Development and Trainability of the Stretch Shortening Cycle in Male Youths
Lloyd, Rhodri S.
University of Wales
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The stretch-shortening cycle (SSC) is an intricate muscle action, which is fundamental to successful plyometric performance. However, there currently exists minimal research examining the effects of age, maturation and trainability on the development of SSC in male youths. Study one of this thesis examined the reliability and validity of a series of SSC-type activities in young adolescents, as assessed by a mobile contact mat in comparison with force plate analysis. Squat jump height was the most reliable measure (CV = 8.64%), while reactive strength index (RSI) during maximal hopping and leg stiffness during sub-maximal hopping produced moderate reliability (CV = 10.17% and 13.98% respectively). Measures of leg stiffness were in agreement (TEE = 6.5-7.5%) with force plate data during sub-maximal hopping, but not during maximal hopping (TEE = 41.9%). Test reliability was moderate to good for the range of performance variables, and leg stiffness measures during sub-maximal hopping were valid. Subsequently, it was concluded that the contact mat served as a replicable assessment tool to assess SSC function in youths. Previous research has identified that a number of physical abilities undergo nonlinear development throughout childhood, however research for SSC development is lacking. Study two of the thesis examined the effects of age and maturation on the natural development of SSC function in boys aged 7-17 years. Peak height velocity (PHV) was used as a central reference mark to assess maturational status, and significant differences were apparent three years either side of (PHV) for SJ, CMJ, RSI and leg stiffness development (P <0.05). Results suggested the possible existence of periods of accelerated adaptation, however it remained unclear as to whether exposure to the correct training stimulus during these timeframes would promote adaptation above and beyond that of natural growth alone. The third study examined the neural regulation of SSC function in youths during both sub-maximal and maximal hopping. Fifteen-year olds produced greater levels of absolute and relative leg stiffness than both twelve- and nine-year olds during sub-maximal hopping. During the sub-maximal hopping condition, the youngest children spent longer in contact with ground, less time in the air, and utilised less muscle activity contribution during the short-latency stretch-reflex phase. When performing maximal hopping, both 15- and 12- year olds recorded greater RSI values than nine-year olds. When expressed as a percentage of total feedforward activity, preactivation of the soleus muscle in the 100 ms prior to ground contact was significantly greater in 15-year olds compared to the younger children when hopping maximally. Therefore, results would suggest a greater reliance on feedforward and immediate feedback mechanisms with advancing age, which help to increase absolute and relative stiffness. The final study examined changes in SSC performance following a 4-week plyometric training programme. Exposure to the plyometric training resulted in 12- and 15- year olds making significant improvements in both absolute and relative leg stiffness, and 12-year olds achieving significant training adaptations in RSI. Control groups revealed no performance changes, and in some cases a significant decline in performance. This study demonstrated that a 4-week training programme can have a positive effect on SSC function, and that trainability may be age or maturation dependent for various measures associated with leg stiffness. The research has enhanced our understanding of the impact of age and maturity on the natural development of SSC function in male youths. It has highlighted that natural development alone can result in performance changes and that, amongst other variables, neural regulation appears to be a confounding factor. Additionally, it was identified that the SSC is sensitive to change when exposed to the correct training stimulus, suggesting the possible existence of trainability.
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