Background: After performing strenuous physical exercises such as repeated sprint exercise (RSE), the leukocyte morphology undergoes changes suggesting immunodepression. Furthermore, it has been previously suggested that, in pediatrics, leukocyte changes may be influenced by the rates of the biological maturation (BM) process, which varies among individuals of similar chronological age. Objective: To investigate the influence of RSE on leukocyte morphology in adolescent athletes at different rates of BM. Methods: We conducted an experimental trial with a final sample consisting 21 adolescent athletes (male sex, age = 12.7 ± 1.2) underwent an RSE protocol, and blood samples were collected at "before, after, and 2 h post" moments. Based on the BM rates assessed by a predictive equation of skeletal age, participants were divided into two groups (Accelerated [n = 10] and Synchronized [n = 11]). Leukocyte morphology was analyzed microscopically and by leukogram. Results: Regardless of the BM rates, a time effect was observed on the absolute levels of total leukocytes (g/L: n2p = 0.36), lymphocytes (g/L: n2p = 0.50, %: n2p = 0.29), segmented neutrophils (g/L: n2p = 0.16, %: n2p = 0.43), and neutrophil-to-lymphocyte ratio [NLR] (g/L: n2p = 0.30). The synchronized BM group showed higher values than the accelerated group for total leukocyte levels (moments after and 2 h post [n2p = 0.10; p < 0.001]) and segmented neutrophils (moments after and 2 h post [n2p = 0.10; p < 0.001]). Conclusion: In adolescent athletes, changes caused by RSE in leukocyte morphology appear to be dependent on the BM rates.
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