The Achilles tendon stores and releases strain energy, influencing running economy. The present study aims to verify the influence of the Achilles tendon tangent modulus, as a material property, on running economy by comparing two groups of elite endurance-performance athletes undergoing different running training volumes. Twelve athletes, six long-distance runners and six pentathletes, were studied. Long-distance runners had a higher weekly running training volume (116.7±13.7 vs. 58.3±20.4 km, p<0.05) and a better running economy (204.3±12.0 vs. 222.0±8.7 O₂ mL ∙ kg⁻¹ ∙ km⁻¹, p<0.05) evaluated in a treadmill at 16 km·h^–1, 1% inclination. Both groups presented similar VO_2max (68.5±3.8 vs. 65.7±5.0 mL ∙ min⁻¹ ∙ kg⁻¹, p>0.05). Achilles tendon tangent modulus was estimated from ultrasound-measured deformations, with the ankle passively mobilized by a dynamometer. True stress was calculated from the measured torque. The long-distance runners had a higher maximum tangent modulus (380.6±92.2 vs. 236.2±82.6 MPa, p<0.05) and maximum true stress than pentathletes (24.2±5.1 vs. 16.0±3.5 MPa, p<0.05). The correlation coefficient between tangent modulus at larger deformations was R=–0.7447 (p<0.05). Quantifying tendon tissue adaptations associated with different running training volumes will support subject and modality-specific workouts prescription of elite endurance athletes.