Investigating the effect of manual physical activity on the form of human hand entheses

Abstract

Reconstructing physical activity based on human skeletal remains comprises a fundamental objective of anthropological sciences. Entheses, the areas of muscle attachment on the bone surfaces, have been widely utilized as occupational stress markers. However, most previous methods for analyzing entheseal morphology (size and/or shape) are characterized by substantially low precision, lack of three-dimensional (3D) multivariate statistical analysis, and absence of individuals documented for their long-term physical activities. Furthermore, no previous study used histological methods for assessing the effect of physical activity on the interindividual variability of entheseal surfaces. My past research put forth a precise methodology for measuring the 3D size of hand entheseal areas, identifying two main multivariate patterns among entheses. On this basis, this PhD thesis performed a multi-disciplinary approach to the analysis of hand entheseal form (a term encompassing both size and shape) and its potential relationship with habitual physical activities.

In Paper I, the hand entheses of a thoroughly documented sample were analyzed using a highly precise 3D method of quantification, followed by multivariate statistical analysis. The utilized material is part of an anthropological collection which is gradually becoming a unique universal reference for the most detailed documentation of the specimens’ lifelong physical activities. The results revealed a close statistical association between multivariate patterns of hand entheses and the nature of individuals’ long-term occupational profiles.

In the framework of an ongoing multidisciplinary research project, the developed method of entheseal analysis was applied for reconstructing the occupational profile of an unidentified individual from Basel (Paper II). The results indicated that this specimen was involved in precise manual activities relying on thumb-index finger interactions. This outcome came in agreement with other research on the physical activities of this individual.

Paper III introduced a new and precise geometric morphometric approach for investigating hand entheseal 3D shape, identifying a statistically significant interaction between 3D size and shape variation in three hand entheses. In this way, it set a novel basis for future research on both aspects of entheseal form (3D size and shape), bridging the gap between quantitative and qualitative methods of entheseal analysis. Furthermore, the results showed that larger phalangeal entheses present a proportionally more projecting surface, which is linked to greater moment arm and biomechanical efficiency for the attaching muscle.

Finally, paper IV presented a microscopic histological analysis of hand entheses, which reported an interaction between entheseal morphology and the levels of applied biomechanical forces. Particularly, the elevated bone areas of hand entheses were associated with greater concentrations of calcified fibrocartilage, which is widely considered as a direct indicator of biomechanical stress. As a consequence, individual entheses with elevated marginal areas comprised evidence of greater biomechanical stress.

Overall, this PhD thesis identified a clear interaction between physical activity and human hand entheses based on three methodological approaches and four separate analyses. The multivariate 3D analysis showed that the patterns among different hand entheses reflect the nature of an individual’s lifelong occupational activities. At the same time, the shape of larger entheseal areas is proportionally projecting, providing greater biomechanical efficiency for the attaching muscle. Finally, higher entheseal bone elevation seems to be related with greater biomechanical stress (i.e., higher concentrations of calcified fibrocartilage).