The consequences of a single bout of heavy eccentric exercise with and without repeated concentric exercises on MRI images, serum CK levels and markers of inflammation were studied. Two groups (ECC and ECCON), each consisting of 18 male volunteers, performed 70 eccentic contractions of the quadriceps femoris muscle. The study group (ECCON) performed additional concentric contractions on a dynamometer (Cybex II+) one day before and two hours, 1, 2, 3, 6 and 9 days after eccentric loading. Serum levels of creatine kinase (CK) were examined as a function of time, and correlated with measurements of magnetic resonance imaging (MRI) of the involved muscle groups. T2-weighted images of the thigh muscles were studied. Serum C-reactive protein, complement factors C3c and C4, haptoglobin and transferrin were measured as markers of inflammation. Additional concentric contractions (ECCON group) significantly increased CK, compared to the ECC group. However, it has no apparent effect on MRI signal intensity changes, which were of equal magnitude in the loaded vastus intermedius and deep parts of the vastus lateralis in both groups. Likewise, the serum markers of inflammation of the exercised muscles appeared to be absent. Based on MRI-images, additional concentric contractions had no statistically significant effect on muscle damage and breakdown of connective tissue. The five-fold increase in CK in the ECCON group could be a reflection of "massaging out" of the CK from the muscles into the circulation by additional concentric exercises. However, it could also be an indication for a superior sensitivity of assessing muscle fiber damage in comparison to the MRI.