The morphology and evolutionary history of the glenohumeral joint of hominoids: A review

An ape partial postcranial skeleton (KNM-NP 64631) from the Middle Miocene of Napudet, northern Kenya

An ape partial postcranial skeleton (KNM-NP 64631) was recovered during the 2015-2021 field seasons at Napudet, a Middle Miocene (∼13 Ma) locality in northern Kenya. Bony elements representing the shoulder, elbow, hip, and ankle joints, thoracic and lumbar vertebral column, and hands and feet, offer valuable new information about the body plan and positional behaviors of Middle Miocene apes. Body mass estimates from femoral head dimensions suggest that the KNM-NP 64631 individual was smaller-bodied (c. 13-17 kg) than some Miocene taxa from eastern Africa, including Ekembo nyanzae, and probably Equatorius africanus or Kenyapithecus wickeri, and was more comparable to smaller-bodied male Nacholapithecus kerioi individuals. Similar to many Miocene apes, the KNM-NP 64631 individual had hip and hallucal tarsometatarsal joints reflecting habitual hindlimb loading in a variety of postures, a distal tibia with a large medial malleolus, an inflated humeral capitulum, probably a long lumbar spine, and a long pollical proximal phalanx relative to femoral head dimensions. The KNM-NP 64631 individual departs from most Early Miocene apes in its possession of a more steeply beveled radial head and deeper humeral zona conoidea, reflecting enhanced supinating-pronating abilities at the humeroradial joint. The KNM-NP 64631 individual also differs from Early Miocene Ekembo heseloni in having a larger elbow joint (inferred from radial head size) relative to the mediolateral width of the lumbar vertebral bodies and a more asymmetrical talar trochlea, and in these ways recalls inferred joint proportions for, and talocrural morphology of, N. kerioi. Compared to most Early Miocene apes, the KNM-NP 64631 individual likely relied on more forelimb-dominated arboreal behaviors, perhaps including vertical climbing (e.g., extended elbow, hoisting). Moreover, the Napudet ape partial postcranial skeleton suggests that an arboreally adapted body plan characterized by relatively large (here, based on joint size) forelimbs, but lacking orthograde suspensory adaptations, may not have been 'unusual' among Middle Miocene apes.

Evolution of the throwing shoulder: why apes don't throw well and how that applies to throwing athletes

BACKGROUND

Humans have unique characteristics making us the only primate that can throw well while most other primates throw predominately underhand with poor speed and accuracy. The purpose of this study is to illuminate the uniquely human characteristics that allow us to throw so well. When treating an injury such as a labral tear or capsule tear, this study hopes the reader can gain a better understanding of the issues that lead to the tear and those that may determine the success of treatment besides the actual repair.

METHODS

In addition to a review of scientific and medical literature, information was obtained from interviews and experience with primate veterinarians, anthropologists, archeologists, and professional baseball players. These sources were used to study the connection between evolutionary throwing activities and current sports medicine issues.

RESULTS

Arm acceleration requires a functional kinetic chain, rapid motor sequences, and the ability to absorb elastic energy in the shoulder. Successful treatment of the throwing shoulder requires awareness of the shoulder's position in the kinetic chain and correction of defects in the ability to execute the kinetic chain. Some problems in the shoulder could reflect regression to a more primitive anatomy or dyskinesis. Return of performance requires regaining the elasticity in the tissues of the shoulder to temporarily store kinetic energy. For example, tissue remodeling after rotator cuff repair continues for months to years; however, the newly formed tissue lacks the same elasticity of the native tendon. This suggests why throwing performance typically does not return for 7 or more months after repair even though there may be structural integrity at 3-4 months.

CONCLUSION

The shoulder has developed uniquely in modern man for the act of throwing. The anatomic deficiencies in primates for throwing provide an illustration of the more subtle changes that a throwing athlete might have that are detrimental to throwing. Nonhuman primates have been unable to demonstrate the kinetic chain sequence for throwing secondary to the lack of neurologic pathways required. Humans are more sophisticated and precise in their movements but lack robusticity in their bone and muscle architecture, seen especially in the human rotator cuff. Successful treatment of a throwing injury requires familiarity with the conditions that cause the injury or affect the rehabilitation process. The return of performance following injury or surgery requires regaining the elasticity in the tissues of the shoulder to temporarily store kinetic energy from the kinetic chain.

