Stiffness During Sub-maximal Hopping and Sprinting in Youth Female Athletes

Abstract

Explosive rebounding movements such as hopping and sprinting that rely on the stretch-shortening cycle (SSC) and elastic properties such as stiffness, are essential for sports performance. While fast-SSC tasks like sub-maximal hopping and sprinting have been assessed for reliability, maturity-related differences and training-related differences in boys, similar investigations remain unexplored in girls. This thesis sought to understand the natural development of rebounding ability of young girls, assessed by leg stiffness, and how leg stiffness develops through training. The introduction and literature review provide an overview of stiffness behavior, how it is regulated, how global measures of stiffness change with maturation and training during sub-maximal hopping and sprinting. These topics established the thesis framework and justification for further investigation in stiffness behavior.

Chapter 3 examined the measurement error of leg stiffness during the most suitable assessment task, sub-maximal hopping, across multiple hopping frequencies and calculation methods in young girls. This chapter revealed an acceptable average coefficient of variation percentage (CV%) range of 3.89% to 9.44%, and excellent intraclass correlation coefficient (ICC) (ICC > 0.9) values across all hopping frequencies and calculation methods. These findings also revealed that hopping at 2.5 Hz on the dominant leg and using the ‘all contacts’ calculation method were the most reliable leg stiffness conditions. The reliability of additional explosive tasks across maturity stages were examined in Chapter 4. It was shown that horizontal jumping tasks were more reliable than horizontal hopping tasks, with coefficients of variation (CVs) of 2.82%-3.13% for jumping and 3.15%-7.46% for hopping. Single hops proved more reliable than triple hops, with CVs of 3.21%-4.5% compared to 3.15%-7.46%. All average intraclass correlation coefficients (ICCs) were above 0.9, indicating high reliability for both tasks when an additional familiarization session is included.

Chapters 5 and 6 examined maturity-related differences during sprinting and unilateral sub-maximal hopping respectively. Chapter 5 found significant differences (g = 1.51-1.64) for vertical stiffness (Kvert), relative vertical stiffness (RelKvert), leg stiffness (Kleg), and relative leg stiffness (RelKleg) between the PRE and MPUB stages. Significant differences (g = 1.66-1.79) for also discovered for speed and step length (SL) between the PRE and MPUB stages. However, due to high CV values for Kleg and RelKleg (11-16%) and systematic changes in SL during reliability tests, these results should be interpreted cautiously. Overall, stiffness and SL changes between PRE and MPUB stages may enhance sprint performance in young girls. Chapter 6 examined leg stiffness differences of a combined pre-pubertal (PRE) and early pubertal group of girls (EPUBPRE), compared to mid-pubertal girls (MPUB), and late pubertal girls (LPUB). The findings demonstrated large and significant differences in absolute leg stiffness between EPUBPRE and MPUB (g = 1.27, 95% CI = 0.81, 1.73) and between MPUB and LPUB (g = 1.27, 95% CI = 0.85, 1.69) for unilateral sub-maximal hopping at 2.5 Hz on the dominant leg. Significant differences were also found between EPUBPRE and MPUB for relative leg stiffness (g = 0.48, 95% CI = 0.16, 0.80). Thus, maturity status appears to impact both absolute and relative leg stiffness development substantially in young girls.

In Chapter 7, plyometric and sprint training were used to target stiffness enhancement, with plyometric training inducing stiffness adaptations at 2.5 Hz on the dominant leg. Furthermore, this adaptation (4.4%) exceeded the within subject error of 3.89% established in Chapter 3. Chapter 8 demonstrated a similar effect of plyometric training on leg stiffness during sub-maximal at 2.5 Hz on the dominant leg. The results of these intervention studies demonstrate that plyometric training is beneficial for improving leg stiffness in young girls. Finally, Chapter 9 outlined a summary of the findings, practical applications, limitations and future research directions of stiffness in young girls.