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The purpose of this study was to evaluate changes in muscle soreness and serum enzyme activity following consecutive drop jumps. Seven male subjects (mean age 30.6 years) performed drop jumps from a 80-cm box height every 7 s until exhaustion (mean = 114 drop jumps). A questionnaire was used to assess muscle soreness (0 = no pain, 7 = unbearable painful) both pre- and post-exercise (0, 12, 24, 36 and 48 h, and 3, 4 and 5 days after the exercise). Blood samples were also taken from three subjects at each of these times. For the other four subjects, blood samples were taken pre-exercise and 0, 12 and 36 h and 5 days post-exercise only. Although there was large inter-subject variability in the development of muscle soreness, all the subjects reported muscle soreness in their lower extremity muscles, especially in the quadriceps femoris. Muscle soreness developed significantly (P less than 0.01) over time, its peak (mean +/- S.E. = 3.7 +/- 0.7) occurring 12-48 h post-exercise. Serum enzyme activity changed significantly over time (P less than 0.05), but the changes were small. Not one subject showed a large increase in creatine kinase, and the average increase was less than 1.3 times as much as the pre-exercise level throughout the period of study. These results suggest that the muscle damage that occurs after drop jumping is not associated with a large release of muscle enzymes into the blood, and muscle soreness is not necessarily related to enzyme elevation following drop jumps.