Muscle architecture of the upper limb in the orangutan

The comparative and functional anatomy of the forelimb muscle architecture of Humboldt's woolly monkey (Lagothrix lagotricha)

Humboldt's woolly monkey (Lagothrix lagortricha) is a ceboid primate that more frequently engages in plantigrade quadrupedalism (~89%) but is, like most other members of the subfamily Atelinae, capable of suspensory postures and "tail assisted" brachiation. That taxon's decreased reliance on suspension is reflected in the skeletal anatomy of the upper limb which is less derived relative to more frequently suspensory atelines (Ateles, Brachyteles) but is in many ways (i.e., phalangeal curvature, enlarged joint surfaces, elongated diaphyses) intermediate between highly suspensory and quadrupedal anthropoids. Although it has been suggested that muscle may have morphogenetic primacy with respect to bone this has not been explicitly tested. The present study employs analyses of Lagothrix upper limb muscle fiber length, relative physiological cross-sectional area and relative muscle mass to test whether muscular adaptations for suspensory postures and locomotion in Lagothrix precede adaptive refinements in the skeletal tissues or appear more gradually in conjunction with related skeletal adaptations. Results demonstrate that Lagothrix upper limb musculature is most like committed quadrupeds but that limited aspects of the relative distribution of segmental muscle mass may approach suspensory hylobatids consistent with only a limited adaptive response in musculature prior to bone. Results specific to the shoulder were inconclusive owing to under-representation of quadrupedal shoulder musculature and future work should be focused more specifically on the adaptive and functional morphology of the muscular anatomy and microstructure of the scapulothoracic joint complex.

From fibre to function: are we accurately representing muscle architecture and performance?

The size and arrangement of fibres play a determinate role in the kinetic and energetic performance of muscles. Extrapolations between fibre architecture and performance underpin our understanding of how muscles function and how they are adapted to power specific motions within and across species. Here we provide a synopsis of how this 'fibre to function' paradigm has been applied to understand muscle design, performance and adaptation in animals. Our review highlights the widespread application of the fibre to function paradigm across a diverse breadth of biological disciplines but also reveals a potential and highly prevalent limitation running through past studies. Specifically, we find that quantification of muscle architectural properties is almost universally based on an extremely small number of fibre measurements. Despite the volume of research into muscle properties, across a diverse breadth of research disciplines, the fundamental assumption that a small proportion of fibre measurements can accurately represent the architectural properties of a muscle has never been quantitatively tested. Subsequently, we use a combination of medical imaging, statistical analysis, and physics-based computer simulation to address this issue for the first time. By combining diffusion tensor imaging (DTI) and deterministic fibre tractography we generated a large number of fibre measurements (>3000) rapidly for individual human lower limb muscles. Through statistical subsampling simulations of these measurements, we demonstrate that analysing a small number of fibres (n < 25) typically used in previous studies may lead to extremely large errors in the characterisation of overall muscle architectural properties such as mean fibre length and physiological cross-sectional area. Through dynamic musculoskeletal simulations of human walking and jumping, we demonstrate that recovered errors in fibre architecture characterisation have significant implications for quantitative predictions of in-vivo dynamics and muscle fibre function within a species. Furthermore, by applying data-subsampling simulations to comparisons of muscle function in humans and chimpanzees, we demonstrate that error magnitudes significantly impact both qualitative and quantitative assessment of muscle specialisation, potentially generating highly erroneous conclusions about the absolute and relative adaption of muscles across species and evolutionary transitions. Our findings have profound implications for how a broad diversity of research fields quantify muscle architecture and interpret muscle function.

