Based on prior research, the standard degree of humeral retroversion (the rotation of the head relative to the distal articulation) has been agreed to be between 25°- 35°, and increased retroversion is potentially correlated with several factors. Madison Hubbart’s research evaluates retroversion angles among adult male and female humeri from the Windover population to test for significant differences associated with a possible cause, such as biological sex, age, and activity. The assessment of the humeral retroversion is one of the most commonly used methods for discerning and analyzing the amount of stress on the humerus. The degrees of humeral retroversion is often greater in individuals who habitually perform activities that unnaturally rotate the shoulder such as overhand throwing and rowing (1).
“Retroversion is an adaptive change and is the result of the inclination to use one arm over the other. The external rotation of the limb becomes greater as an individual makes throwing a habitual act and is commonly seen in sports players of modern populations” (1). As the humeral head makes the adaptive change, mechanical stress placed on the limb decreases, allowing for prolonged activity and an extended range of motion at the articulation between the humeral head and glenoid fossa. Thus, more disparities are seen between the dominant and non-dominant limbs. The researcher, Hubbart, hones in on the Windover population in the Early Archaic Period. “Though they were atlatl users, their upper limb symmetry is more similar to that of agricultural populations, reflecting more bilateral activities,” (2). This unusual reality deviates Windover from the perceived norm. In fact, by examining the juxtaposition between Windover and other Archaic sites, Windover displays fewer significant asymmetries between males and females. Hubbart’s thesis analyzes the differences in right and left humeral retroversion amongst adult males and females from Windover to test for anatomical indications of unilateral throwing in males and potentially reaffirm the highly symmetrical nature of the population.
The retroversion rotation has shown to be correlated with biological sex, age, and the presence of joint disease and injury, but is most influenced by culturally specific activities, such as subsistence and gender roles (5). For instance, the angle of retroversion is greater in younger individuals than in adults. Numerous studies indicate that there is no causal effect based on biological sex alone. The variations of the angle of the humeral head that begin to appear are attributable to dominant and nondominant limbs, along with activities that involve a definitive gendered activity pattern, such as a strict division of labor (6). Past research shows that individuals engaging in habitual throwing activities often have a greater degree of retroversion unilaterally. Studies today support this finding, as they found that sports players, such as baseball pitchers, who make the use of overhand throwing also display increased retroversion unilaterally. Studies focusing on prehistoric agricultural and hunter-gatherer societies employ a similar mode of measurement and a majority come to the same conclusion regarding the average degree of retroversion.
Hubbart’s findings show that retroversion of both the left and right humeri are statistically greater in males than females. Additionally, there is no statistically significant difference between the different age cohorts at Windover. Thus, it is reasonable to assume that age has no direct relationship with retroversion, regardless of sex and subsistence (24). Age does not work as a cause of retroversion at Windover because the ages are too variable. Activity patterns are the most reasonable cause for retroversion in the Windover population. “The Windover male sample is set apart because they are more likely to continue in the repetitive behavior of atlatl throwing. Windover females and the outgroups participate in largely bilateral activities that are not putting the same amount of strain on the glenohumeral joint,” (25). Overall, Windover males display a significant difference from Windover females and both sexes of the outgroup population for both left and right humeri.
Madison Hubbart is a graduate of the College of Social Sciences and Public Policy at Florida State University. This post was based on Madison’s honors thesis, written by COSSPP Blog Intern, Jillian Kaplan.