally applicable to fossil material. © Wiley-Liss, Inc.     

Functional aspects of strepsirrhine lumbar vertebral bodies and spinous processes

The relationship between form and function in the lumbar vertebral column has been well documented among platyrrhines and especially catarrhines, while functional studies of postcranial morphology among strepsirrhines have concentrated predominantly on the limbs. This morphometric study investigates biomechanically relevant attributes of the lumbar vertebral morphology of 20 species of extant strepsirrhines. With this extensive sample, our goal is to address the influence of positional behavior on lumbar vertebral form while also assessing the effects of body size and phylogenetic history. The results reveal distinctions in lumbar vertebral morphology among strepsirrhines in functional association with their habitual postures and primary locomotor behaviors. In general, strepsirrhines that emphasize pronograde posture and quadrupedal locomotion combined with leaping (from a pronograde position) have the relatively longest lumbar regions and lumbar vertebral bodies, features promoting sagittal spinal flexibility. Indrids and galagonids that rely primarily on vertical clinging and leaping with orthograde posture share a relatively short (i.e., stable and resistant to bending) lumbar region, although the length of individual lumbar vertebral bodies varies phylogenetically and possibly allometrically. The other two vertical clingers and leapers, Hapalemur and Lepilemur, more closely resemble the pronograde, quadrupedal taxa. The specialized, suspensory lorids have relatively short lumbar regions as well, but the lengths of their lumbar regions are influenced by body size, and Arctocebus has dramatically longer vertebral bodies than do the other lorids. Lumbar morphology among galagonids appears to reflect a strong phylogenetic signal superimposed on a functional one. In general, relative length of the spinous processes follows a positively allometric trend, although lorids (especially the larger-bodied forms) have relatively short spinous processes for their body size, in accordance with their positional repertoire. The results of the study broaden our understanding of postcranial adaptation in primates, while providing an extensive comparative database for interpreting vertebral morphology in fossil primates.     

Do nonhuman primates comprise appropriate experimental models for studying the function of human leukocyte antigen-G?

The expression and function of the human major histocompatibility complex (MHC) class Ia genes, human leukocyte antigen (HLA)-A, -B, and -C, is well-established; they are expressed in most nucleated cells and present endogenous peptides to CD8+ T cells. However, MHC class Ib genes are poorly characterized and have unknown functions. In humans, the best-characterized class Ib gene is HLA-G. This gene has a restricted tissue expression of the mRNA and a unique pattern of protein expression; it is expressed mainly in the extravillous cytotrophoblast cells in the placenta. The function of HLA-G is not clear, but its presence at the maternal-fetal interface suggests a role in protection of the semiallogeneic fetus. Whereas functional studies using in vitro models and transgenic mice provide useful insights regarding the potential function of this molecule, in vivo studies cannot be performed in humans. Nonhuman primates that are closely related to humans phylogenetically contain homologues of HLA-G. The MHC-G loci in nonhuman primates appear to have diverged from the human HLA-G. However, in the rhesus monkey (Macaca mulatta) and olive baboon (Papio anubis), a novel class Ia-related locus has been described. This gene encodes glycoproteins with characteristics that resemble those of HLA-G, including restricted tissue distribution, alternative splicing of mRNA, truncated cytoplasmic domain, and limited polymorphism. Thus, this molecule may be the functional homologue of HLA-G, and these two species may comprise appropriate models for elucidating the function of HLA-G.     

A comparison of the ontogeny of shape variation in the anthropoid scapula: functional and phylogenetic signal

This article compares ontogenetic shape variation in the scapula of 17 anthropoid species using three-dimensional landmark-based geometric morphometrics. These data are used to investigate functional and phylogenetic signal in the major components of scapular variation and to evaluate the degree to which postnatal growth contributes to interspecific differences in shape. Results indicate that the shape of the infant and adult scapula is primarily associated with positional behavior (e.g., orthograde suspensory nonquadrupeds versus pronograde quadrupeds), but within this functional structure there is phylogenetic signal, particularly at infant stages. Growth most closely correlates with infant/adult shape and locomotor function. These results suggest that the shape of the infant scapula drives the pattern of postnatal scapular growth and adult morphology. As such, variation in postnatal growth is not the primary source of interspecific variation in adult shape. Instead, interspecific differences in scapular morphology are hypothesized to be the result of selection for variation in embryonic developmental processes that affect shape.     

