Leukocyte response to repeated sprint exercise: Differences between adolescent and adult athletes

Repeated sprint exercise (RSE) can significantly affecting leukocytes dynamics, influenced by fitness, lifestyle, and biological maturation (BM), especially in adolescents. This experimental trial examined RSE effects on leukocyte responses in 34 male-athletes (18 adolescents, age = [12.9 ± 1.1] years; 16 adults, age = [22.5 ± 2.2] years). Participants performed 3 sets of 6 sprints (35 m with 10 s of passive rest and 5 min between sets), with blood samples collected before, immediately after, 2 h , and 24 h after-RSE. Leukocyte levels were analyzed microscopically and via leukogram, and adolescents' BM was assessed using peak-height-velocity (PHV). Statistical analyses included Friedman and Mann-Whitney U tests. Total leukocyte counts increased immediately after and at 2 h after-RSE in both groups ( p < 0.001 56). Lymphocytes increased immediately after-RSE (absolute and relative, p < 0.001 56). Absolute segmented neutrophils and the neutrophil-to-lymphocyte-ratio (NLR) peaked at 2 h ( p < 0.001). Large granular lymphocytes (LGLs) increased only immediately after-RSE ( p < 0.001 56). Absolute and relative eosinophils decreased at 2 h and rose again 24 h ( p < 0.001 56). Regarding the condition factor, 2 h after-RSE adults showed higher levels of NLR and segmented neutrophils compared to adolescents ( p < 0.000 1); while adolescents were higher in relative lymphocyte values ( p < 0.000 1). Eosinophil levels were higher in adolescents 2 h after-RSE, extending up to 24 h after-RSE ( p < 0.000 1). In PHV-based sub-analyses, for the time factor, circum-PHV (circum = during-PHV) individuals showed more evident responses in total leukocytes, absolute lymphocytes, LGLs, and segmented neutrophils ( p < 0.001 56), reflecting greater immunological sensitivity to exercise. No significant condition-related differences were observed between pre- and circum-PHV subgroups. It is concluded that RSE promotes transient immune mobilization, more pronounced in adults, and modulated by the stage of biological maturation. Visual Abstract . Leukocyte response to repeated sprints exercise (Main Results). This figure was created in https://BioRender.com • The present study investigated leukocyte counts and subpopulations in adolescent athletes, comparing the findings with those of adult athletes to elucidate differences in leukocyte behavior following repeated sprint exercise (RSE). • Sub-analyses were performed to compare adolescent athletes at different stages of biological maturation (BM), using peak-height-velocity (PHV) as a reference. Apparently, time-dependent effects appeared more prominent in biologically more mature individuals (circum-PHV [circum=during PHV]) relationship to their less mature peers (pre-PHV [Before the occurrence of PHV, i.e., before the highest rate of the growth spurt]), suggesting greater physiological sensitivity to exercise-induced metabolic stress. • The findings provide valuable reference values for monitoring leukocyte dynamics in both adolescent and adult athletes, offering a foundation for future research in sport and exercise immunology. Notably, optical microscopy proved to be a reliable and accessible method for analyzing leukocyte concentrations, presenting a practical alternative for field-based studies in this domain. • In both groups, leukocyte counts returned to baseline levels within 24 h after-RSE, indicating a full recovery. This recovery timeline suggests the feasibility of scheduling subsequent RSE sessions at 24 h intervals without compromising the immune system.