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Stretch-shortening cycle (SSC) function, recognized as fundamental for effective plyometrics and sprinting, can be estimated from measures of leg stiffness and reactive strength index (RSI). Whilst published data for these measures exist for adult populations, limited data is available within paediatric populations. Given the significance of SSC to explosive ability, it is deemed necessary to identify potential sensitive periods of growth and development which may produce accelerated adaptations. (i) determine differences in leg stiffness and RSI between chronological- and maturity-divided groups of schoolchildren (2nd-11 th Grade) and (ii) investigate whether age- or maturity-related factors were dominant contributors to RSI and leg stiffness performance. Two hundred and fifty high school-aged boys performed four maximal hopping trials and a single trial of sub-maximal hopping (2.5Hz). RSI was calculated from the maximal hopping task, whilst leg stiffness was calculated from the sub-maximal test. Additionally, four trials of both squat- (SJ) and countermovement-jumps (CMJ) were performed, representing concentric strength and slow SSC respectively, with the mean of the best two jump heights being used for analysis. One-way ANOVA's revealed significant increases in leg stiffness between grades 3-5 and 7-10; and between grades 4-6 and 7-10 for RSI. When grouped according to estimated maturity, significant differences were found for both leg stiffness and RSI between-3 years pre-PHV (mean -2.97 yr) and PHV(mean -0.05 yr), and between PHV and + 3 years post PHV (mean 2.99 yr). Multiple stepwise regression analysis revealed that body mass (R2 change = 0.62), RSI (R2 change = 0.02) and maturity status (R2 change = 0.02) produced the greatest explained variance for leg stiffness (R2 = 0.66; p < 0.001). When leg stiffness was normalised to body mass, height (R2 change = 0.28), RSI (R2 change = 0.09) and maturity (R2 change = 0.03) produced the greatest explained variance (R2 = 0.40; p < 0.001). For RSI, SJ (R2 change = 0.54), stiffness (R2 change = 0.03), body mass (R2 change = 0.02), CMJ (R2 change = 0.01) and age (R2 change = 0.01) produced the largest explained variance (R2 = 0.61; p < 0.02). Statistics for both tolerance (> 0.1) and VIF (< 10) suggested minimal risk of multicolinearity for both models. The greatest variance in leg stiffness and RSI can be explained by maturational factors (body mass and concentric strength respectively). Therefore the windows of accelerated adaptation should be expressed in relation to the onset of PHV as opposed to chronological age. Three years pre- and post-PHV should be considered as suitable windows in which to maximise SSC adaptation. Periods of accelerated adaptation have been suggested to reflect a time when a system(s) is most sensitive to manipulation, therefore SSC-type training may be most beneficial during the identified maturity related periods of accelerated adaptation. Within these identified periods training emphasis should be placed on concentric strength expression for RSI development. Whilst body mass was identified as the main contributor to leg stiffness, owing to the limited amount of explained variance for relative leg stiffness alternative biomechanical, neuromuscular or motor control variables should be considered in future research. [ABSTRACT FROM AUTHOR]