Date of Graduation

5-2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Anthropology (PhD)

Degree Level

Graduate

Department

Anthropology

Advisor/Mentor

Plavcan, Joseph M.

Committee Member

Delezene, Lucas K.

Second Committee Member

Paul, Kathleen S.

Keywords

asymmetry; enthesis; handedness; metacarpal; myology; opponens pollicis

Abstract

Muscle attachment sites (entheses) are often used to infer soft tissue anatomy and reconstruct behaviors within skeletal populations, but there remains a significant debate about whether and to what degree muscle use is accurately reflected in bony morphology. One of the most difficult problems in establishing the relationship between variation in muscle use and entheseal development is the inability to exclude a variety of external factors thought to influence enthesis development, including age, sex, body mass, and activity patterns. One way around this is to take advantage of the fact that humans are handed (preferentially using one hand over another, regardless of task), therefore showing asymmetrical patterns of muscle recruitment in the hands. If a direct relationship between bony morphology and muscle recruitment does exist, one would expect to find strong asymmetry in entheseal development within human hands. Conversely, non-human primates do not exhibit handedness and should not show asymmetrical manual entheseal development. The m. opponens pollicis is heavily used in many key grips employed during precision manipulation and is active during flexion, abduction, and adduction during thumb opposition. As such, the muscle insertion on the radial aspect of the first metacarpal is an ideal site for testing if enthesis development is reflective of repetitive muscle recruitment. This study examines if lifetime activity patterns influence bony morphology by comparing opponens pollicis muscle enthesis asymmetry between handed (Homo sapiens) and non-handed (Hylobates lar, Gorilla spp., Macaca fascicularis) primates to identify if differences exist between dominant and non-dominant hands. Additionally, since thumb flexion and opposition span all three joints, the functional signal from precision pinch and grasps should not be confined to a single bony region. Enthesis asymmetry is therefore hypothesized to serve as a predictor for asymmetry in the proximal and distal ends due to their role in flexion and stabilization during key grips. This study measured enthesis size between the right and left first metacarpals in a sample (n=197 pairs) of adult modern humans and catarrhine primates using both measurements obtained from calipers and from distance-based heat maps generated from 3-D renderings. Results demonstrated right-directional asymmetry in the opponens pollicis enthesis in modern humans and symmetry for non-human primates for enthesis size. However, it demonstrated that increased muscle recruitment alone is not sufficient to induce significant differences between sides; rather, this study established there is a threshold for enthesis size that most individuals do not exceed, and that, to infer muscle use based on enthesis size, the individual must exceed this threshold. Further, this study found that asymmetry in the opponens pollicis enthesis can predict a moderate amount of asymmetry within the proximal and distal ends of the metacarpal, but also found that ligamentous recruitment contributes to asymmetry within those areas. Overall, this study found support for inferring muscle use based on enthesis size but cautions against making large generalizations based on size alone, as further research is needed to better understand the roles that both muscle recruitment frequency and intensity play in enthesis development.

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