The forearm and hand musculature of semi-terrestrial rhesus macaques (Macaca mulatta) and arboreal gibbons (fam.Hylobatidae). Part II. Quantitative analysis

Nonhuman primates have a highly diverse locomotor repertoire defined by an equally diverse hand use. Based on how primates use their hands during locomotion, we can distinguish between terrestrial and arboreal taxa. The 'arboreal' hand is likely adapted towards high wrist mobility and grasping, whereas the 'terrestrial' hand will show adaptations to loading. While the morphology of the forearm and hand bones have been studied extensively, functional adaptations in the forearm and hand musculature to locomotor behaviour have been documented only scarcely. In this paper, we investigate the forelimb musculature of the highly arboreal gibbons (including Hylobates lar,Hylobates pileatus,Nomascus leucogenys,Nomascus concolor and Symphalangus syndactylus) and compare this with the musculature of the semi-terrestrial rhesus macaques (Macaca mulatta). Anatomical data from previous dissections on knuckle-walking bonobos (Pan paniscus) and bipedal humans (Homo sapiens) are also included to further integrate the analyses in the scope of catarrhine hand adaptation. This study indicates that the overall configuration of the arm and hand musculature of these primates is very similar but there are some apparent differences in relative size which can be linked to differences in forelimb function and which might be related to their specific locomotor behaviour. In macaques, there is a large development of wrist deviators, wrist and digital flexors, and m. triceps brachii, as these muscles are important during the different phases of palmi- and digitigrade quadrupedal walking to stabilize the wrist and elbow. In addition, their m. flexor carpi ulnaris is the most important contributor to the total force-generating capacity of the wrist flexors and deviators, and is needed to counteract the adducting torque at the elbow joint during quadrupedal walking. Gibbons show a relatively high force-generating capacity in their forearm rotators, wrist and digital flexors, which are important muscles in brachiation to actively regulate forward movement of the body. The results also stress the importance of the digital flexors in bonobos, during climbing and clambering, and in humans, which is likely linked to our advanced manipulation skills.

Anatomical and ontogenetic influences on muscle density

Physiological cross-sectional area (PCSA), an important biomechanical variable, is an estimate of a muscle's contractile force potential and is derived from dividing muscle mass by the product of a muscle's average fascicle length and a theoretical constant representing the density of mammalian skeletal muscle. This density constant is usually taken from experimental studies of small samples of several model taxa using tissues collected predominantly from the lower limbs of adult animals. The generalized application of this constant to broader analyses of mammalian myology assumes that muscle density (1) is consistent across anatomical regions and (2) is unaffected by the aging process. To investigate the validity of these assumptions, we studied muscles of rabbits (Oryctolagus cuniculus) in the largest sample heretofore investigated explicitly for these variables, and we did so from numerous anatomical regions and from three different age-cohorts. Differences in muscle density and histology as a consequence of age and anatomical region were evaluated using Tukey's HSD tests. Overall, we observed that older individuals tend to have denser muscles than younger individuals. Our findings also demonstrated significant differences in muscle density between anatomic regions within the older cohorts, though none in the youngest cohort. Approximately 50% of the variation in muscle density can be explained histologically by the average muscle fiber area and the average percent fiber area. That is, muscles with larger average fiber areas and a higher proportion of fiber area tend to be denser. Importantly, using the age and region dependent measurements of muscle density that we provide may increase the accuracy of PCSA estimations. Although we found statistically significant differences related to ontogeny and anatomical region, if density cannot be measured directly, the specific values presented herein should be used to improve accuracy. If a single muscle density constant that has been better validated than the ones presented in the previous literature is preferred, then 1.0558 and 1.0502 g/cm³ would be reasonable constants to use across all adult and juvenile muscles respectively.

Review of the methods used for calculating physiological cross-sectional area (PCSA) for ecological questions

This review examines literature that used physiological cross-sectional area (PCSA) as a representative measure of an individual muscle's maximal isometric force production. PCSA is used to understand the muscle architecture and how a trade-off between muscle force and muscle contractile velocity reflect adaptations of the musculoskeletal system as a reflection of functional demands. Over the decades, methods have been developed to measure muscle volume, fascicle lengths, and pennation angle to calculate PCSA. The advantages and limitations of these methods (especially the inclusion/elimination of pennation angle) are discussed frequently; however, these method descriptions are scattered throughout the literature. Here, we reviewed and summarised the different approaches to collecting and recording muscle architectural properties to subsequently calculate PCSA. By critically discussing the advantages and limitations of each methodology, we aim to provide readers with an overview of repeatable methods to assess muscle architecture. This review may serve as a guide to facilitate readers searching for the appropriate techniques to calculate PCSA and measure muscle architecture to be applied in ecomorphology research. RESEARCH HIGHLIGHTS: Discuss the theories behind PCSA in a synthesised review to inform researchers about PCSA methodology.

