What are the main findings? a) Players improved their intermittent endurance capacity (vIFT) after the 8-week SSG-based training, with no differential adaptation between maturity groups despite clear baseline performance differences. b) External load metrics were comparable across maturity levels, yet less mature players consistently reported higher internal load, indicating maturational differences in the perceptual and physiological response to identical training demands. What are the implications of the main findings? a) SSG-based training can be effectively implemented across heterogeneous maturity groups without compromising aerobic development, supporting its utility as an ecologically valid method for youth football conditioning. b) Coaches should consider biological maturation when interpreting internal load measures, as less mature players may experience disproportionate physiological strain despite similar external loads, with potential implications for load management and injury risk. Abstract: Biological maturation strongly influences youth players` physical performance, yet its role in shaping training load responses remains unclear. This study examined how maturation status affects physical adaptations and the relationship between internal load (IL) and external load (EL) during an 8-week small-sided game (SSG) training program in youth football. Fifty-three players were allocated to an experimental group (EG) or control group (CG). EL during SSGs was continuously monitored using 10 Hz GPS units with inertial sensors, while session-RPE quantified IL. Intermittent endurance (vIFT) and 5 m and 30 m sprint performance were assessed before and after the intervention. Players were categorized by years from peak height velocity (PHV). No between-group differences were found in EL variables; however, IL was significantly lower in more mature players. The EG showed a significant improvement in vIFT, whereas sprint performance remained unchanged and the CG showed no improvements. Both maturity groups increased vIFT similarly, with no interaction between maturation status and training adaptation. These findings indicate that SSG-based training effectively enhances intermittent endurance regardless of biological maturity, although less mature players experience higher perceived exertion under equal mechanical demands. Maturation status should therefore be considered when prescribing and interpreting training loads in youth athletes.
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