Stretch-shortening cycle development and trainability in girls during maturation

McGarrigal, Lee David (2025) Stretch-shortening cycle development and trainability in girls during maturation. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

The stretch-shortening cycle (SSC) is an intricate cyclical muscle action fundamental in many explosive sporting movements such as sprinting and jumping. Plyometric training (PT) is often considered the training method of choice when aiming to increase slow (>250 ms) and fast (<250 ms) SSC function, power, jump height and sprinting performance. Maturation appears to affect the SSC through childhood and into adolescence. However, most studies into the effect of maturation in SSC tasks has included boys and not girls. PT is considered the bridge between strength and speed, with the ability to develop force in the shortest time possible. Many neuro-musculoskeletal adaptations can occur following PT, which positively influences athletic ability while mitigating the risk of serious knee injuries, which is more prevalent in girls compared to boys. However, there currently exists limited research examining the effects of maturation and plyometric trainability on the development of SSC in girls. Study one (Chapter 4) examined the between-device agreement and the inter-day test-retest reliability of the MyJump 2 app (Apple Inc. Cupertino, USA) and the Optojump Next system (Microgate, Bolzano, Italy) during a series of SSC tasks, including squat jump (SJ), countermovement jump (CMJ), a drop jump (DJ), reactive strength index (RSI) and its derivatives of jump height and ground contact time (GCT), and leg stiffness and its derivatives of flight time (FT) and GCT. The agreement study involved 34 recreational post-PHV female soccer players, 24 of which also took part in the reliability study. To the author’s knowledge, this was the first time such studies have been conducted in this population, which is pertinent given that girls and boys may record differences in SSC function (Pedley et al., 2020; Pedley et al., 2021) due to hormonal and physiological differences between the sexes. It was also novel to measure the reliability of the MyJump 2 app in measuring leg stiffness, which to date has not been measured in any population. Intraclass correlation coefficient (ICC) demonstrated moderate to excellent agreement between the devices (ICC ≥ 0.735 – 0.964) but was dependent on the task involved. Coefficient of variance (CV) values reflected strong agreement and low bias between the MyJump 2 app and the Optojump Next DJ RSI, leg stiffness and relative leg stiffness (CV ≤ 8.0%) and excellent agreement in all other dependent variables (CV ≤ 4.4%). Test-retest reliability of the MyJump 2 app was good-to-excellent in all variables (ICC ≥ 0.856), accept DJ GCT (ICC = 0.682). CV values for the MyJump 2 app were acceptable-to-excellent in all variables (CV 2.0 - 10.7%). The reliability of the Optojump Next was moderate-to-excellent in SJ, CMJ, RSI variables (ICC ≥ 0.730), but not in DJ GCT and flight time during 5 maximal hopping (5max), jump height during 20 submaximal hopping (20submax), relative leg stiffness during a DJ, 5max, and 20submax hopping (ICC ≥ 0.613). CV values were acceptable-to-excellent, with the lower CV during DJ flight time (CV = 3.3%), and the highest during DJ relative stiffness (CV = 12.4%). Bland-Altman plots revealed a systematic bias between the devices, with the MyJump 2 recording greater GCT, RSI, leg stiffness and relative stiffness during DJ, and the Optojump reporting greater measures during a SJ, CMJ, DJ jump height and flight time. Consequently, it was concluded that while the MyJump 2 and Optojump next offered agreement, due to the flexibility of the Optojump Next in terms of instantaneous feedback and the measurement of RSI and leg stiffness during hopping as well as a DJ, this device should be used in all subsequent chapters. Study two (Chapter 5) compared the measurement of the fast SSC during a DJ and hopping using the Optojump Next involving 34 recreational post- PHV female soccer players. The data demonstrated that while both the 5max and 20submax hopping recorded fast GCTs (≤ 206.4 ms), the 30cm DJ did not (255 ms). This finding contradicts literature in elite post-PHV female soccer players and other populations, who can record GCTs <250 ms during a 30cm DJ. As fast GCTs are an integral determinant of leg stiffness, it was unsurprising that both hopping tests demonstrated significantly greater leg stiffness and relative leg stiffness than the DJ (P ≤ .001). This finding suggests that strength and conditioning practitioners should test leg stiffness during maximal and submaximal hopping and include these protocols in training. Significantly greater RSI during the DJ compared to both hopping tasks (P ≤ .05) highlighted that girls can increase RSI values by