Quantitative shape analysis of the deltoid tuberosity of modern humans (Homo sapiens) and common chimpanzees (Pan troglodytes)

PURPOSE

To identify anatomical differences in the deltoid tuberosity of Homo sapiens and Pan troglodytes, potentially relating to the different uses of the forelimb in these two phylogenetically related species.

BASIC PROCEDURES

We have used three-dimensional geometric morphometrics (3D GM) to analyze the deltoid tuberosity of scanned humeri from 30 H. sapiens and 27 P. troglodytes. We also used the 3D scans of the humeri to calculate the surface area of the deltoid tuberosity. Finally, we dissected the deltoid muscles of three H. sapiens and three P. troglodytes to determine the relative mass and the physiological cross-sectional area (PCSA) of each part of the muscle.

MAIN FINDINGS

The 3D GM analysis of the deltoid tuberosity identified an anteroposterior enlargement of the P. troglodytes tuberosity, with a lateral displacement of the middle segment, whereas in H. sapiens, there was a distal displacement of the middle segment. Muscle architecture analysis indicated higher normalized values ​​of the PCSA of the clavicular and acromial deltoid in P. troglodytes.

PRINCIPAL CONCLUSIONS

The anatomical features observed in our P. troglodytes specimens serve to strengthen the three parts of the deltoid muscle. This fact can be related to the use of the forelimb in locomotion, both arboreal and knuckle-walking, in this species. Humans use the forelimb mainly in manipulative tasks, so they do not develop - as do chimpanzees - the anatomical features that increase the deltoid force. Our findings have shown that the different uses of the forelimb in modern humans and common chimpanzees can affect both muscle architecture and bone morphology, either jointly or separately.

The impact of hand proportions on tool grip abilities in humans, great apes and fossil hominins: A biomechanical analysis using musculoskeletal simulation

Differences in grip techniques used across primates are usually attributed to variation in thumb-finger proportions and muscular anatomy of the hand. However, this cause-effect relationship is not fully understood because little is known about the biomechanical functioning and mechanical loads (e.g., muscle or joint forces) of the non-human primate hand compared to that of humans during object manipulation. This study aims to understand the importance of hand proportions on the use of different grip strategies used by humans, extant great apes (bonobos, gorillas and orangutans) and, potentially, fossil hominins (Homo naledi and Australopithecus sediba) using a musculoskeletal model of the hand. Results show that certain grips are more challenging for some species, particularly orangutans, than others, such that they require stronger muscle forces for a given range of motion. Assuming a human-like range of motion at each hand joint, simulation results show that H. naledi and A. sediba had the biomechanical potential to use the grip techniques considered important for stone tool-related behaviors in humans. These musculoskeletal simulation results shed light on the functional consequences of the different hand proportions among extant and extinct hominids and the different manipulative abilities found in humans and great apes.

Forearm pronation efficiency in A.L. 288-1 (Australopithecus afarensis) and MH2 (Australopithecus sediba): Insights into their locomotor and manipulative habits

OBJECTIVES

The locomotor and manipulative abilities of australopithecines are highly debated in the paleoanthropological context. Australopithecus afarensis and Australopithecus sediba likely engaged in arboreal locomotion and, especially the latter, in certain activities implying manipulation. Nevertheless, their degree of arboreality and the relevance of their manipulative skills remain unclear. Here we calculate the pronation efficiency of the forearm (E_rot ) in these taxa to explore their arboreal and manipulative capabilities using a biomechanical approach.