A comparative approach for characterizing the relationship among morphology, range-of-motion and locomotor behaviour in the primate shoulder

Shoulder shape directly impacts forelimb function by contributing to glenohumeral (GH) range-of-motion (ROM). However, identifying traits that contribute most to ROM and visualizing how they do so remains challenging, ultimately limiting our ability to reconstruct function and behaviour in fossil species. To address these limitations, we developed an in silico proximity-driven model to simulate and visualize three-dimensional (3D) GH rotations in living primate species with diverse locomotor profiles, identify those shapes that are most predictive of ROM using geometric morphometrics, and apply subsequent insights to interpret function and behaviour in the fossil hominin Australopithecus sediba. We found that ROM metrics that incorporated 3D rotations best discriminated locomotor groups, and the magnitude of ROM (mobility) was decoupled from the anatomical location of ROM (e.g. high abduction versus low abduction). Morphological traits that enhanced mobility were decoupled from those that enabled overhead positions, and all non-human apes possessed the latter but not necessarily the former. Model simulation in A. sediba predicted high mobility and a ROM centred at lower abduction levels than in living apes but higher than in modern humans. Together these results identify novel form-to-function relationships in the shoulder and enhance visualization tools to reconstruct past function and behaviour.

Downclimbing and the evolution of ape forelimb morphologies

The forelimbs of hominoid primates (apes) are decidedly more flexible than those of monkeys, especially at the shoulder, elbow and wrist joints. It is tempting to link the greater mobility of these joints to the functional demands of vertical climbing and below-branch suspension, but field-based kinematic studies have found few differences between chimpanzees and monkeys when comparing forelimb excursion angles during vertical ascent (upclimbing). There is, however, a strong theoretical argument for focusing instead on vertical descent (downclimbing), which motivated us to quantify the effects of climbing directionality on the forelimb kinematics of wild chimpanzees (Pan troglodytes) and sooty mangabeys (Cercocebus atys). We found that the shoulders and elbows of chimpanzees and sooty mangabeys subtended larger joint angles during bouts of downclimbing, and that the magnitude of this difference was greatest among chimpanzees. Our results cast new light on the functional importance of downclimbing, while also burnishing functional hypotheses that emphasize the role of vertical climbing during the evolution of apes, including the human lineage.

The morphofunctional implications of the glenoid labrum of the glenohumeral joint in hominoids

OBJECTIVES

A morphocline of the glenoid cavity has been used to infer differences in locomotor behaviors; however, the glenoid cavity is surrounded by the glenoid labrum, a fibrocartilaginous structure that could influence the functionality of the glenoid. The objectives of this study are to explore the effects of the glenoid labrum on the area, depth, and morphology of the glenoid cavity in primates.

MATERIALS AND METHODS

Photogrammetry was used to build 3D models of the glenoid, with and without the labrum, and three- (3D) and two-dimensional (2D) geometric morphometrics (GM) was applied. 2D areas were collected from zenithal images for glenoids with and without labrum to evaluate the availability of articular surface area.

RESULTS

In the 2D GM the morphocline is present in the dry-bone sample but not with the presence of the glenoid labrum. In the 3D GM there are differences between species mainly concerning the depth of the glenoid cavity. 2D areas reveal that the amount of articular area of the glenoid cavity increases with the presence of the labrum, particularly in humans.

DISCUSSION

The glenoid labrum changes the shape, increases the depth and the surface area of the glenoid cavity, particularly in humans. Therefore, the glenoid labrum might hold a functional role, increasing the stability of the glenohumeral joint of primates in general, and especially in humans.

