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Purpose: Eccentric calf training for Achilles tendinopathy shows variable success in athletes. Recent insights suggest a role for tendon fluid flow (exudation or redistribution) during exercise, which explains post-exercise reductions in thickness and increases in stiffness of the tendon. This fluid flow is thought to be beneficial as it may promote tendon remodeling, reduce intratendinous pressure, and alleviate pain. In this perspective, slow, high-load exercises are promoted as they theoretically facilitate tendon fluid flow. However, evidence supporting this assumption is lacking. Therefore, this study aimed to investigate whether loading speed and intensity during eccentric calf training impact acute changes in midportion Achilles tendon thickness and stiffness, reflecting alterations in local tendon fluid content., Methods: A randomized, assessor-blinded, crossover trial was conducted with 34 healthy athletes (17 men, 17 women, age: 23.7 ± 6 years). Participants underwent 3 single-leg eccentric heel-drop interventions with 20% additional bodyweight, varying in loading speed (fast: 1 s, slow: 3 s) and loading intensity (low: to plantigrade, high: to maximal dorsiflexion). Achilles tendon anteroposterior diameter (APD), cross-sectional area (CSA), and shear-wave velocity (SWV) were assessed in the midportion region using ultrasonography and shear-wave elastography pre-and immediately post-intervention., Results: The slow, high-load intervention produced greater immediate reductions in tendon APD and CSA (8.7% and 10.1%), compared to the slow, low-load (4.0% and 4.7%) and fast, high-load (2.9% and 3.4%) interventions (p < 0.001). Moreover, only the slow, high-load intervention increased tendon SWV (52.4%, p < 0.001)., Conclusions: These findings provide the first evidence that both loading speed and intensity during eccentric calf training impact acute changes in Achilles tendon thickness and stiffness, likely mediated by changes in fluid flow, which could be relevant for tendinopathy rehabilitation., Competing Interests: Conflict of Interest and Funding Source: This study was internally funded by Ghent University Hospital. The authors have no conflicts of interest to disclose., (Copyright © 2024 by the American College of Sports Medicine.)