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To evaluate the tendon response after acute strength training in chronic Achilles tendinosis using magnetic resonance imaging (MRI).Twenty-two patients (44 Achilles tendons, 15 males, 8 patients with bilateral symptoms) with a median age of 45 yr (range 28–57 yr) were included in the study. In all patients, both Achilles tendons were examined with MRI before and immediately after a standardized training program. The most painful side underwent 6 sets and 15 repetitions of heavy-loaded eccentric training. The contralateral tendons underwent only concentric loading during the training program. The tendon volume and the intratendinous signal were evaluated and calculated by MRI using a seed-growing technique.The immediate response of eccentric loading on the symptomatic tendons resulted in a 12% increase of the tendon volume, evident on T2-WI, from 7.8 ± 2.0 to 8.8 ± 2.7 cm3 (P < 0.001), and a 31% increase of the intratendinous signal evident on PD-WI, from 221 ± 74 to 278 ± 78 signal units (SU) (P < 0.001). The corresponding sequences on the contralateral concentrically loaded tendons showed an increase of 17% of tendon volume, from 6.1 ± 1.5 to 7.0 ± 1.6 cm3 (P < 0.001), and an increase of 27% of the intratendinous signal, from 170 ± 55 to 211 ±57 SU (P < 0.001). There was no significant difference of the mean of the increased tendon volume and the intratendinous signal between the eccentrically heavily loaded symptomatic tendons and the concentrically loaded contralateral tendons.Both eccentric and concentric loading of the Achilles tendon resulted in increased total tendon volume and intratendinous signal. This increase may be explained by a higher water content and/or hyperemia in the Achilles tendon during and/or immediately after strength training of the gastrocnemius-soleus complex.