Background The Achilles tendon undergoes prolonged, repetitive, submaximal loading during endurance running. The acute effects of endurance running on the Achilles tendon are unclear.

Hypothesis Structural abnormalities and Doppler flow are common in highly trained endurance runners. A marathon may lead to acute structural changes in the Achilles, including change in diameter and increased Doppler flow.

Study design Prospective cohort study

Methods 41 Achilles tendons in 21 marathon runners were examined with grey-scale and colour Doppler ultrasound (US) before and immediately after the London Marathon, 2010. Six females and 15 males, mean age 40 years (±13) were included. 17 tendons were re-examined at 16 days (±1.6) postmarathon. Anteroposterior (AP) diameter (mm), grey-scale pathology (present, absent) and Doppler flow (present, absent) were measured.

Results Mean race time was 03:45 h (±33 min). US examination revealed 14 of 41 tendons (34%) had abnormalities before the marathon.

After the marathon, mean tendon diameter decreased significantly in both normal and abnormal tendons (53 mm–47 mm, p=0.005). Tendon diameter returned to premarathon thickness after 16 days (±1.6 days). Doppler flow was present in seven of 41 tendons (17%) before the race. This did not significantly increase in direct response to the marathon (p=0.41). There was an association between abnormal tendons and faster marathon times (p=0.005).

Conclusions About 1/3 of the runners had structural abnormalities at baseline which did not increase immediately after the marathon. AP tendon diameter significantly decreased in normal and abnormal tendons immediately after the race. Tendon thickness appears to return to baseline at 2 weeks postmarathon.