MATERIALS AND METHODS

Three-dimensional humeral images and upper limb measurements of A.L. 288-1 (Au. afarensis) and MH2 (Au. sediba) were used to calculate E_rot using a previously described biomechanical model.

RESULTS

Maximal E_rot in elbow flexion occurs in a rather supinated position of the forearm in Au. afarensis, similarly to Pan troglodytes. In elbow extension, maximal E_rot in this fossil taxon occurs in the same forearm position as in Pongo spp. In Au. sediba the forearm positions where E_rot is maximal are largely coincident with those for Hylobatidae.

CONCLUSIONS

The pattern in Au. afarensis suggests relevant arboreal capabilities, which would include vertical climbing, although it is suggestive of poorer manipulative skills than in modern humans. The similarity between Au. sediba and Hylobatidae is difficult to interpret, but the differences between Au. sediba and Au. afarensis suggest that the capacity to rotate the forearm followed different evolutionary processes in these australopithecine species. Although functional inferences from the upper limb are complex, the observed differences between both taxa point to the existence of two distinct anatomical models.

Age-related structural changes in upper extremity muscle tissue in a nonhuman primate model

BACKGROUND

Longitudinal studies of upper extremity aging in humans include logistical concerns that animal models can overcome. The vervet is a promising species with which to study aging-related processes. However, age-related changes in upper extremity muscle structure have not been quantified in this species. This study measured age-related changes to muscle structure, examined relationships between muscle structure and measures of physical performance, and evaluated the presence of rotator cuff tears.

METHODS

Muscle structure (volume, optimal fiber length, and physiologic cross-sectional area (PCSA)) of 10 upper extremity muscles was quantified from the right upper limb of 5 middle-aged and 6 older adult female vervets.

RESULTS

Total measured PCSA was smaller (P = .001) in the older adult vervets than in the middle-aged vervets. Muscle volume reduction predominate the age-related reductions in PCSA. Total measured PCSA was not correlated to any measures of physical performance. No rotator cuff tears were observed. Supraspinatus volume was relatively larger and deltoid volume relatively smaller in the vervet compared with a human.

CONCLUSIONS

The vervet is an appropriate translational model for age-related upper extremity muscle volume loss. Functional measures were not correlated to PCSA, suggesting the vervets may have enough strength for normal function despite loss of muscle tissue. Reduced relative demand on the supraspinatus may be responsible for the lack of naturally occurring rotator cuff tears.

Rotator cuff muscle size and the interpretation of scapular shape in primates

Scapular shape variation among primates is widely viewed as being strongly related to locomotor differences. The relative importance of overhead forelimb elevation in the locomotor repertoire of a species, as reflected in muscular leverage for scapular rotation or in the sizes of attachment areas for muscles involved in glenohumeral elevation, has proven to be a useful organizing principle for understanding this variation. While generally successful in sorting primate scapulae into functional groups, the scapulae of some species do not entirely match predictions based on the perceived importance of forelimb elevation. A recent study has shown that scapular fossa sizes in apes are not as accurate predictors of the sizes of the muscles arising from them as has been assumed. To further explore the degree of correspondence between actual and predicted muscle size based on the perceived importance of forelimb elevation, the current study examines the relative sizes of the rotator cuff muscles in a wider sample of primate taxa using published data on muscle mass and cross-sectional area. The results do not support some of the accepted generalizations about the relative sizes of members of the rotator cuff based on measurements of the sizes of scapular fossae. For example, orthograde apes do not display enlarged supraspinatus muscles compared to pronograde monkeys. Differences in assessments of relative muscle size based on mass compared to those based on cross-sectional area suggest that poor correspondence between muscle size predicted from scapular fossa size and actual muscle size may be related to constraints on scapular form associated with muscular leverage for scapular rotation and with scapular position on the thorax.