Fast myosin heavy chain expression during the early and late embryonic stages of chicken skeletal muscle development

The development of embryonic skeletal muscles in the chick can be divided into two periods of fiber specialization--an early one during which the different muscles of the limb are formed and an initial round of fiber specialization occurs and a late or fetal period during which there is extensive growth of this previously established fiber pattern. This latter period of growth is dependent on the establishment and maintenance of functional neuromuscular contacts. As has been described for other developmental stages, we show here that there are different embryonic fast skeletal muscle myosin heavy chain (MHC) isoforms expressed during the different embryonic periods of muscle growth. The identification of these isoforms was based on differences in their reactivity with various fast MHC monoclonal antibodies and on their different peptide banding patterns. The in ovo accumulation of the late embryonic MHC isoform pattern was similar to the time course of the previously described changes in alpha-actin and troponin T isotype switching during embryogenesis. The appearances of the late embryonic isoforms were blocked by chronic treatment with the neuromuscular blocking agent, d-tubocurarine, and cell cultures of embryonic chicken skeletal muscle which differentiated in the absence of motorneurons expressed little of the late embryonic isoform, indicating that the expression of the late embryonic isoform was dependent on functional nerve-muscle interactions. These different embryonic fast MHC isoforms provide important markers for monitoring the progression of muscle through its embryonic stages and its interaction with motorneurons.     

Apparent density of the primate calcaneo‐cuboid joint and its association with locomotor mode,foot posture,and the “midtarsal break”

Primates use a range of locomotor modes during which they incorporate various foot postures. Humans are unique compared with other primates in that humans lack a mobile fore‐ and midfoot. Rigidity in the human foot is often attributed to increased propulsive and stability requirements during bipedalism. Conversely, fore‐ and midfoot mobility in nonhuman primates facilitates locomotion in arboreal settings. Here, we evaluated apparent density (AD) in the subchondral bone of human, ape, and monkey calcanei exhibiting different types of foot loading. We used computed tomography osteoabsorptiometry and maximum intensity projection (MIP) maps to visualize AD in subchondral bone at the cuboid articular surface of calcanei. MIPs represent 3D volumes (of subchondral bone) condensed into 2D images by extracting AD maxima from columns of voxels comprising the volumes. False‐color maps are assigned to MIPs by binning pixels in the 2D images according to brightness values. We compared quantities and distributions of AD pixels in the highest bin to test predictions relating AD patterns to habitual locomotor modes and foot posture categories of humans and several nonhuman primates. Nonhuman primates exhibit dorsally positioned high AD concentrations, where maximum compressive loading between the calcaneus and cuboid likely occurs during “midtarsal break” of support. Humans exhibit less widespread areas of high AD, which could reflect reduced fore‐ and midfoot mobility. Analysis of the internal morphology of the tarsus, such as subchondral bone AD, potentially offers new insights for evaluating primate foot function during locomotion. Am J Phys Anthropol, 2010. © 2009 Wiley‐Liss, Inc.     

Effects of rotator cuff ruptures on the cellular and intracellular composition of the human supraspinatus muscle

Ruptures of the rotator cuff tendons of the human shoulder are a common incidence and lead to functional impairment of the four muscles connected to the cuff, entailing profound changes of their cellular tissue composition. Most importantly, such tendon tears lead to atrophy, fatty degeneration and fibrosis of the corresponding muscles. The muscle most commonly affected with such changes is the M. supraspinatus. The present study uses biopsy samples from the supraspinatus muscle of 12 elderly patients and 6 controls to examine the rupture-induced muscle change at both the cellular and the intracellular (ultrastructural) levels. Amounts of fatty tissue, connective tissue and muscle were assessed by light microscopy-based morphometry and stereology. Stereology of electron micrographs was employed to determine volume densities of muscle fibre mitochondria, myofibrils and intracellular lipid. Results demonstrate that the supraspinatus muscles of patients with a massive rupture contain significantly higher amounts not only of fatty tissue but also of intracellular lipid than those of control subjects. These patients further exhibit a major decrease in relative amounts of myofibrils, thus confirming that change of intracellular composition is a major component of the observed muscle degeneration. The results contribute to establish the true spectrum of supraspinatus muscle damage in humans induced by tendon rupture.     

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.     

