Studies of comparative morphology clearly distinguish the shoulder morphology of Homo from that of the other hominoids. While the shoulder morphology of non-human hominoids is thought to signal adaptations to arboreal locomotion, human shoulder morphology is understood to have lost this adaptation during hominin evolution. Ideas how non-human hominoid shoulder morphology is advantageous in an arboreal context suggest that the specific shoulder morphological traits enhance
the arm-raising mechanism. However, this idea has not been biomechanically tested. This thesis constitutes the first analysis of the biomechanical consequences of two distinct shoulder morphologies within Hominoidea by comparing the glenohumeral muscle capabilities of Gorilla to Homo. The biomechanical capabilities are evaluated by constructing a computational musculoskeletal model of a gorilla thorax, shoulder girdle and upper arm, which is used to predict relevant biomechanical metrics such as muscle moments and moment arms. Muscle moments and moment arms are predicted for two important mechanisms, arm-raising and arm-lowering. The predictions are compared to those of an already existing human musculoskeletal model in order to evaluate differences in arm-raising and arm-lowering capability based on the two distinct thorax and shoulder girdle morphologies. The results of the biomechanical analyses show that the arm-lowering mechanism is enhanced in Gorilla compared to Homo, instead of the arm-raising mechanism. The enhanced arm-lowering mechanism is evident by greater moment capacities of two important arm-lowering muscles, pectoralis major and teres major. The greater moments are the result of greater muscle force capacities and greater moment arms, due to the beneficial musculoskeletal geometry of Gorilla. The results highlight that a more distal muscle insertion along the humerus has the greatest enhancing effect on the arm-lowering moment arms of teres major and pectoralis major. Furthermore, thorax and shoulder girdle morphological traits that are well known to distinguish non-human apes from humans were found to contribute to the enhancement of the arm-lowering mechanism. The more cranially oriented glenoid, obliquely oriented scapular spine and cranial scapula position on the thorax enabled certain muscles to act as arm-lowering muscles in Gorilla, contrary to the arm-raising action capability that is predicted for Homo. The enhanced arm-lowering capability is likely advantageous for the arboreal locomotion of apes. During hoisting behaviours that
are known to occur during suspension and vertical climbing, arm-lowering is used to lift the heavy body of the apes upward. The results of this thesis in conjunction with earlier EMG studies suggest those muscles which are highly activated during these hoisting behaviours also have enhanced arm-lowering capacities in Gorilla and potentially other non-human hominoids compared to Homo. As such, the results highlight shoulder morphological traits that are biomechanically important for the arboreal locomotor behaviour of apes. By this, the thesis demonstrates a link between the conformation of shoulder morphological traits and their biomechanical capability, which will aid future functional interpretations of extant and extinct species.:Acknowledgements
Bibliographische Darstellung
Summary
Zusammenfassung
Chapter 1: Exploring the functional morphology of the Gorilla shoulder through musculoskeletal modelling
Chapter 2: Comparison of the arm-lowering performance between Gorilla and Homo through musculoskeletal modeling
Conclusion
Appendix A: Supplementary Information for Chapter 1
Appendix B: Supplementary Information for Chapter 2
Appendix C: Curriculum Vitae
Appendix D: Author Contributions
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:79847 |
| Date | 08 July 2022 |
| Creators | van Beesel, Julia |
| Contributors | Universität Leipzig |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
| Relation | https://doi.org/10.1111/joa.13412, https://doi.org/10.1002/ajpa.24511 |
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