Growing together: Developmental integration and modularity in the human talus-calcaneus complex

This study investigates age-related shape changes, morphological integration, and modularity in the human talus-calcaneus complex throughout postnatal development (0-10 years). Geometric morphometric analyses were performed on three-dimensional landmark data from 23 individuals binned into four age groups, considering both the talus and calcaneus. Statistical analyses included Procrustes ANOVA to assess age effects, two-block partial least squares to quantify inter-bone integration, and Covariance Ratio tests to evaluate modular organization within each bone. Trabecular bone architectural parameters from micro-CT scans were analyzed in parallel. Marked age-related shape changes were detected in both the talus and calcaneus, with pronounced morphological transitions observed between the middle age groups (1-3 and 3-6 years). The two bones exhibited strong morphological integration across all age classes, representing the first quantitative assessment of talus-calcaneus integration during ontogeny. Modular organization was confirmed also within each bone. Trabecular bone architectural analysis via micro-CT revealed distinct developmental patterns: the talus exhibited higher initial bone volume fraction with subsequent stabilization and stable trabecular thickness, while the calcaneus showed progressive increases in both bone volume fraction and trabecular thickness, alongside higher anisotropy values, presumably reflecting adaptation to its more direct role in experiencing impact forces during heel strike. The talus-calcaneus complex exhibits highly coordinated ontogenetic timing and strong morphological integration across both external morphology and internal trabecular architecture. These adaptations highlight the sophisticated interplay within this skeletal unit, supporting its function as an integrated biomechanical system critical for understanding human locomotor evolution and foot development.