Precision-grasping muscles & and power-grasping muscles... Reconstructing occupations thru muscles to allow us to look into our ancestral past

New horizons in reconstructing past human behavior: Introducing the “Tübingen University Validated Entheses-based Reconstruction of Activity” method. Fotios Alexandros Karakostis, Katerina Harvati. Evolutionary Anthropology: Issues, News, and Reviews, March 25 2021. https://doi.org/10.1002/evan.21892

Abstract: An accurate reconstruction of habitual activities in past populations and extinct hominin species is a paramount goal of paleoanthropological research, as it can elucidate the evolution of human behavior and the relationship between culture and biology. Variation in muscle attachment (entheseal) morphology has been considered an indicator of habitual activity, and many attempts have been made to use it for this purpose. However, its interpretation remains equivocal due to methodological shortcomings and a paucity of supportive experimental data. Through a series of studies, we have introduced a novel and precise methodology that focuses on reconstructing muscle synergies based on three-dimensional and multivariate analyses among entheses. This approach was validated using uniquely documented anthropological samples, experimental animal studies, histological observations, and geometric morphometrics. Here, we detail, synthesize, and critically discuss the findings of these studies, which overall point to the great potential of entheses in elucidating aspects of past human behavior.

1 INTRODUCTION

Muscle attachment sites (i.e., “entheses”) are the areas of the bone where muscles, tendons, or ligaments attach.1 They represent the only direct evidence of the musculotendinous, soft tissue system on skeletal remains. As such, they are commonly thought to broadly reflect muscular activity and have been used for reconstructing habitual physical activity among past human populations and extinct hominin species.2-4 The methods of entheseal analysis can broadly be divided into those relying on observational scoring systems describing robusticity or potentially pathological entheseal changes (EC) of the surface on the one hand3-7; and those quantifying entire entheseal size or shape based on three-dimensional (3D) surface models on the other.8-12 The interpretations of the latter approaches typically rely on the assumption that entheseal morphology may vary with changes of biomechanical load.2, 13, 14 However, most visual scoring systems for analyzing these activity markers have often been criticized for a lack of precision and statistical rigor.15, 16 At the same time, several studies focusing on entheseal 3D morphology in laboratory animals and human cadavers have found no evidence to support the hypothesized functional character of entheses,12, 17-19 questioning their reliability in reconstructing past behavior.

In this framework, we recently put forth a new and repeatable methodology for analyzing entheses based on multivariate analyses targeting the reconstruction of muscle synergy groups.10 As our approach relies on 3D surface area measurements of entheses without potential pathological lesions (i.e., distinctive osteophytic or osteolytic lesions),10 it can be considered more comparable to visual scoring techniques focusing on entheseal robusticity, rather than those incorporating potentially pathological ECs.20, 21 Here, we conduct a critical review of the interdisciplinary body of literature that validated this 3D approach on the basis of anthropological samples with uniquely detailed occupational documentation22 as well as two blindly-conducted experimental studies on animals.23, 24 These results are discussed together with our relevant findings from histological research on human cadaveric material25 and 3D geometric morphometric applications for analyzing muscle attachment form.11 Taken together, our synthesis supports the new approach's efficiency as an important tool for reconstructing past human behavior,26 while also describing its limitations and potential pathways for improvement in the future. We term this new methodology the Tübingen University Validated Entheses-based Reconstruction of Activity (hereafter V.E.R.A.).

1.1 Types of entheses and biomechanical implications

Muscle attachment sites are broadly categorized as fibrous or fibrocartilaginous, depending on the nature of their insertion to the bone.1 In fibrous attachments, the muscle/tendon or ligament attaches directly onto the periosteum, which is itself attached to the bone via Sharpey fibers.27, 28 Fibrocartilaginous entheses, on the other hand, describe a gradient structure of attachment involving four transition zones: tendon, uncalcified fibrocartilage, calcified fibrocartilage, and bone.1, 27, 29, 30 Some studies of EC scores analyze fibrous and fibrocartilaginous entheses together.20 However, most researchers consider fibrous entheses less useful for reconstructing physical activity, mainly due to the poor understanding of their reaction to biomechanical stress, lower measurement repeatability, weak correlation with occupational types, lower level of bilateral asymmetry in the upper limbs, and higher correlation with body size proxies.5, 6, 31-34 This viewpoint is also supported by histological research suggesting that, in fibrous entheses (typically along the metaphysis or diaphysis of long bones), biomechanical forces seem to be dissipated over a wider area of attachment.17, 27 By contrast, in fibrocartilaginous entheses (typically at apophyses or epiphyses of long bones), all biomechanical stress is concentrated on to relatively smaller attachment surfaces, potentially leading to higher stress energy density and greater deformations of the comparatively thin cortical layer within these areas.1, 17, 29 Nonetheless, it should be emphasized that recent histological research showed that fibrous and fibrocartilaginous attachments may co-exist in the same enthesis,27, 35, 36 suggesting that the nature of muscle attachment to bone is much more complex than previously believed.

The anatomical area where an enthesis is located is also considered important, given that the nature and intensity of the biomechanical forces at play are bound to differ across the skeleton. As a result, there is substantial variation among entheses both in gross morphology and micro-architecture.5, 25, 30 From a biomechanical point of view, entheseal variability has been suggested to be broadly associated with an interaction between body weight (compressive) and muscle tendon (tensile) forces, which substantially vary across anatomical regions.25, 30, 37, 38 In this regard, the hand shares the smallest proportion of body weight in the human skeleton,39 suggesting that perhaps the effects of muscle recruitment on entheses may possibly be greater in hand bones compared to elements from high-weight-bearing anatomical areas.25 Based on this concept, as well as on the fundamental role of manual tasks in human daily activities and subsistence strategies,40 most of our research to date has focused on the entheses of the human hand.