Introduction More than half a million athletes were enrolled in a marathon run during the last years. During running the Achilles tendon is submitted to high loads, and multiple tensile strain cycles. There are no studies investigating the effect of a marathon in the structure of the Achilles tendon. Thus, the aim of this study was to investigate the short-term response of the Achilles tendon structure after running a marathon.

Materials and methods Ten male non-elite runners who participated in a marathon were included. Tendon structure was assessed before, 2 days and 7 days after a marathon using the ultrasound tissue characterization (UTC), an imaging tool which quantifies tendon organization dividing the structure into four different echo types (I–IV).

Results Two days after a marathon, no significant changes in tendon structure were observed. However, after 7 days both insertional and midportion structure changed significantly. At the insertion of the tendon there was a significant decrease in the percentage of echo types I, III and IV and a significant increase in the percentage of echo type II. The midportion of the tendon showed a significant decrease in the percentage of echo types III and IV and a significant increase in the percentage of echo type II.

Conclusion We observed that the effects of running a marathon on the Achilles tendon structure occured 7 days after the event. In a population of runners without tendon injuries, there seems to be a positive adaptation of the tendon to the load after running a marathon.