Biological maturation may contribute to difference in neutrophil mobilization and IL-8 responses following repeated sprint exercise in athletes: a quasi-experimental study

Repeated sprint exercise (RSE) is commonly used in intermittent sports, including combat and court-based disciplines. This type of high-intensity exercise induces substantial physiological stress and may elicit acute immune responses, including neutrophil mobilization and transient increases in circulating interleukin-8 (IL-8). Because biological maturation (BM) is accompanied by endocrine, metabolic, and hematological changes throughout development, athletes at different maturational stages may exhibit distinct immune and recovery responses to the same exercise stimulus. This study aimed to investigate changes in the profile of circulating neutrophils and IL-8 levels in adolescent athletes at different BM stages and adults after RSE. This was a non-randomized, blinded quasi-experimental study. Thirty trained athletes participated, including 20 adolescents categorized by BM stage based on peak height velocity (PHV): pre-PHV ( n = 10), circum-PHV ( n = 10), and 10 adults ( n = 10). Participants completed a standardized RSE protocol. Blood samples were collected at rest and during recovery. Leukocyte counts were determined using an automated hematology analyzer. Circulating neutrophil phenotype was evaluated through flow cytometry (CD45 identification and side scatter area—SSC-A) and optical microscopy to classify segmented and band (rod) neutrophils and to assess pseudopod formation. Plasma IL-8 concentrations were measured using an immunoassay. RSE induced a transient increase in circulating neutrophil counts across groups ( p < 0.05). Adults and circum-PHV athletes demonstrated greater magnitude and/or longer persistence of selected neutrophil responses compared with pre-PHV athletes. Exercise was associated with changes in circulating neutrophil phenotype, including SSC-A, distribution of segmented and rod neutrophils, and pseudopod formation during recovery. Segmented neutrophils peaked immediately after RSE (time effect: p < 0.001, η² p = 0.236), whereas pseudopod formation reached its highest values 2 h post-RSE (time effect: p < 0.001, η² p = 0.815). Plasma IL-8 increased acutely and followed temporal changes in circulating neutrophil responses (time effect: p < 0.001, η² p = 0.140). Adjustment for chronological age did not alter the main

Findings: . Athletes classified at different BM stages exhibited distinct temporal patterns of neutrophil mobilization and circulating morphological responses following repeated sprint exercise. These differences remained after adjustment for chronological age but should be interpreted within the developmental context that includes both maturation and age. Combined assessment of neutrophil count, circulating phenotype, and IL-8 may contribute to the physiological monitoring of acute exercise responses across developmental stages.