The Ponseti method corrects a clubfoot by manipulation and casting which causes stress relaxation on the tendons. Here, we examined the effect of long-term stress relaxation on tendon extracellular matrix (ECM) by (1) anex vivostress relaxation test, (2) anin vitrotenocyte culture with stress relaxation and (3) anin vivorabbit study. Time-dependent tendon lengthening and ECM alterations including crimp angle reduction and cleaved elastin were observed, which illustrated the mechanism of tissue lengthening behind the treatment—a material-based crimp angle reduction resulted from elastin cleavage. Additionally,in vitroandin vivoresults observed restoration of these ECM alterations along with increased elastin level after 7 days of treatment, and the existence of neovascularization and inflammation, indicating the recovery and adaptation from the tendon in reaction to the treatment. Overall, this study provides the scientific background and information that helps explain the Ponseti method.