Mammalian forelimb evolution is driven by uneven proximal-to-distal morphological diversity

Vertebrate limb morphology often reflects the environment due to variation in locomotor requirements. However, proximal and distal limb segments may evolve differently from one another, reflecting an anatomical gradient of functional specialization that has been suggested to be impacted by the timing of development. Here, we explore whether the temporal sequence of bone condensation predicts variation in the capacity of evolution to generate morphological diversity in proximal and distal forelimb segments across more than 600 species of mammals. Distal elements not only exhibit greater shape diversity, but also show stronger within-element integration and, on average, faster evolutionary responses than intermediate and upper limb segments. Results are consistent with the hypothesis that late developing distal bones display greater morphological variation than more proximal limb elements. However, the higher integration observed within the autopod deviates from such developmental predictions, suggesting that functional specialization plays an important role in driving within-element covariation. Proximal and distal limb segments also show different macroevolutionary patterns, albeit not showing a perfect proximo-distal gradient. The high disparity of the mammalian autopod, reported here, is consistent with the higher potential of development to generate variation in more distal limb structures, as well as functional specialization of the distal elements.

Effects of Different Orthoses on Neuromuscular Activity of Superficial and Deep Shoulder Muscles during Activities of Daily Living and Physiotherapeutic Exercises in Healthy Participants

BACKGROUND

This study aimed to investigate the effects of different shoulder orthoses on the neuromuscular activity of superficial and deep shoulder muscles during activities of daily living (ADL) and physiotherapeutic exercises.

METHODS

Ten participants with healthy shoulders (31 ± 3 years, 23.1 ± 3.8 kg/m²) were randomized to receive a "shoulder sling", an "abduction pillow" and a "variably adjustable orthosis" on the dominant side. With each orthosis, they completed seven ADL with and four physiotherapeutic exercises without wearing the orthoses. An electromyographic system was used to record the neuromuscular activity of three superficial (trapezius, deltoid, pectoralis major) and two deep shoulder muscles (infraspinatus, supraspinatus) using surface and intramuscular fine-wire electrodes.

RESULTS

The neuromuscular activity differs between the orthoses during ADL (p ≤ 0.045), whereby the "variably adjustable orthosis" mostly showed the highest activation levels associated with the worst subjective wearing comfort rated on a visual analog scale. In addition, differences exist between the physiotherapeutic exercises (p ≤ 0.006) demonstrating the highest activations of the infra- and supraspinatus muscles for assistive elevation and wipe across a table, middle for pendulum and lowest for continuous passive motion exercises.

CONCLUSIONS

The neuromuscular activity of superficial and deep shoulder muscles differs between the orthoses during ADL and also between the physiotherapeutic exercises.

On the importance of integrating comparative anatomy and One Health perspectives in anatomy education

As a result of many factors, including climate change, unrestricted population growth, widespread deforestation and intensive agriculture, a new pattern of diseases in humans is emerging. With increasing encroachment by human societies into wild domains, the interfaces between human and animal ecosystems are gradually eroding. Such changes have led to zoonoses, vector‐borne diseases, infectious diseases and, most importantly, the emergence of antimicrobial‐resistant microbial strains as challenges for human health. Now would seem to be an opportune time to revisit old concepts of health and redefine some of these in the light of emerging challenges. The One Health concept addresses some of the demands of modern medical education by providing a holistic approach to explaining diseases that result from a complex set of interactions between humans, environment and animals, rather than just an amalgamation of isolated signs and symptoms. An added advantage is that the scope of One Health concepts has now expanded to include genetic diseases due to advancements in omics technology. Inspired by such ideas, a symposium was organised as part of the 19^th International Federation of Associations of Anatomists (IFAA) Congress (August 2019) to investigate the scope of One Health concepts and comparative anatomy in contemporary medical education. Speakers with expertise in both human and veterinary anatomy participated in the symposium and provided examples where these two disciplines, which have so far evolved largely independent of each other, can collaborate for mutual benefit. Finally, the speakers identified some key concepts of One Health that should be prioritised and discussed the diverse opportunities available to integrate these priorities into a broader perspective that would attempt to explain and manage diseases within the scopes of human and veterinary medicine.

Unknown variant of the accessory subscapularis muscle?

