The 81st Annual Meeting of the American Association of Physical Anthropologists (2012)

The unique bilateral strength asymmetry and cross-sectional shape of Neandertal humeri may reflect adaptation to scraping tasks, but not spear thrusting


1McDonald Institute for Archaeological Research, Cambridge University, 2Department of Kinesiology, Pennsylvania State University, 3Department of Biological Anthropology, Cambridge University

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Aspects of Neandertal skeletal anatomy that fall outside the range of modern human morphological variation may reflect osseous adaptation to specific activities. Indirect experimental evidence has supported the hypothesis that spear thrusting plausibly explains two of these skeletal attributes: a) pronounced (right-side dominant) humeral bilateral strength (J) asymmetry and, b) anteroposteriorly reinforced mid-diaphyseal cross-sectional shape (Ix/Iy) in both the right and left arm. However, alternative habitual tasks have not been evaluated. To extend this line of inquiry, muscle activity was measured using surface electromyography at the right and left pectoralis major (PM) as well as the anterior (AD) and posterior (PD) deltoid during the performance of various ‘spear thrusting’ and ‘hide scraping’ tasks. Contrary to published predictions of greater dominant limb (right-side) muscle activity during spear thrusting, bilateral comparisons measured during three separate spearing activities reveal significantly greater non-dominant (left-side) innervation of the shoulder (AD, PD) and chest (PM) musculature. Similar bilateral comparisons performed during single-handed, ‘pushing’, ‘pulling’ and ‘hacking’ scraping tasks reveal significantly greater dominant-limb muscle activity at these same muscles (AD and PM). In contrast, the performance of a two-handed ‘vertical pull-down’ scraping technique caused significantly greater non-dominant-limb muscle activity. These measures of in vivo muscle activity do not support previous attempts to link unique skeletal attributes of the Neandertal and Palaeolithic Homo sapiens humeri to bimanual spearing tasks. The question remains whether a more repetitive, yet still physically demanding, task might better explain these unique humeral adaptations.

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