Rotator cuff muscle function and its relation to scapular morphology in apes

It is widely held that many differences among primate species in scapular morphology can be functionally related to differing demands on the shoulder associated with particular locomotor habits. This perspective is largely based on broad scale studies, while more narrow comparisons of scapular form often fail to follow predictions based on inferred differences in shoulder function. For example, the ratio of supraspinous fossa/infraspinous fossa size in apes is commonly viewed as an indicator of the importance of overhead use of the forelimb, yet paradoxically, the African apes, the most terrestrial of the great apes, have higher scapular fossa ratios than the more suspensory orangutan. The recent discovery of several nearly complete early hominin scapular specimens, and their apparent morphological affinities to scapulae of orangutans and gorillas rather than chimpanzees, has led to renewed interest in the comparative analysis of human and extant ape scapular form. To facilitate the functional interpretation of differences in ape scapulae, particularly in regard to relative scapular fossa size, we used electromyography (EMG) to document the activity patterns in all four rotator cuff muscles in orangutans and gibbons, comparing the results with previously published data for chimpanzees. The EMG results indicate that the distinctive contributions of each cuff muscle to locomotion are the same in the three ape species, failing to support inferences of differences in rotator cuff function based on relative scapular fossa size comparisons. It is also shown that relative scapular fossa size is not in fact a good predictor of either the relative masses or cross-sectional areas of the rotator cuff muscles in apes, and relative fossa size gives a false impression of the importance of individual cuff muscles to locomotor differences among apes. A possible explanation for the disparity between fossa and muscle size relates to the underappreciated role of the scapular spine in structural reinforcement of the blade.

Muscle dimensions of the foot in the orangutan and the chimpanzee

The hindlimbs of two orangutans and four chimpanzees were dissected, and muscle parameters (mass, fascicle length, and physiological cross-sectional area: PCSA) were determined to explore possible interspecies variation in muscle dimensions. Muscle mass and PCSA were divided by the total mass and total PCSA of the entire foot muscles for normalization. The results indicate that the pedal interosseous and the intrinsic pedal digital extensor muscles in the orangutans probably have higher capacity for force production due to their relatively larger PCSAs than in chimpanzees. Moreover, the medial components of the intrinsic muscles exhibited relatively larger mass and PCSA ratios in orangutans. The mass and PCSA ratios of the hallucal muscles were larger in chimpanzees. These differences in foot muscle dimensions of the two species suggest that the orangutan is more specialized for hook-like digital gripping without involvement of the rudimentary hallux, while the chimpanzee is adapted to hallux-assisted power gripping in arboreal locomotion.

Functional adaptations in the forelimb muscles of non-human great apes

The maximum capability of a muscle can be estimated from simple measurements of muscle architecture such as muscle belly mass, fascicle length and physiological cross-sectional area. While the hindlimb anatomy of the non-human apes has been studied in some detail, a comparative study of the forelimb architecture across a number of species has never been undertaken. Here we present data from chimpanzees, bonobos, gorillas and an orangutan to ascertain if, and where, there are functional differences relating to their different locomotor repertoires and habitat usage. We employed a combination of analyses including allometric scaling and ancovas to explore the data, as the sample size was relatively small and heterogeneous (specimens of different sizes, ages and sex). Overall, subject to possible unidentified, confounding factors such as age effects, it appears that the non-human great apes in this sample (the largest assembled to date) do not vary greatly across different muscle architecture parameters, even though they perform different locomotor behaviours at different frequencies. Therefore, it currently appears that the time spent performing a particular behaviour does not necessarily impose a dominating selective influence on the soft-tissue portion of the musculoskeletal system; rather, the overall consistency of muscle architectural properties both between and within the Asian and African apes strengthens the case for the hypothesis of a possible ancient shared evolutionary origin for orthogrady under compressive and/or suspensory loading in the great apes.