Acting in medial rotation of the arm, the subscapularis (SM) is the most powerful and largest muscle of the rotator cuff. It is morphologically variable, especially in the number of tendons, place of insertion, and number of bellies, and it is sometimes fused with another muscle. An accessory subscapularis muscle (ASM) is among the morphological variations of the SM, but it is a really rare variant. The present case describes a very rare ASM that is divided into proximal tendinous attachment, intermediate fleshy muscular belly and distal tendinous attachment. Its origin is located on the lateral border of the scapula, but some fibers are connected with the muscular part of the SM. Its distal attachment is fused with the capsule of shoulder joint, above the tendinous insertion of the SM. Such an arrangement allows for greater stabilization of the joint. Moreover, there is a possibility that it could be used during treatment of ruptured SM tendons.

Glenohumeral joint and muscles functions during a lifting task

The mobility of the healthy shoulder depends on complex interactions between the muscles spanning its glenohumeral joint. These interactions ensure the stability of this joint. While previous studies emphasized the complexity of the glenohumeral stability, it is still not clear how the kinematics and muscles interact and adapt to ensure a healthy function of the glenohumeral joint. To understand the function of each muscle and degree of freedom of the glenohumeral joint in executing an above-the shoulder box handling task while ensuring stability, we adapted an index-based approach previously used to characterize the functions of the lower limb joints and muscles during locomotion. Forty participants lifted two loads (6 Vs. 12 kg) from hip to eye level. We computed the mechanical powers of the glenohumeral joint and its spanning muscles. We characterized the function of muscles and degrees of freedom using function indices. The function of the glenohumeral joint underlined its compliancy and design for a large range of motion, while the rotator cuff indices emphasized their stabilizing function. The overall muscle functions underlined the complexity of the glenohumeral stability that goes beyond the rotator cuff. Additionally, the load increase was compensated with changes in the functions that seem to favor joint stability. The implemented approach represents a synthetized tool that could quantify the glenohumeral joint and muscles behavior during tridimensional upper limb tasks, which might offer additional insight into motor control strategies and functional alterations related to pathologies or external parameters (e.g., load).

The pectoral girdle of StW 573 ('Little Foot') and its implications for shoulder evolution in the Hominina

The ca. 3.67 Ma adult skeleton known as 'Little Foot' (StW 573), recovered from Sterkfontein Member 2 breccia in the Silberberg Grotto, is remarkable for its morphology and completeness. Preservation of clavicles and scapulae, including essentially complete right-side elements, offers opportunities to assess morphological and functional aspects of a nearly complete Australopithecus pectoral girdle. Here we describe the StW 573 pectoral girdle and offer quantitative comparisons to those of extant hominoids and selected homininans. The StW 573 pectoral girdle combines features intermediate between those of humans and other apes: a long and curved clavicle, suggesting a relatively dorsally positioned scapula; an enlarged and uniquely proportioned supraspinous fossa; a relatively cranially oriented glenoid fossa; and ape-like reinforcement of the axillary margin by a stout ventral bar. StW 573 scapulae are as follows: smaller than those of some homininans (i.e., KSD-VP-1/1 and KNM-ER 47000A), larger than others (i.e., A.L. 288-1, Sts 7, and MH2), and most similar in size to another australopith from Sterkfontein, StW 431. Moreover, StW 573 and StW 431 exhibit similar structural features along their axillary margins and inferior angles. As the StW 573 pectoral girdle (e.g., scapular configuration) has a greater affinity to that of apes-Gorilla in particular-rather than modern humans, we suggest that the StW 573 morphological pattern appears to reflect adaptations to arboreal behaviors, especially those with the hand positioned above the head, more than human-like manipulatory capabilities. When compared with less complete pectoral girdles from middle/late Miocene apes and that of the penecontemporaneous KSD-VP-1/1 (Australopithecus afarensis), and mindful of consensus views on the adaptiveness of arboreal positional behaviors soliciting abducted glenohumeral joints in early Pliocene taxa, we propose that the StW 573 pectoral girdle is a reasonable model for hypothesizing pectoral girdle configuration of the crown hominin last common ancestor.

