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Context: Achilles tendons (ATs) adapt to increased loading generated by long-term adoption of a minimalist shoe running style. There may be difference in the chronology and extent of adaptation between the sexes. Objective: To learn the chronology of AT adaptations in female and male runners who transitioned to a minimalist running style through a planned, progressive 12-wk transition program. Design: Prospective cohort study of well-trained, traditionally shod runners who transitioned to minimalist shoe running. Setting: Repeated laboratory assessment at baseline and 3, 12, and 24 wk after initiating transition program. Participants: Fifteen women and 7 men (of 29 enrolled) completed the study. Main Outcome Measures: The authors used diagnostic ultrasound and isokinetic dynamometry to generate a force elongation curve and its derivatives at each time point. Results: Greater adaptations were observed in men than in women, with men generating more force and having greater increases in CSA, stiffness, and Young's modulus and less elongation after 12 wk of training. Conclusion: Men demonstrated changes in AT properties that were consistent with increased loading of the triceps surae during exercise. The women demonstrated far smaller changes. Further investigation is warranted to understand when adaptations may occur in women and the implications of altered AT mechanical properties for performance and injury risk. [ABSTRACT FROM AUTHOR]