Comparison of Calf Muscle and Achilles Tendon Stiffness Between Triathletes and Physically Active Controls: A Cross-Sectional Study Using Shear Wave Elastography.

Introduction: Triathlon's global popularity, with over 120 national federations and millions of athletes, has led to an increase in injuries, particularly Achilles tendinopathy, affecting 12-24% of long-distance and 7.7% of short-distance triathletes. Understanding the morphological adaptations of the Achilles tendon and calf muscles is crucial. Objective: This study compares the stiffness of the Achilles tendon and calf muscles between triathletes and physically active controls, while identifying differences in the superficial, middle, and deep layers of the calf muscles across and within these groups. Methods: A cross-sectional study was conducted with 42 participants divided into four groups: 10 male triathletes, 10 male controls, 11 female triathletes, and 11 female controls. Shear wave elastography assessed the stiffness of the Achilles tendon and calf muscles. Results: No between-group differences were found for the overall stiffness of the Achilles tendon and calf muscles. In the soleus muscle, the stiffness of the superficial layer was greater in the male control group compared to female triathletes (p=0.002). Also, the middle layer was greater in the male control group compared to the male triathletes (p=0.023) and female triathletes (p=0.028). Finally, the middle layer was greater in male controls compared to female triathletes (p=0.008). Within-group differences showed that the superficial layer was generally stiffer than the middle and deep layers across all groups and muscles. Notably, in the lateral gastrocnemius, the deep layer showed greater stiffness compared to the middle layer only for female controls (p=0.014). Conclusion: Triathlon does not affect the overall stiffness of the Achilles tendon and calf muscles, but differences in muscle layers highlight the need for a segmented approach in elastography, which may reveal specific training effects or injury risks.