Exploring the functional morphology of the Gorilla shoulder through musculoskeletal modelling

Musculoskeletal computer models allow us to quantitatively relate morphological features to biomechanical performance. In non‐human apes, certain morphological features have long been linked to greater arm abduction potential and increased arm‐raising performance, compared to humans. Here, we present the first musculoskeletal model of a western lowland gorilla shoulder to test some of these long‐standing proposals. Estimates of moment arms and moments of the glenohumeral abductors (deltoid, supraspinatus and infraspinatus muscles) over arm abduction were conducted for the gorilla model and a previously published human shoulder model. Contrary to previous assumptions, we found that overall glenohumeral abduction potential is similar between Gorilla and Homo. However, gorillas differ by maintaining high abduction moment capacity with the arm raised above horizontal. This difference is linked to a disparity in soft tissue properties, indicating that scapular morphological features like a cranially oriented scapular spine and glenoid do not enhance the abductor function of the gorilla glenohumeral muscles. A functional enhancement due to differences in skeletal morphology was only demonstrated in the gorilla supraspinatus muscle. Contrary to earlier ideas linking a more obliquely oriented scapular spine to greater supraspinatus leverage, our results suggest that increased lateral projection of the greater tubercle of the humerus accounts for the greater biomechanical performance in Gorilla. This study enhances our understanding of the evolution of gorilla locomotion, as well as providing greater insight into the general interaction between anatomy, function and locomotor biomechanics.

Development of a comparative chimpanzee musculoskeletal glenohumeral model: implications for human function

Modern human shoulder function is affected by the evolutionary adaptations that have occurred to ensure survival and prosperity of the species. Robust examination of behavioral shoulder performance and injury risk can be holistically improved through an interdisciplinary approach that integrates anthropology and biomechanics. Coordination of these fields can allow different perspectives to contribute to a more complete interpretation of biomechanics of the modern human shoulder. The purpose of this study was to develop a novel biomechanical and comparative chimpanzee glenohumeral model, designed to parallel an existing human glenohumeral model, and compare predicted musculoskeletal outputs between the two models. The chimpanzee glenohumeral model consists of three modules - an external torque module, a musculoskeletal geometric module and an internal muscle force prediction module. Together, these modules use postural kinematics, subject-specific anthropometrics, a novel shoulder rhythm, glenohumeral stability ratios, hand forces, musculoskeletal geometry and an optimization routine to estimate joint reaction forces and moments, subacromial space dimensions, and muscle and tissue forces. Using static postural data of a horizontal bimanual suspension task, predicted muscle forces and subacromial space were compared between chimpanzees and humans. Compared with chimpanzees, the human model predicted a 2 mm narrower subacromial space, deltoid muscle forces that were often double those of chimpanzees and a strong reliance on infraspinatus and teres minor (60-100% maximal force) over other rotator cuff muscles. These results agree with previous work on inter-species differences that inform basic human rotator cuff function and pathology.

The subscapularis tendon: A proposed classification system

BACKGROUND

The subscapularis muscle originates from the medial two-thirds and from the lower two-thirds of the groove on the subscapular fossa of the scapula and inserts into the lesser tubercle of the humerus. Our initial hypothesis is that it shows little morphological variation. The aim of this study is to demonstrate and classify the morphological variability of the subscapularis muscle.

METHODS

Classical anatomical dissection was performed on 64 upper limbs (44 females, 20 males, 30 left and 34 right, fixed in 10% formalin). The mean age "at death" of the cadavers was 75.6 years (range 48-95), and the group comprised equal numbers of female and male adults (Central European population). Upon dissection, the following morphological features were assessed: the number of tendons of the SM, the type of insertion of each tendon of the SM, morphometric measurements of the SM.

RESULTS

Four types of morphology (based on number of tendons) were observed in the cadavers. Type I was characterized by a single band. This was the most common type, occurring in 43.7% of all cases. Type II was characterized by a double tendon (superior and inferior); it occurred in 9.4%. Type III had three tendons (superior, middle, and inferior). It was the rarest type (7.8% of cases). Type IV, called "multiband", was the second most common (39.1%) and was divided into five subtypes.