Diversity of fast myosin heavy chain expression during development of gastrocnemius, bicep brachii, and posterior latissimus dorsi muscles in normal and dystrophic chickens

The expression of fast myosin heavy chain (MHC) isoforms was examined in developing bicep brachii, lateral gastrocnemius, and posterior latissimus dorsi (PLD) muscles of inbred normal White Leghorn chickens (Line 03) and genetically related inbred dystrophic White Leghorn chickens (Line 433). Utilizing a highly characterized monoclonal antibody library we employed ELISA, Western blot, immunocytochemical, and MHC epitope mapping techniques to determine which MHCs were present in the fibers of these muscles at different stages of development. The developmental pattern of MHC expression in the normal bicep brachii was uniform with all fibers initially accumulating embryonic MHC similar to that of the pectoralis muscle. At hatching the neonatal isoform was expressed in all fibers; however, unlike in the pectoralis muscle the embryonic MHC isoform did not disappear. With increasing age the neonatal MHC was repressed leaving the embryonic MHC as the only detectable isoform present in the adult bicep brachii muscle. While initially expressing embryonic MHC in ovo, the post-hatch normal gastrocnemius expressed both embryonic and neonatal MHCs. However, unlike the bicep brachii muscle, this pattern of expression continued in the adult muscle. The adult normal gastrocnemius stained heterogeneously with anti-embryonic and anti-neonatal antibodies indicating that mature fibers could contain either isoform or both. Neither the bicep brachii muscle nor the lateral gastrocnemius muscle reacted with the adult specific antibody at any stage of development. In the developing posterior latissimus dorsi muscle (PLD), embryonic, neonatal, and adult isoforms sequentially appeared; however, expression of the embryonic isoform continued throughout development. In the adult PLD, both embryonic and adult MHCs were expressed, with most fibers expressing both isoforms. In dystrophic neonates and adults virtually all fibers of the bicep brachii, gastrocnemius, and PLD muscles were identical and contained embryonic and neonatal MHCs. These results corroborate previous observations that there are alternative programs of fast MHC expression to that found in the pectoralis muscle of the chicken (M.T. Crow and F.E. Stockdale, 1986, Dev. Biol. 118, 333-342), and that diversification into fibers containing specific MHCs fails to occur in the fast muscle fibers of the dystrophic chicken. These results are consistent with the hypothesis that avian muscular dystrophy is a developmental disorder that is associated with alterations in isoform switching during muscle maturation.     

Immunochemical analysis of protein isoforms in thick myofilaments of regenerating skeletal muscle

The expression of myosin heavy chain (MHC) and C-protein isoforms has been examined immunocytochemically in regenerating skeletal muscles of adult chickens. Two, five, and eight days after focal freeze injury to the anterior latissimus dorsi (ALD) and posterior latissimus dorsi (PLD) muscles, cryostat sections of injured and control tissues were reacted with a series of monoclonal antibodies previously shown to specifically bind MHC or C-protein isoforms in adult or embryonic muscles. We observed that during the course of regeneration in each of these muscles there was a reproducible sequence of antigenic changes consistent with differential isoform expression for these two proteins. These isoform switches appear to be tissue specific; i.e., the isoforms of MHC and C-protein which are expressed during the regeneration of a "slow" muscle (ALD) differ from those which are synthesized in a regenerating "fast" muscle (PLD). Evidence has been obtained for the transient expression of a "fast-type" MHC and C-protein during ALD regeneration. Furthermore, during early stages of PLD regeneration this muscle contains MHCs which antigenically resemble those found in the pectoralis muscle at embryonic and early posthatch stages of development. Both regenerating muscles express an isoform of C-protein which appears immunochemically identical to that normally expressed in embryonic and adult cardiac muscle. These results support the concept that isoform transitions in regenerating skeletal muscles qualitatively resemble those found in developing muscles but differences may exist in temporal and tissue-specific patterns of gene expression.     

Fiber type distribution in the shoulder muscles of the tree shrew, the cotton-top tamarin, and the squirrel monkey related to shoulder movements and forelimb loading

