Relationships and agreement between Yo-Yo intermittent recovery test level 1 performance and laboratory VO 2max in adolescent male soccer players: roles of maturity status, playing position, and running economy

YYIR1 is strongly associated with laboratory VO2max but systematically underestimates it. Coaches may use YYIR1 distance to monitor intermittent endurance, while interpreting estimated VO2max with caution. Larger, maturity-balanced samples are needed to clarify subgroup differences and improve youth-specific prediction accuracy.