Countermovement Jump Responses During an Academy Rugby League In-Season

ABSTRACT: Moore, DA, Darrall-Jones, JD, Weakley, JJS, Sawczuk, TS, Heyward, OW, Jones, BL, and Till, K. Countermovement jump responses during an academy rugby league in-season. J Strength Cond Res XX(X): 000-000, 2026-The countermovement jump (CMJ) is widely used to assess lower-body force-time characteristics in rugby league; however, how these characteristics change across the in-season period remains unclear. The purpose of this study was to investigate changes in lower-body force-time characteristics over an 8-week in-season period using CMJ outcome, driver, and strategy metrics derived from force plates. Twenty-five male academy rugby league players (17.0 ± 1.9 years) completed CMJ assessments twice weekly (2 trials per session): 24 hours prematch and 24-72 hours postmatch, while engaging in regular field training, resistance training, and weekly competition. Force plate-derived variables included outcome (i.e., jump height, relative peak power), driver (phase-specific mean force, impulse), and strategy (i.e., countermovement depth, contraction time) metrics. Linear mixed-effects models were used to assess acute (24-72 hours) and preobservation to postobservation window (8 weeks) changes, with "week" included as a fixed effect and "athlete identity" as a random effect. Hedges' g effect sizes with 95% confidence intervals were reported. Results demonstrated trivial reductions in CMJ performance across the observation window. Jump height and concentric impulse remained unchanged, but countermovement depth exhibited a moderate increase (Δ = 3.46 ± 1.60 cm, g = 0.61 (0.33-0.89); p < 0.001). These findings suggest athletes adopt compensatory patterns, characterized by greater ranges of motion and contraction time, to sustain performance under conditions of accumulating load. Incorporating outcome, driver and strategy metrics enhances the utility of CMJ testing for athlete monitoring, informing training adjustments, guiding recovery strategies and supporting long-term athlete development.