Muscle fiber type composition of intrinsic shoulder muscles was examined in tree shrews, cotton-top tamarins, and squirrel monkeys with respect to their shoulder kinematics and forelimb loading during locomotion. Enzyme- and immunohistochemical techniques were applied to differentiate muscle fiber types on serial cross-sections of the shoulder. In the majority of the shoulder muscles, the proportions of fatigue resistant slow-twitch fibers (SO) and fatigable fast-twitch fibers (FG) were inversely related to each other, whereas the percentage of intermediate FOG-fibers varied independently. A segregation of fatigue resistant SO-fibers into deep muscle regions is indicative of differential activation of histochemically distinct muscle regions in which deep regions stabilize the joint against gravitational loading. In all three species, this antigravity function was demonstrated for both the supraspinatus and the cranial subscapularis muscle, which prevent passive joint flexion during the support phase of the limb. The infraspinatus muscle showed a high content of SO-fibers in the primate species but not in the tree shrew, which demonstrates the "new" role of the infraspinatus muscle in joint stabilization related to the higher degree of humeral protraction in primates. In the tree shrew and the cotton-top tamarin, a greater proportion of the body weight is carried on the forelimb, but the squirrel monkey exhibits a weight shift to the hind limbs. The lower amount of forelimb loading is reflected by an overall lower proportion of fatigue resistant muscle fibers in the shoulder muscles of the squirrel monkey. Several muscles such as the deltoid no longer function as joint stabilizers and allow the humerus to move beyond the scapular plane. These differences among species demonstrate the high plasticity of the internal muscle architecture and physiology which is suggested to be the underlying reason for different muscle activity patterns in homologous muscles. Implications for the evolution of new locomotor modes in primates are discussed.     

Neural control of the sequence of expression of myosin heavy chain isoforms in foetal mammalian muscles

The expression of myosin isoforms was studied during development of calf muscles in foetal and neonatal rats, using monoclonal antibodies against slow, embryonic and neonatal isoforms of myosin heavy chain (MHC). Primary myotubes had appeared in all prospective rat calf muscles by embryonic day 16 (E16). On both E16 and E17, primary myotubes in all muscles with the exception of soleus stained for slow, embryonic and neonatal MHC isoforms; soleus did not express neonatal MHC. In earlier stages of muscle formation staining for the neonatal isoform was absent or faint. Secondary myotubes were present in all muscles by E18, and these stained for both embryonic and neonatal MHCs, but not slow. In mixed muscles, primary myotubes destined to differentiate into fast muscle fibres began to lose expression of slow MHC, and primary myotubes destined to become slow muscle fibres began to lose expression of neonatal MHC. This pattern was further accentuated by E19, when many primary myotubes stained for only one of these two isoforms. Chronic paralysis or denervation from E15 or earlier did not disrupt the normal sequence of maturation of primary myotubes up until E18, but secondary myotubes did not form. By E19, however, most primary myotubes in aneural or paralyzed tibialis anterior muscles had lost expression of slow MHC and expressed only embryonic and neonatal MHCs. Similar changes occurred in other muscles, except for soleus which never expressed neonatal MHC, as in controls. Paralysis or denervation commencing later than E15 did not have these effects, even though it was initiated well before the period of change in expression of MHC isoforms. In this case, some secondary myotubes appeared in treated muscles. Paralysis initiated on E15, followed by recovery 2 days later so that animals were motile during the period of change in expression of MHC isoforms, was as effective as full paralysis. These experiments define a critical period (E15-17) during which foetuses must be active if slow muscle fibres are to differentiate during E19-20. We suggest that changes in expression of MHC isoforms in primary myotubes depend on different populations of myoblasts fusing with the myotubes, and that the normal sequence of appearance of these myoblasts has a stage-dependent reliance on active innervation of foetal muscles. A critical period of nerve-dependence for these myoblasts occurs several days before their action can be noted.     

Enzyme‐ and immunohistochemical aspects of skeletal muscle fibers in brown bear (Ursus arctos)

To further elucidate the pattern of MHC isoform expression in skeletal muscles of large mammals, in this study the skeletal muscles of brown bear, one of the largest mammalian predators with an extraordinary locomotor capacity, were analyzed. Fiber types in longissimus dorsi, triceps brachii caput longum, and rectus femoris muscles were determined according to the myofibrillar ATPase (mATPase) histochemistry and MHC isoform expression, revealed by a set of antibodies specific to MHC isoforms. The oxidative (SDH) and glycolytic enzyme (α‐GPDH) capacity of fibers was demonstrated as well. By mATPase histochemistry five fiber types, i.e., I, IIC, IIA, IIAX, IIX were distinguished. Analyzing the MHC isoform expression, we assume that MHC‐I, ‐IIa, and ‐IIx are expressed in the muscles of adolescent bears. MHC‐I isoform was expressed in Type‐I fibers and coexpressed with presumably ‐IIa isoform, in Type‐IIC fibers. Surprisingly, two antibodies specific to rat MHC‐IIa stained those fast fibers, that were histochemically and immunohistochemically classified as Type IIX. This assumption was additionally confirmed by complete absence of fiber staining with antibody specific to rat MHC‐IIb and all fast fiber staining with antibody that according to our experience recognizes MHC‐IIa and ‐IIx of rat. Furthermore, quite high‐oxidative capacity of all fast fiber types and their weak glycolytic capacity also imply for MHC‐IIa and ‐IIx isoform expression in fast fibers of bear. However, in adult, full‐grown animal, only MHC‐I and MHC‐IIa isoforms were expressed. The expression of only two fast isoforms in bear, like in many other large mammals (humans, cat, dog, goat, cattle, and horse) obviously meets the weight‐bearing and locomotor demands of these mammals. J. Morphol., 2009. © 2008 Wiley‐Liss, Inc.     

