Sex Differences in Achilles Tendon Loading in Healthy Recreational Runners: A Cross-Sectional Study

CONTEXT: Running-related Achilles tendon injuries have been observed more frequently in males, which may reflect sex differences in tendon morphology or mechanics during running. Males exhibit a larger Achilles tendon cross-sectional area and some measures of Achilles tendon loading in running, but sex differences in measures of tendon loading have not been thoroughly characterized. This study examines sex differences in the Achilles tendon cross-sectional area and Achilles tendon loading variables associated with running kinetics and kinematics. DESIGN: Cross-sectional. METHODS: Recruited from a university population, 26 female and 23 male healthy recreational runners ran at 3.35 m/s on an instrumented treadmill and were assessed using 3-dimensional motion capture, musculoskeletal modeling, and diagnostic ultrasound. A multivariate analysis examined differences between sexes in Achilles tendon loading and additional running variables with alpha set to .05. Mean differences (MD) and confidence intervals (CIs) were reported. RESULTS: Males displayed greater peak Achilles tendon force (MD = 0.94 body weight [BW]; 95% CI, -1.48 to -0.33), peak gastrocnemius force (MD = 0.52 BW; 95% CI, -0.75 to -0.29), and Achilles tendon cross-sectional area (MD = 0.1 cm2; 95% CI, -0.15 to 0.04) compared with females (P < .05). No group differences were observed in peak Achilles tendon stress (MD = 4.13 kPa; 95% CI, -16.25 to 7.98), peak Achilles tendon strain (MD = 0.41; 95% CI, -1.60 to 0.78), peak soleus force (MD = 0.34 BW; 95% CI, -0.76 to 0.078), foot strike angle (MD = 3.1; 95% CI, -1.65 to 7.85), or peak vertical ground reaction force (MD = 0.106; 95% CI, -0.24 to 0.03) (P > .05). CONCLUSIONS: Male runners produced greater peak Achilles tendon force when running at the same speed, but similar peak Achilles tendon stress was observed between male and female recreation runners. Male runners had a greater Achilles tendon cross-sectional area. These findings may have implications to running-related injury based on sex.