The Trail Runners’ Tendon—How Do Weekly Mileage and Elevation Gain Affect Achilles and Patellar Tendon Morphology?

Background: Unlike road running, mountain and trail running typically cover longer distances and include uphill and downhill segments that impose unique physiological and mechanical demands on athletes. Objectives: This study aimed to identify morphological differences in the patellar and Achilles tendons between trail and road runners. Moreover, the potential influence of weekly mileage and accumulated positive elevation gain on the morphology of both tendons was obtained. Design: Cross-sectional comparative study. Methods: Thirty-three road runners (11 women, 22 men) and thirty-three trail runners (13 women, 20 men) were recruited and their weekly mileage and elevation gain collected. All participants had a weekly training volume exceeding 20 km. The thickness and cross-sectional area (CSA) of their patellar and Achilles tendons were evaluated using ultrasound. Results: Independent samples t-tests revealed significant differences between groups for the Achilles tendon (p < 0.003) but not for the patellar tendon (p > 0.330). Further, Spearman’s correlation coefficients indicated moderate positive correlations for the thickness and CSA of the Achilles tendon with weekly running volume (0.256 and 0.291, respectively) and with elevation gain (0.332 and 0.334, respectively), suggesting a tendency for the tendon to adapt to greater training loads, enhancing its structural integrity and resilience. Conclusions: Trail runners exhibit larger and thicker Achilles tendons, likely due to increased weekly mileage and elevation gain, highlighting the adaptive response to mechanical overload from uphill running.