Enhancing functional interpretations of shoulder morphology: an investigation of the range of motion and biomechanical capacity of the hominoid shoulder complex

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Introduction. The scapulae (shoulder blades) of apes (excluding humans) display considerable morphological differences compared to those of humans. These long known differences are well documented. Traditionally, ape-specific scapular morphology is understood as an adaptation to the arboreal habitat (climbing and suspensory locomotion), which was lost during the course of human evolution. However, this understanding is based on mere correlations of similar scapular morphologies and locomotor behaviours, while evidence for the functional advantage of the ape scapula is missing. As a result, current functional interpretations of (fossil) scapulae are based on assumptions; a tool for evidence-based interpretation is non-existent. Objectives. The aim of this project is to determine the effect of different shoulder morphologies on forelimb function within the Hominoidea (humans and apes), which will provide insight into the adaptation of the shoulder to different locomotor behaviours. In humans, the well-documented shoulder mechanism consists of a complicated interaction of all joints during arm raising and lowering. However, in apes, the contribution of the individual joints is currently unknown. The missing information on scapulothoracic movement (the gliding of the scapula across the thorax) is particularly problematic, as without knowing which scapular movements are necessary for apes to lift their arm, an investigation of the biomechanical properties of different shoulder anatomies is impossible. Work programme. As a first step, this project will investigate the shoulder mechanism and the range of motion of the individual joints in the shoulder girdle of two arboreal apes (gibbons and chimpanzees). This is addressed using state-of-the-art marker-based videography. The experiments take place post mortem; the upper arms of the apes are passively moved through their full range of motion, while the position of the individual bones is recorded via the markers. The marker data is used to investigate the range of motion of the individual elements and the contribution of each joint towards the overall shoulder mechanism. As a second step, the biomechanical properties of anatomically different shoulders will be investigated using musculoskeletal modelling. Firstly, a gibbon and a chimpanzee shoulder model will be developed. Together with already existing models of a gorilla and a human, these models will be used to evaluate and compare the biomechanical capacities of the shoulder muscles. The comparison of the capacities of different species together with the knowledge of the species-specific shoulder mechanism will allow us to enhance our understanding of the biomechanical effect of different shoulder anatomies.