offsetting slow GCTs with greater jumping heights. Considering that RSI and leg stiffness require effective elastic energy reutilisation and activation of the stretch reflex indicative of the fast SSC, it was deemed that 5max and 20submax hopping should be used in subsequent chapters to measure RSI and leg stiffness, respectively. Research in the SSC development in girls during growth and maturation is lacking and often contradictory. Study three (Chapter 6) examined the effects of maturation on SJ and CMJ (slow SSC), and RSI, and leg stiffness (fast SSC function) in active girls aged 7-17 years. Peak height velocity (PHV) and years from PHV (YPHV) were used to assess biological maturational status. As expected, height, body mass and leg length increased during maturation (pre-PHV < mid-PHV < post-PHV), with body mass index also greater in the post-PHV subgroup compared to the pre- and mid-PHV subgroups. Unsurprisingly, the post-PHV outperformed the pre-PHV subgroup in all other dependent variables (P ≤ .05). The mid-PHV subgroup also outperformed the pre-PHV subgroup in 20submax flight time (P ≤ .0.5), with the mid-PHV outperforming both the pre- and post-PHV subgroups in relative leg stiffness (P ≤ 0.05). Unexpectedly, RSI did not increase during maturation. Further analysis based on consecutive YPHV subgroups revealed that the -0.5 YPHV maturity offset subgroup generated greater leg stiffness than the -1.5 YPHV maturity offset subgroup (P = .014), and that the 0.5 YPHV maturity offset subgroup recorded greater relative leg stiffness than the 1.5 YPHV maturity offset subgroup (P = .002). For the first time, in girls based on maturity, results suggested the possible existence of periods of accelerated adaptation in leg stiffness only. This extends what is currently known in the literature, particularly because the data of leg stiffness in maturing girls is sparse and that the data comparing the SSC ability of maturing girls rarely includes pre-PHV populations. However, it remained unclear whether exposure to the correct training stimulus during these timeframes would promote adaptation above and beyond that of natural growth alone. Study four (Chapter 7) examined the influence of biological maturity of girls following plyometric training (PT). Currently, the literature in this area is sparse and contradictory. To the author’s knowledge, this was the first time that PT has been used to increase the slow and fast SSC of maturing recreational female soccer players using CMJ, RSI, leg stiffness, and sprinting. Chapter 7 examined the trainability of pre-, mid-, and post-PHV girls following 8-week (once per week) soccer-only training, followed immediately by 8-week soccer training (once per week) supplemented with 8-weeks vertical and horizontal PT (also once per week). The main findings revealed that exposure to PT resulted in all maturity groups making significant improvements in all SSC-related performance variables (P ≤ .05), except for RSI in the pre-PHV group (P = .333) and 20submax flight time in all subgroups (P ≥ .616). For the first time, this chapter demonstrated that 8-week low frequency PT can have a positive effect on SSC function in recreational female soccer players, and that trainability may not be maturation dependent. The research has enhanced our understanding of the impact of maturity and PT on the SSC function and athletic development in girls who participate in recreational soccer. Additionally, it was identified that the SSC is sensitive to change when exposed to the correct training stimulus, suggesting the possible existence of trainability. In summary, this thesis found that strength and conditioning practitioners working girls and with a limited budget, could use a cost-effective app to test the SSC function of this population. However, the versatility of the Optojump Next could be an option for strength and conditioning practitioners working in a professional setting. This thesis also highlighted that strength and conditioning practitioners should use hopping to test the fast SSC, which is important given the positive impact the RSI and leg stiffness has on athletic ability and the risk of injury. Additionally, unlike boys, girls may not demonstrate ‘accelerated adaptation’ in slow and fast SSC function during maturity, but that following PT, girls demonstrate ‘synergistic adaptation’, which refers to the symbiotic relationship between the adaptations caused by a training stimulus and the SSC development due to growth and maturation. Given that PT studies involving girls, and in girls participating in soccer, is limited, this thesis has added valuable information to strength and conditioning practitioners working with girls, particularly in SSC-based tasks such as reactive strength, fast GCT and speed, which often determine the difference between winning and losing in many sports, including soccer.