Relationship between humeral geometry and shoulder muscle power among suspensory, knuckle-walking, and digitigrade/palmigrade quadrupedal primates

Shoulder morphology is functionally related to different patterns of locomotion in primates. To investigate this we performed a quantitative analysis of the relationship between cortical bone thickness (Cbt) of the muscle/tendon attachment site on the humerus and physiological cross-sectional area (PCSA) of the shoulder muscle in primates with different locomotory habits. The deltoid, subscapularis, supraspinatus, and infraspinatus were investigated. A chimpanzee, a gibbon, a baboon, two species of macaque, a lutong, a capuchin, and a squirrel monkey were included in the study. The total length of the humerus was measured and the values were converted into three-dimensional reconstructed data on a computer by computed tomography. The Cbt values were obtained from the volumes divided by the areas of the muscle/tendon attachment sites of the humerus by computer analysis. Muscle mass, muscle fascicle length, and muscle pennation angle were measured and PCSA was calculated using these parameters. A relatively high Cbt and small PCSA were characteristic of the gibbon. The gibbon's high Cbt suggests that passive tension in the muscle/tendon attachment site of suspensory primates (brachiators) may be greater than that of quadrupedal primates, whereas the relatively small PCSA indicates an association with a large amount of internal muscle fascia to endure the passive stress of brachiation. Although chimpanzees undertake some suspensory locomotion, the results for this species resemble those of the digitigrade/palmigrade quadrupedal primates rather than those of the suspensory primate. However, the deltoid and subscapularis in chimpanzee differ from those of the other primates and appear to be affected by the peculiar locomotion of knuckle-walking, i.e. the moment arm of forelimb in chimpanzees is relatively longer than that of digitigrade/palmigrade quadrupedal primates. Hence, a large PCSA in the deltoid and subscapularis may contribute to sustaining the body weight during locomotion. On the other hand, a thin cortical bone relative to a large PCSA was a feature of the cercopithecids, indicating that digitigrade/palmigrade quadrupedal locomotion produces less tension at the muscle/tendon attachment sites compared with that produced by brachiators.

Soft-tissue anatomy of the primates: phylogenetic analyses based on the muscles of the head, neck, pectoral region and upper limb, with notes on the evolution of these muscles

Apart from molecular data, nearly all the evidence used to study primate relationships comes from hard tissues. Here, we provide details of the first parsimony and Bayesian cladistic analyses of the order Primates based exclusively on muscle data. The most parsimonious tree obtained from the cladistic analysis of 166 characters taken from the head, neck, pectoral and upper limb musculature is fully congruent with the most recent evolutionary molecular tree of Primates. That is, this tree recovers not only the relationships among the major groups of primates, i.e. Strepsirrhini {Tarsiiformes [Platyrrhini (Cercopithecidae, Hominoidea)]}, but it also recovers the relationships within each of these inclusive groups. Of the 301 character state changes occurring in this tree, ca. 30% are non-homoplasic evolutionary transitions; within the 220 changes that are unambiguously optimized in the tree, ca. 15% are reversions. The trees obtained by using characters derived from the muscles of the head and neck are more similar to the most recent evolutionary molecular tree than are the trees obtained by using characters derived from the pectoral and upper limb muscles. It was recently argued that since the Pan/Homo split, chimpanzees accumulated more phenotypic adaptations than humans, but our results indicate that modern humans accumulated more muscle character state changes than chimpanzees, and that both these taxa accumulated more changes than gorillas. This overview of the evolution of the primate head, neck, pectoral and upper limb musculature suggests that the only muscle groups for which modern humans have more muscles than most other extant primates are the muscles of the face, larynx and forearm.

