Introduction: Good drop jump performance is characterised by spring-mass like behaviour underpinned by effective use of the stretch-shortening cycle (SSC) to store kinetic energy in the elastic components of the muscle-tendon unit [1-3]. Drop jump ground reaction forces have been shown to be sensitive to the physiological changes that occur as a result of maturation [4] though little is currently known about the effect of maturity status on these variables. Purpose: To assess the effects of maturity status on a range of novel and traditional drop jump ground reaction force variables in elite male youth soccer players. Methods: 264 subjects (age: 13.95 ± 1.99 years; height; 162.3 ± 15.2 cm; body mass; 52.27 ± 13.99 kg; maturity offset: -0.19 ± 1.85 years) from the academies of 6 elite English soccer clubs agreed to take part in the study. Maturity offset was calculated using the equation of Mirwald et al. [7]; subjects were then determined to be either pre-PHV (maturity offset < -1.0), circa-PHV (-0.5 < maturity offset < +0.5) or post-PHV (maturity offset > +1.0). Following familiarization, subjects performed the testing session which required 3 trials of a 30 cm drop jump onto a force plate (Pasco, USA) sampling at 1,000 Hz. Ground reaction force data was processed to calculate a variety of variables; jump height (JH), ground contact time (GCT), reactive strength index (RSI), center of mass displacement (CoM), vertical stiffness, peak force (PF), peak braking force (PBF), peak propulsive force (PPF), timing of PBF and PPF and spring-like behaviour correlation (SLC). Results: There were no significant differences between any of the groups for GCT (p < 0.05). Significant increases in JH (pre-PHV: 23.74 ± 4.53 cm; circa-PHV: 28.36 ± 6.61 cm; 32.10 ± 4.78 cm) and RSI (pre-PHV: 0.79 ± 0.22; circa-PHV: 0.91 ± 0.24; post-PHV: 1.09 ± 0.31) were observed between consecutive maturity groups (p < 0.05). PBF was significantly greater in pre-PHV (4.17 ± 1.03 BW) than both circa- (3.66 ± 0.75 BW) and post-PHV (3.63 ± 0.80 BW) (p < 0.05). There were no significant differences between maturity groups for PPF (p < 0.05). PBF occurred significantly earlier in pre-PHV (57 ± 14 ms) than circa-PHV (68 ± 20 ms) and post-PHV (74 ± 18 ms) (p < 0.05). There were no differences between any maturity groups for the timing of peak propulsive force (p < 0.05). SLC increased significantly between each maturity group (pre-PHV: 0.82 ± 0.16; circa-PHV: 0.89 ± 0.09; post-PHV: 0.91 ± 0.09) (p < 0.05). Conclusions: Pre-PHV elite male soccer players tend to display poor spring-like behaviour which is characterised by greater ground reaction force in the early period of GCT; this is typically considerably larger than the PPF. Improved SSC function would facilitate improved reutilisation of elastic energy which appears to manifest as greater jump height rather than shorter ground contact times as young athletes mature. Practical Application: RSI is traditionally used to determine drop jump performance but the underlying kinetic variables that drive SSC function might offer greater insight into suitable coaching and training interventions in order to direct athletes towards a more spring-like profile. Research addressing the effect of cueing styles on drop jump performance demonstrates that directing an athlete's attention towards minimising ground contact time results in a larger peak landing force that occurs earlier during ground contact [8, 9].
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