The 82nd Annual Meeting of the American Association of Physical Anthropologists (2013)

The primate upper arm: A study on the deltoid index


1Centre for Evolutionary Medicine, Institute for Anatomy, University of Zurich, 2Institute for Human Evolution, University of the Witwatersrand

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On the proximal humerus we find several features that help making the shoulder joint stable and at the same time more mobile and it provides the attachment place for the main rotator cuff muscles.

The humerus of Hylobates lar (n=67), Pongo sp. (n=21), Gorilla gorilla (N=37), Pan troglodytes (n=43) and Homo sapiens (n=89) was analyzed and compared to casts of the humerus MH2 (A. sediba) and KNM WT 15000 (H. erectus). A number of 9 landmarks were defined on the humerus and digitized with a MicroScribe-3DX. Based on this, indices have been calculated in order to enable a comparison with older studies.

The deltoid index is an indicator for the position of the most distal insertion of the deltoid muscle. The deep insertion of the deltoid muscle in humans and great apes helps to increase the lever advantage of the arm. Hylobates lar, in contrary, shows a much higher insertion of the deltoid muscle. This higher position of the insertion of the deltoid muscle results from the elongation of the hylobatid arm and the special distribution of the muscle mass as an adaptation for the much specialized form of locomotion in this species.

For Australopithecus sediba and Homo erectus, nearly equal values have been calculated, both of which fall into the range of Homo sapiens and are significantly different from the values of Hylobates lar.

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