Humeral torsion and activity-related change in the human upper limb and pectoral girdle. A biomechanical investigation and social implications.

Abstract

This project investigas humeral torsion and activity-related change in the human upper limb. Increased humeral torsion angles have been identified in the professional throwing athlete and may be associated with strenuous activity. The nature of humeral torsion as an osteogenic response to the strain environment is investigated to identify its role in the behavioural morphology of the upper limb. These physical manifestations of strenuous physical activity provide an insight into the make-up of medieval armies prior to the establishment of standing armies. Populations analysed include two blade-injured samples, Towton and a subsample of blade-injured men from the Priory of St. Andrew, Fishergate, York. The men from the Mary Rose, a Tudor warship are also investigated. Other samples analysed include the rural sites of Wharram Percy and Hickleton, the urban cemeteries from the Priory of St. Andrew, Fishergate,York and the leprosarium of Sts. James and Mary Magdalene, Chichester, the modern cadaver-based Terry collection and non-human primates, Gorilla sp., Pan sp., Pongo sp., and Macaca sp.. Measurement of the humeral torsion angle and external measurements and indices of architecture, articulations and robusticity are employed. Cross-sectional geometric properties are investigated using CT imaging of the paired humeri from a sub-sample of blade-injured individuals and a comparative sample of those who were not. Bilateral asymmetry is investigated to identify the role of plasticity within the humerus and to reveal aspects of limb dominance. The results are compared with non-human primate species to obtain insight into inter-species differences. Results indicate the humeral torsion is not ontogenetically constrained, but is highly variable between and within populations, individuals and even between sides. Biomechanical analyses indicate that in the Towton population, humeral torsion may serve as part of a two-stage adaptation, in which the architecture is modified to enable greater biomechanical efficiency in distributing strain, reducing the need of increased cortical thickness. Changes in humeral torsion related to strenuous activity have been identified, although in the blade-injured samples it is decreased torsion angles, w hile in the comparative sample it is increased torsion angles that significantly correlate with limb hypertrophy. Humeral torsion appears to be influenced by other measurementd of humeral architecture, specifically, the amount of anterior bowing and anterior curvature to the distal humeral shaft. This work demonstrates the need for individual rather than population-based analyses, as the heterogeneity within population samples obscures individual variation in activity patterns. This analysis provides baseline data for typical populations of the Middle Ages. From this, it is then possible to investigate the individual within this baseline, to identify those who stand out from their samples through habitual, strenuous activity patterns. Movement patterns identified related to warfare include those consistent with the use of the longbow in the Towton sample and the use of a sword in the Fishergate blade-injured sample. These men, and those of the Mary Rose, appear to have either been selected for combat based on size, or benefited from a more nutritious diet during growth.