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Achilles tendon injuries are problematic for triathletes ranked as the most severe injuries in club and development athletes. As an overuse injury, Achilles tendon injures were proposed to be the result of a combination of risk factors, requiring measurements which incorporated multiple risk factors. Stiffness was identified to be a measure that was influenced by many of the risk factors for Achilles injury. In a one-year prospective study, 75 triathletes were followed to determine the association between measures of lower extremity stiffness and the risk of developing an Achilles injury. Triathletes who developed a new or reoccurring injury during the surveillance period had higher leg and knee to ankle stiffness ratio compared to Uninjured athletes. The influence of transitioning from cycling to running, on lower extremity stiffness were assessed. Transitioning from cycling caused an increase in ankle stiffness (ES=0.55) but a decrease in knee stiffness (ES=-0.38). Individual responses are likely to be important when assessing injury risk.