“We Are the Sons of Our Own Deeds”: Comparing Skeletal Health and Frailty Indices in Deceased Individuals Across 2000 Years of Milanese History

Objectives: In bioarchaeology, the concepts of resilience and frailty, and their quantification through indices, have gathered significant attention. This study is the first to apply, evaluate, and compare skeletal frailty indices and aims to trace frailty over time while identifying methodological challenges in their use on a sample representative of urban Milan's history. Materials and methods: Two-hundred fifty individuals from five historical periods over 2000 years in urban Milan, equally represented by estimated males and females, were analyzed. Three skeletal frailty indices were applied-the "Health Index" GHHP, "Skeletal Frailty Index" (SFI), and "Biological Index of Frailty" (BIF)-and their diachronic variations interpreted. Index values were compared to each other through Spearman's correlations, and frailty values were assessed by periods (overall and by estimated sex) and by estimated sex through ANOVA and General Linear Models. Results: Diachronic analyses revealed a gradual increase in frailty from the Roman era to the Late Middle Ages, which then progressively decreased, corroborating historical sources. While all methods identified the Late Middle Ages sample as the frailest, discrepancies arose when defining the least frail group, especially when considering estimated biological sex and age variables. Discussion: Our study found practical and conceptual limitations in the GHHP. Most noticeably, criteria for GHHP and SFI limited sample size (and consequently) representation, while the more inclusive BIF proved overly permissive, allowing direct comparisons between skeletons with differential preservation. This study highlights common challenges and prospects, defines common criteria to standardize methodologies, and further investigates the relevance of stress markers in relation to frailty.