title: Gastrocnemius medialis muscle-tendon properties and their relationship with power and reactive strength in elite youth soccer players
Creators: name: Robshaw, D
Publisher: name: Liverpool John Moores University
Year: 2021

description: There are several key physical performance indicators (KPIs) in soccer, and these include lower limb power and reactive strength, which can be evaluated using a variety of jump assessments, e.g. countermovement jumps (CMJs) and drop jumps (DJs). These KPIs are thought to be underpinned by the physiological characteristics of the respective lower limb muscle-tendon units (MTUs). However, the influence of the gastrocnemius medialis (GM) MTU on power and reactive strength in elite youth soccer players (ESP) is currently unknown. Before assessing MTU properties and KPIs in ESP, it is important to understand the reproducibility of the respective assessment methods. In chapter 3, the aim was to identify the between-session reliability of all methodological techniques involved in the subsequent chapters. For this, 12 physically active, healthy, young men were recruited, the findings demonstrated all morphological properties of the GM muscle and GM tendon could be measured with high levels of reproducibility (coefficient of variation (CV)=1.2-6.6%; mean bias ratio (MBR)=1.00-1.09, ratio limits of agreement (RLoA)=1.04-1.22). Mechanical and material properties of the GM tendon could also be measured reliably (CV=2.6-17.3%; MBR=1.00-1.13, RLoA=1.08-1.80), however, the reproducibility of these techniques was slightly lower than for the morphological properties. Furthermore, a simplified method for calculating GM tendon mechanical/material properties proved reliable and agreed with the more comprehensive method, when assessed in 13 ESP and 12 non-athletes. This provides researchers/practitioners with a valid, reliable, and efficient method to monitor GM MTU properties when time is limited, e.g. when working with elite athletes. During jump-based assessments, peak power was measured more reliably during bilateral than unilateral CMJ, while reactive strength temporal variables during a DJ were more reliable than kinetic parameters. The second experimental study (chapter 4) aimed to determine whether the physiological characteristics of the GM muscle, as well as lower-limb power and reactive strength (the ability to develop maximal forces in minimal time while changing from an eccentric to concentric contraction) during CMJ and DJ, differed between ESP and recreationally active control participants (CON). A second aim was to investigate the relationship between GM muscle properties with lower limb power and reactive strength. ESP (17.4 ± 2.5) displayed a larger GM fascicle pennation angle (θp) compared to CON (14.3 ± 1.2, P&lt;0.001), while projectile range and horizontal peak power during bilateral and unilateral horizontal CMJ were greater in the ESP. Furthermore, GM θp and cross-sectional area correlated with power generated in the vertical and horizontal plane of motion during unilateral vertical and bilateral horizontal CMJ, respectively. Perhaps surprisingly, GM muscle fascicle length and muscle volume both correlated inversely with power during bilateral vertical and horizontal jumps, although this may represent the differing biomechanical demands between unilateral and bilateral jumping tasks. The third experimental study (chapter 5) aimed to determine whether the mechanical and material properties of the GM tendon differed between ESP and CON, and whether GM tendon properties were related to lower limb power and reactive strength during CMJs and DJ. Tendon properties did not differ between ESP and CON but tendon elongation and strain correlated inversely with power during unilateral vertical CMJ, whilst higher force and vertical stiffness outputs were achieved during a DJ by stronger individuals with stiffer and larger GM tendons. Together with the findings from chapter 4, the results from chapter 5 suggest that specific physiological characteristics of the GM MTU could be deemed favourable for lower limb power and reactive strength capabilities. Through identifying (and perhaps developing) these characteristics in ESP, such KPIs in soccer players may be improved. The purpose of the fourth and final experimental study (chapter 6) was to investigate the effects of a 12-week in-season soccer training programme on GM MTU properties and jump performance in 14 ESP. Post-training, GM θp (pre-training: 18.0 ± 2.5 ; post-training: 17.2 ± 2.6 ; P=0.038), maximal tendon elongation (pre-training: 2.68 ± 0.58 cm; post-training: 2.30 ± 0.46 cm; P=0.019) and maximal tendon strain (pre-training: 12.8 ± 2.4 %; post-training: 11.1 ± 2.5 %; P=0.019) all reduced. Furthermore, projectile range during a unilateral horizontal CMJ increased from pre- to post-training (pre-training: 0.93 ± 0.15 m; post-training: 1.07 ± 0.12 m; P=0.001) but no other jump metrics changed. The physiological reasons for these changes are unclear but it is plausible that GM θp decreased to facilitate a faster shortening of the overall muscle in response to pitch-based training. Alternatively, it may have occurred in response to the limited progressive overload applied to the plantar flexors during in-season training (potentially leading to a detraining effect). Further, the reductions in tendon elongation and strain may have been due to a change in the material properties of the tendon (e.g. increased density of collagen fibrils) that did not quite translate to changes in stiffness or modulus. Crucially, however, these changes did not correlate (neither individually, nor in combination) with the change in projectile range during a unilateral horizontal CMJ, thus suggesting they did not contribute to this increase in jump performance. These results demonstrate that a 12-week period of in-season soccer training is not a sufficient chronic stimulus to elicit meaningful adaptation of the GM MTU to cause significant improvements in power and reactive strength during different jump-based assessments. In summary, this thesis demonstrates that the reproducibility of the techniques used to characterise GM MTU properties is generally acceptable to very good. Furthermore, specific GM MTU properties are related to lower limb power and reactive strength. Moreover, this thesis demonstrates that a three-month period of in-season soccer training is insufficient to elicit meaningful GM MTU adaptations required to improve physical performance. Therefore, whilst multiple factors contribute to influencing KPIs in ESP, this thesis presents novel information regarding the GM MTU and highlights potential physiological determinants that can be developed in youth soccer players.