Background: It has been reported that strenuous exercise increases the number of bone marrow-derived progenitor cells such as CD34+ cells in the blood, but no previous studies have investigated the changes in circulating CD34+ cells following resistance exercise. This study tested the hypothesis that the number of CD34+ cells in the blood would increase after eccentric exercise of the elbow flexors, but decrease in recovery, and the magnitude of the changes would be dependent on the magnitude of muscle damage. Methods: Nine men (28.0 ± 6.6 years) performed exercises consisting of 10 sets of six maximal voluntary eccentric contractions of the elbow flexors with their non-dominant arm. Six of them performed the same exercise with the same arm 4 weeks later. Changes in indirect markers of muscle damage were measured before, within 10 min after, and at 24, 48, 72, and 96 h after eccentric exercise. Differential leukocyte counts (total leukocytes, neutrophils, lymphocytes, monocytes) and CD34+ cells in the blood were measured before, immediately after, and at 2, 24, 48, 72, and 96 h following the exercises. Results: After eccentric exercise, significant (p < 0.05) decreases in maximal voluntary isometric contraction torque and increases in delayed onset muscle soreness and plasma creatine kinase activity were observed. However, no significant changes in leukocytes and CD34+ cells were evident. The changes in muscle damage markers were significantly (p < 0.05) smaller following the second exercise session as compared with the first exercise session, but the changes in leukocytes and CD34+ cells were not significantly different between sessions. Conclusion: These results did not support the hypothesis, and showed that eccentric exercise-induced muscle damage to the elbow flexors did not influence the number of circulating CD34+ cells.
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