Hindlimb muscle architecture in non-human great apes and a comparison of methods for analysing inter-species variation

By relating an animal's morphology to its functional role and the behaviours performed, we can further develop our understanding of the selective factors and constraints acting on the adaptations of great apes. Comparison of muscle architecture between different ape species, however, is difficult because only small sample sizes are ever available. Further, such samples are often comprised of different age-sex classes, so studies have to rely on scaling techniques to remove body mass differences. However, the reliability of such scaling techniques has been questioned. As datasets increase in size, more reliable statistical analysis may eventually become possible. Here we employ geometric and allometric scaling techniques, and ancovas (a form of general linear model, GLM) to highlight and explore the different methods available for comparing functional morphology in the non-human great apes. Our results underline the importance of regressing data against a suitable body size variable to ascertain the relationship (geometric or allometric) and of choosing appropriate exponents by which to scale data. ancova models, while likely to be more robust than scaling for species comparisons when sample sizes are high, suffer from reduced power when sample sizes are low. Therefore, until sample sizes are radically increased it is preferable to include scaling analyses along with ancovas in data exploration. Overall, the results obtained from the different methods show little significant variation, whether in muscle belly mass, fascicle length or physiological cross-sectional area between the different species. This may reflect relatively close evolutionary relationships of the non-human great apes; a universal influence on morphology of generalised orthograde locomotor behaviours or, quite likely, both.

Distribution patterns of fibre types in the triceps surae muscle group of chimpanzees and orangutans

Different locomotor and postural demands are met partly due to the varying properties and proportions of the muscle fibre types within the skeletal muscles. Such data are therefore important in understanding the subtle relationships between morphology, function and behaviour. The triceps surae muscle group is of particular interest when studying our closest living relatives, the non-human great apes, as they lack a significant external Achilles tendon, crucial to running locomotion in humans and other cursorial species. The aim of this study, therefore, was to determine the proportions of type I (slow) and type II (fast) fibres throughout these muscles in chimpanzees and orangutans using immunohistochemistry. The orangutan had a higher proportion of type I fibres in all muscles compared with the chimpanzees, related to their slower, more controlled movements in their arboreal habitat. The higher proportion of type II fibres in the chimpanzees likely reflects a compromise between their need for controlled mobility when arboreal, and greater speed and power when terrestrial. Overall, the proportion of slow fibres was greater in the soleus muscle compared with the gastrocnemius muscles, and there was some evidence of proximal to distal and medial to lateral variations within some muscles. This study has shown that not only do orangutans and chimpanzees have very different muscle fibre populations that reflect their locomotor repertoires, but it also shows how the proportion of fibre types provides an additional mechanism by which the performance of a muscle can be modulated to suit the needs of a species.

Dimensions of forelimb muscles in orangutans and chimpanzees

Eight forelimbs of three orangutans and four chimpanzees were dissected and the muscle mass, fascicle length and physiological cross-sectional area (PCSA) of all forelimb muscles were systematically recorded to explore possible interspecies variation in muscle dimensions. Muscle mass and PCSA were divided by the total mass and total PCSA of the entire forelimb muscles for normalization. The results indicate that the mass and PCSA ratios of the monoarticular elbow flexors (M. brachialis and M. brachioradialis) are significantly larger in orangutans. In contrast, the mass ratios of the biarticular muscles in the upper arm (the short head of M. biceps brachii and the long head of M. triceps brachii) are significantly larger in chimpanzees. For the rotator cuff muscles, the force-generating capacity of M. subscapularis is significantly larger in orangutans, whereas the opposite rotator cuff muscle, M. infraspinatus, is larger in chimpanzees. These differences in forelimb muscle dimensions of the two species may reflect functional specialization for their different positional and locomotor behaviors.

Dimensions of the foot muscles in the lowland gorilla

We dissected the hindlimb of a female western lowland gorilla and determined the muscle dimensions (mass, fascicle length, and physiological cross-sectional area: PCSA). Comparisons of the muscle parameters of the measured gorilla with corresponding reported human data demonstrated that the triceps surae muscles were larger and had more capacity to generate force than the other muscle groups in both species, but this tendency was more prominent in the human, probably as an adaptation to strong toe-off during bipedal walking. On the other hand, PCSAs of the extrinsic pedal digital flexors and digiti minimi muscles were larger in the western lowland gorilla, suggesting that the foot, particularly the fifth toe, has a relatively high grasping capability in the lowland gorilla.