CONCLUSIONS

The subscapularis muscle is highly morphologically variable. Knowledge of particular types of insertion is essential for both clinicians (for example orthopedists, physiotherapists) and anatomists.

Postcranial elements of small mammals as indicators of locomotion and habitat

Many studies have shown a correlation between postcranial anatomy and locomotor behavior in mammals, but the postcrania of small mammals (<5 kg) is often considered to be uninformative of their mode of locomotion due to their more generalized overall anatomy. Such small body size was true of all mammals during the Mesozoic. Anatomical correlates of locomotor behavior are easier to determine in larger mammals, but useful information can be obtained from the smaller ones. Limb bone proportions (e.g., brachial index) can be useful locomotor indicators; but complete skeletons, or even complete long bones, are rare for Mesozoic mammals, although isolated articular surfaces are often preserved. Here we examine the correlation of the morphology of long bone joint anatomy (specifically articular surfaces) and locomotor behavior in extant small mammals and demonstrate that such anatomy may be useful for determining the locomotor mode of Mesozoic mammals, at least for the therian mammals.

Odd-nosed monkey scapular morphology converges on that of arm-swinging apes

Odd-nosed monkeys 'arm-swing' more frequently than other colobines. They are therefore somewhat behaviorally analogous to atelines and apes. Scapular morphology regularly reflects locomotor mode, with both arm-swinging and climbing anthropoids showing similar characteristics, especially a mediolaterally narrow blade and cranially angled spine and glenoid. However, these traits are not expressed uniformly among anthropoids. Therefore, behavioral convergences in the odd-nosed taxa of Nasalis, Pygathrix, and Rhinopithecus with hominoids may not have resulted in similar structural convergences. We therefore used a broad sample of anthropoids to test how closely odd-nosed monkey scapulae resemble those of other arm-swinging primates. We used principal component analyses on size-corrected linear metrics and angles that reflect scapular size and shape in a broad sample of anthropoids. As in previous studies, our first component separated terrestrial and above-branch quadrupeds from clambering and arm-swinging taxa. On this axis, odd-nosed monkeys were closer than other colobines to modern apes and Ateles. All three odd-nosed genera retain glenoid orientations that are more typical of other colobines, but Pygathrix and Rhinopithecus are closer to hominoids than to other Asian colobines in mediolateral blade breadth, spine angle, and glenoid position. This suggests that scapular morphology of Pygathrix may reflect a significant reliance on arm-swinging and that the morphology of Rhinopithecus may reflect more reliance on general climbing. As 'arm-swinging' features are also found in taxa that only rarely arm-swing, we hypothesize that these features are also adaptive for scrambling and bridging in larger bodied anthropoids that use the fine-branch component of their arboreal niches.

Proximal Humerus Morphology Indicates Divergent Patterns of Locomotion in Extinct Giant Kangaroos

Sthenurine kangaroos, extinct “giant kangaroos” known predominantly from the Plio-Pleistocene, have been proposed to have used bipedal striding as a mode of locomotion, based on the morphology of their hind limbs. However, sthenurine forelimb morphology has not been considered in this context, and has important bearing as to whether these kangaroos employed quadrupedal or pentapedal locomotion as a slow gait, as in extant kangaroos. Study of the correlation of morphology of the proximal humerus in a broad range of therian mammals shows that humeral morphology is indicative of the degree of weight-bearing on the forelimbs during locomotion, with terrestrial species being distinctly different from arboreal ones. Extant kangaroos have a proximal humeral morphology similar to extant scansorial (semi-arboreal) mammals, but sthenurine humeri resemble those of suspensory arboreal taxa, which rarely bear weight on their forelimbs, supporting the hypothesis that they used bipedal striding rather than quadrupedal locomotion at slow gaits. The humeral morphology of the enigmatic extinct “giant wallaby,” Protemnodon, may be indicative of a greater extent of quadrupedal locomotion than in extant kangaroos.