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This study was designed to test the hypothesis that performing repeated bouts of eccentric exercise when muscles were not recovered from previous exercise would exacerbate muscle damage. Twelve nonweight-trained males (21.7+/-2.4 yr) performed three sets of 10 eccentric actions of the elbow flexors (ECC) using a dumbbell that was set at 80% of the preexercise maximal isometric force level. This same exercise was repeated 3 and 6 d after the first exercise. Maximal isometric force, relaxed and flexed elbow joint angle, muscle soreness, plasma creatine kinase, and glutamic-oxaloacetic transaminase activities were assessed. Ultrasound images were taken from the upper arm. These measures (except soreness) were assessed immediately before and after each eccentric exercise bout (ECC1, ECC2, and ECC3) and 3 d after ECC3. Soreness was assessed prior to ECC1 and once a day for 9 d thereafter. All criterion measures changed significantly (P< 0.01) after ECC1. ECC2 and ECC3 performed 3 and 6 d after ECC1 did not exacerbate damage and did not appear to slow the recovery rate. Increased echointensity in ultrasound images was demonstrated following ECC1, but no indication of increased damage was found after ECC2 and ECC3. Strenuous exercise performed with" damaged" muscles did not exacerbate damage or affect the repair process.