The developmental program of fast myosin heavy chain expression in avian skeletal muscles

We have examined the types of fast myosin heavy chains (MHCs) expressed in a number of different developing chicken skeletal muscles by combining peptide mapping and immunoblotting to identify fast MHC-specific peptides among the total mixture of MHC digestion products. Using this technique, we have identified three different fast MHC patterns among the different fast and mixed (i.e., fast and slow) fiber type muscles of the adult. While the different muscles all underwent sequential changes in fast MHC isoform expression during their development, the exact sequence of these changes and the isoform patterns expressed varied from muscle to muscle. During late embryonic or fetal development, all muscles expressed a similar fast MHC pattern (designated here as the fetal pattern) which was replaced shortly after hatching with a different fast MHC pattern (the neonatal pattern). During the transition from the neonatal to the adult state that occurred sometime in the first year after hatching, many of the muscles underwent additional changes in fast MHC isoform expression. In muscles such as the pectoralis major and pectoralis minor, a new fast MHC isoform pattern was seen in the adult so that the developmental program of isoform switching in these muscles involved the sequential appearance of distinct fetal, neonatal, and adult fast MHCs. Other muscles, such as the sartorius and posterior latissimus dorsi, underwent a qualitatively different program of isoform switching and expressed as an adult a fast MHC pattern that was indistinguishable from that expressed during fetal development. Finally, in some muscles, such as the superficial biceps, no change in isoform pattern was detected during the neonatal to adult transition--in these muscles, expression of the neonatal MHC isoform pattern apparently persisted into the adult state. These data indicate that no single scheme or program of fast MHC isoform switching can describe all the developmental changes that occur in fast MHC isoform expression in the chicken and that at least three different programs of isoform switching and expression can be identified.     

Temporalis and masseter muscle function during incision in macaques and humans

Most previously published electromyographic (EMG) studies have indicated that the temporalis muscles in humans become almost electrically quiet during incisai biting. These data have led various workers to conclude that these muscles may contribute little to the incisai bite force. The feeding behavior and comparative anatomy of the incisors and temporalis muscles of certain catarrhine primates, however, suggest that the temporalis muscle is an important and powerful contributor to the bite force during incision. One purpose of this study is to analyze the EMG activity of the masseter and temporalis muscles in both humans and macaques with the intention of focusing on the conflict between published EMG data on humans and inferences of muscle function based on the comparative anatomy and behavior of catarrhine primates. The EMG data collected from humans in the present study indicate that, in five of seven subjects, the masseter,anterior temporalis, and posterior temporalis muscles are very active during apple incision (i.e., relative to EMG activity levels during apple and almond mastication). In the other two human subjects the EMG levels of these muscles are lower during incision than during mastication, but in no instance are these muscles ever close to becoming electrically quiet. The EMG data on macaques indicate that, in all six subjects, the masseter, anterior temporalis, and posterior temporalis muscles are very active during incision. These data are in general agreement with inferences on muscle function that have been drawn from the comparative anatomy and behavior of primates, but they do not agree with previous experimental data. The reason for this disagreement is probably due to differences in the experimental procedure. In previous studies subjects simply bit isometrically on their incisors and the resulting EMG pattern was compared to the pattern associated with powerful clenching in centric occlusion. In the present study the subjects incised into actual food objects, and the resulting EMG pattern was compared to the pattern associated with mastication of various foods. It is not surprising that these two procedures result in markedly different EMG patterns, which in turn result in markedly different interpretations of jaw-muscle function. In an attempt to explain the evolution of the postorbital septum in anthropoids, it has been suggested that the anterior temporalis is more active than the masseter during incision (Cachel, 1979). The human and macaque EMG data do not support this hypothesis; during incision, the two muscles show no consistent differences in humans and the masseter appears to be in fact more active than the anterior temporalis in macaques.