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Purpose: The primary aim was to describe the response of plasma creatine kinase (CK) activity in a sample of African-American men after a bout of eccentric exercise. The study also described signal intensity changes detected by MR in the musculature of the right lower leg., Methods: Subjects were 20 male volunteers of African descent (age = 24 +/- 4 [mean +/- SD] yr). Each walked backward for 60 min at 3 km x h(-1) down a 23% grade. Venous blood was sampled before exercise, immediately (0 d) after and 1, 2, 4, and 7 d after exercise for plasma CK assay. Soreness in the plantar flexor muscles was evaluated in 18 subjects at selected times during the 7 d postexercise. Injury to the plantar flexor muscles was evaluated by magnetic resonance imaging (MRI)., Results: The subjects had high baseline plasma CK activity (187 +/- 127 IU x L(-1); 163 +/- 70 IU x L(-1) with one outlier excluded) compared with typical clinical norms. As a group, CK activity was increased (P < 0.05) 4 d (980 +/- 1331 IU L(-1)) and 7 d (1022 +/- 1031 IU L(-1)) postexercise, compared with preexercise. Eleven (55%) of the subjects had large, delayed increases in plasma CK activity ("hyperresponses"). As a group, the plasma CK response was similar to responses of comparison Caucasian subjects. All subjects reported delayed muscle soreness; there was no association between soreness and plasma CK. Every subject showed MRI evidence of injury to plantar flexor muscles postexercise but varying in degree and time course. Plasma CK activity correlated to MRI signal intensity (rho = 0.445)., Conclusion: Results suggest that changes in plasma CK activity and skeletal muscle injury in African-American men after eccentric exercise do not differ from the responses of Caucasians.