1. THE INFLUENCE OF MATURATION ON SEASONAL CHANGES IN STRENGTH, SPEED AND POWER IN YOUTH SOCCER PLAYERS  R. Morris1, S. Emmonds1, B. Jones1, G, Roe1, J. Lake2, N. D. Clarke3, K. Till1. INTRODUCTION Longitudinal assessment for athlete tracking is very important, especially when trying to understand the effect of maturation has on physical development. However, limited longitudinal evidence exist on the development of strength, speed and power across youth soccer players (12-18 yrs) when considering maturation. This study assessed the influence of maturation on the changes in speed, strength and power across a competitive season in elite youth soccer players. METHODS: One hundred and twelve male youth soccer players from 4 professional soccer academies were assessed at the start- (T1) and post- season (T2) for 30m sprint speed, countermovement jump height and strength (Isometric Mid-Thigh Pull peak force) via force plate analysis. Maturation groups were classified according to peak height velocity (PHV) as ‘Pre-PHV’ (<-1 years PHV; n=55), ‘Circa-PHV’ (between <- 1 to 1> PHV; n=27) and ‘Post-PHV’ (>1 PHV; n=30). A mixed model approach was used to identify mean percentage differences (90% CI) between T1 to T2, across the maturation groups. Magnitude based inferences (MBIs) were then used to assess the magnitude of difference between the changes in performance across the maturation groups (e.g., Pre-PHV vs Circa-PHV). RESULTS: Table 1 shows the mean (±SD) values for performances from T1 to T2 with percentage change in performance (CI 90%) along with MBI’s between the maturation groups. Pre-PHV demonstrated a possibly greater increase in jump height than both Circa- PHV (3.1% [-1.7 to 7.9%]) and Post-PHV (1.2% [-2.9 to 5.3%]) groups, while Circa-PHV exhibited a possibly reduced change in jump height than Post-PHV (-2.0% [-7.1 to 3.0]). There was a possibly greater improvement in sprint speed in both Pre-PHV (-1.5% [-3.0 to 0.0]) and Circa-PHV (-1.5% [-3.4 to 0.5] when compared to Post-PHV, but differences between Pre-PHV and Circa-PHV were unclear. Pre-PHV and Circa-PHV demonstrated possibly (1.8% [-3.0 to 6.5]) and likely (8.8% [2.5 to 15.2] greater increases in peak force than Post-PHV respectively, while Circa-PHV showed a likely greater increase (-7.1% [-13.1 to -1.1]) in comparison to Pre-PHV. Increases in relative peak force were reduced in Pre- PHV, demonstrating possibly (-8.8% [-14.6 to -3.0]) and likely (-5.1% [-9.8 to -0.4]) differences with Circa-PHV and Post-PHV respectively, while Circa-PHV exhibited a possibly greater increase (3.7% [-2.5 to 10.0]) than Post-PHV. CONCLUSION: Changes in performance from pre- to post-season seem to be dependent on maturation and performance variable. Pre-PHV athletes seem to possess a greater ability to increase speed and power whilst greatest increases in absolute and relative strength occur at Circa-PHV. Notably, post-PHV failed to demonstrate any meaningful changes in performance compared to pre-circa-PHV. PRACTICAL APPLICATIONS: Practitioners should be aware of the potential maturity related time periods for development of strength, speed and power, within youth soccer players. This has implications for both talent identification and player development with further research required to understand the impact of training on speed, strength and power development in relation to maturation.   3. 2. DrJLake r.o.morris@leedsbeckett.ac.uk @rhysmorris88