Although the exact mechanisms are still unclear, it is commonly acknowledged that acute eccentric exercise alters muscle performance whereas the repetition of successive bouts leads to the disappearance of the deleterious signs. In order to clarify this issue, we measured blood creatine kinase, lactate dehydrogenase activities and proton T2 relaxation time in various leg muscles 72 hours after a single and repeated bouts of exhausting downhill running sessions (-15 degrees ;1.5km/h), elapsed by either 4 or 7-days. After a single exercise bout, T2 and enzyme activities initially increased and recovered rapidly. When exercise bouts were repeated over a short time period (4-days), initial changes did not recover and endurance time throughout additional exercise sessions was significantly reduced. On the contrary, with a longer resting time between exercises (7-days), endurance time of additional running sessions was significantly longer and muscle changes (T2 increase, muscle oedema and enzyme activities changes) slowly and completely reversed. Significant correlations were found between T2 changes and enzymes activities. T2 changes in the soleus and gastrocnemius muscle heads were differently affected by lengthening contractions suggesting a muscle-specificity and indicating that muscle alterations might be linked to different anatomical properties such as fiber pennation angles, typology and/or to the exhausting nature of the downhill running sessions. We documented a "less muscle injury" effect due to the repetition of exercise bouts at a low frequency (i.e. 1 session per week), in accordance with the delayed muscle inflammation. This effect was not observed when the between-exercise resting time was shorter. Key words: eccentric, skeletal muscle, rat, lengthening, functional MR imaging.