Background/Objectives: Understanding the factors that influence tendon mechanical properties is essential for optimizing performance and preventing injuries in elite athletes. This study aimed to identify the strongest correlates of the biomechanical properties (frequency, stiffness, logarithmic decrement, relaxation, and creep) in the Achilles and patellar tendons in elite international athletes. Methods: A cross-sectional study was conducted with 111 elite athletes from 11 sports disciplines assessed at a high-performance training center. Tendon properties were measured bilaterally using MyotonPRO. Anthropometric (height, weight, age), demographic (sex, limb dominance defined as the preferred limb for sport-specific activities), and sport-specific variables were analyzed using correlation, multiple regression, and machine learning approaches. Results: Height showed the strongest correlations with tendon frequency and stiffness, particularly for the Achilles tendon (r = 0.52 for frequency; r = 0.53 for stiffness; p ≤ 0.001, large effects). Sex differences were evident across all measures, with men showing higher stiffness and frequency, and women greater relaxation and creep (partial η² = 0.35–0.48, Cohen’s d = 0.84–1.16). Sports discipline explained substantial variance in tendon properties (η² > 0.40), and limb dominance influenced Achilles stiffness, with left-dominant athletes showing higher values (p < 0.05). Age showed minimal associations (r < 0.10). Conclusions: Height, sex, and sports discipline were the strongest correlates of Achilles and patellar tendon mechanical properties in elite athletes, with large and practically meaningful effects across sports. This comprehensive analysis, utilizing multivariate and machine learning approaches, provides insights that can inform individualized training, injury prevention, and performance optimization strategies in high-performance sports.

Objective: To analyze and compare the viscoelastic properties of the Achilles and patellar tendons in elite athletes from different sports and determine whether there are significant differences attributable to sport-specific mechanical loading. Methods: This is a cross-sectional study in which a total of 105 elite athletes (33 females; 72 males) from 11 distinct sport disciplines (road cycling, roller hockey, karate, athletics, volleyball, taekwondo, speed skating, judo, archery, handball, and boxing) were evaluated at a national high-performance sports center. The viscoelastic properties (stiffness, relaxation, decrement, among others) of both tendons were measured bilaterally using a portable myotonometry device (Myoton Pro®, Myoton, Tallinn, Estonia). The primary outcome was tendon stiffness. Results: Significant differences were found in all measured variables for both tendons (p < 0.001) across sports, except for the decrement parameter in the patellar tendon (p = 0.45). The Achilles tendon demonstrated greater sport-specific adaptation, with stiffness showing the highest number of inter-sport differences (29 significant pairwise comparisons). Relaxation was the most variable parameter in the patellar tendon (9 significant comparisons). Conclusions: The findings demonstrate that the viscoelastic properties of the Achilles and patellar tendons differ significantly based on the sport practiced by elite athletes. This suggests that long-term, sport-specific mechanical loading plays a crucial role in tendon adaptation, leading to functional specialization. While the cross-sectional design precludes causal inference, these results provide valuable reference data for training prescription and injury prevention strategies tailored to specific athletic disciplines.