Countermovement Jump Performance Across Ten Competitive Microcycles in Under-18 Soccer Players: An Analysis of Delayed and Long-Term Responses to Training and Official Competition

ABSTRACT: Villaseca-Vicuña, R, Freitas, TT, Spyrou, K, Marín-Cascales, E, Comyns, T, Loturco, I, and Zabaloy, S. Countermovement jump performance across 10 competitive microcycles in under-18 soccer players: An analysis of delayed and long-term responses to training and official competition. J Strength Cond Res XX(X): 000-000, 2026-The aims of this study were to (a) determine within-microcycle changes in countermovement jump (CMJ) measures (i.e., jump height, reactive strength index modified [RSImod], and eccentric peak velocity [EPV]) collected 2 days after (MD+2) and 1 day before (MD-1) the match-day across 10 weeks during the competitive period in under-18 soccer players and (b) analyze week-to-week variations in the selected CMJ-metrics collected on MD+2 across the entire mesocycle. Twelve (n = 12) under-18 academy soccer players (age: 17.5 ± 0.5 years; body mass: 71.9 ± 7.7 kg) volunteered to participate in this study. Over 10 competitive weeks, CMJ assessments were recorded on force plates at 2 key microcycle time points: MD+2 (active recovery phase at the start of the microcycle) and MD-1 (end of the microcycle). Results showed significant within-week changes in CMJ height and EPV in most of the training microcycles (p: <0.001 to 0.049) while RSImod showed significant changes in only 4 of the 10 microcycles (p: 0.001 to 0.049). Similarly, the between-week comparisons showed significant variations only in EPV and RSI (p: 0.005 to 0.049). These findings suggest that jump height might be better suited to assess within-week changes, while RSImod appears to hold greater discriminative value for detecting between-week fluctuations. Eccentric peak velocity, in turn, may provide useful information for both within-week and between-week changes in young soccer players.