Consideration of sexual dimorphism, age, and asymmetry in quantitative measurements of muscle insertion sites

Abstract

Variation in entheses has been attributed to repetitive mechanical stress, but age, sex, and genetics may also affect entheses. In this study, the insertion areas of the pronator teres, biceps brachii, deltoid, pectoralis major, latissimus dorsi, teres minor and subscapularis were measured. The skeletal sample included 137 American Blacks and Whites from the Terry and Hamann-Todd Human (20th century) collections and 238 skeletons from Hawikuh, New Mexico (1300–1680 AD); Indian Knoll, Kentucky (3300–2000 BC); Hardin Village, Kentucky (1500–1620 AD); Kushkokwim River, Alaska (protohistoric); and Mummy Cave, Alaska (protohistoric). There was no significant sexual dimorphism at Hawikuh, moderate levels for Indian Knoll and Hardin Village (12.8–15.2%), and high levels for Kushkokwim River and Mummy Cave (22.6–32.8%). All significant differences in these levels occurred between Alaskan and non-Alaskan populations. These findings suggest that cultural or genetic adaptations to climatic differences explain some of the variation in sexual dimorphism of the insertion areas. Lateral asymmetry in insertion areas show reduced levels of right-sided dominance when compared to maximum humeral length. Correlations between the frequency of longer right humeri and the frequency of handedness found in modern populations may be a by-product of developmental processes. The increased symmetry for muscle insertions indicates a closer correspondence to the true pattern of mechanical stress since many tasks involve both limbs. Males and females had smaller insertion areas in their 20s than in their 30s and 40s, but the difference was only significant for males. The greater differences between younger and older males when compared to females can be explained by the delayed physical maturation of males. There were no significant differences in humeral measurements with age. These results suggest that activity-related studies not rely solely on epiphyseal fusion of the long bones to estimate physical maturity. © 1998 John Wiley & Sons, Ltd.