Global Insights into the Role of Strength Training in Soccer, with Particular Reference to Academy Players

Soccer is a global sport, participated in by both men and women, in professional and academy environments. Researchers have proposed that resistance training (RT) interventions are beneficial for each of these demographics, as generating high levels of muscular strength and power are important for success in soccer. While research has focused on a multitude of training methods aiming to improve strength and power in soccer, an understanding of commonalities and differences between strength and conditioning (S&C) coaches working in various demographics is needed so a training intervention can then be designed with greater ecological validity, while aligning with scientific guidelines for using strength training. Initially, taking a wide scope of S&C practice, chapter four aimed to investigate whether differences in current soccer S&C practice existed between different global regions. Overall, relatively more coaches in the United Kingdom (UK) believed bodyweight training was the most important RT modality compared to coaches in South America (SA) (45% vs. 27%, p = 0.040). Conversely, relatively more first team coaches in the United States of America (USA) than in the UK regarded free-weight RT as the most important training method in their programmes (100% vs. 60%, p = 0.033). Further, coaches in Europe conducted fewer formal S&C sessions, placed less importance on free-weight RT and performed less speed and plyometric training compared to coaches in other global regions (all p < 0.05). Based on these findings, the S&C practice of coaches in the USA and SA generally align better with scientific guidelines for strength and power development in soccer compared to those in the UK and other European countries, with an emphasis on free-weight RT alongside regular sprint and plyometric training. However, SA academy players were introduced to S&C later (14 ± 2 years-old) than in the UK (12 ± 3 years-old, p = 0.002), which may limit physical development in SA players. The purpose of chapter five was to investigate the practices of S&C coaches working with male and female soccer players, at first team and academy level. Compared to men’s soccer, much less is known about S&C practice in the women’s game, and consequently, the S&C approaches taken with women’s first team and academy squads may not be appropriate. This investigation highlighted differences in S&C practices between coaches of men’s and women’s soccer squads on a global scale. Women’s academies had fewer weekly in-season S&C sessions than men’s academies (1 ± 1 vs. 2 ± 1, p = 0.005), despite greater injury risk in female players. However, relatively more women’s coaches (39%) used the Nordic hamstring exercise (NHE) compared to men’s coaches (18%, p = 0.008), suggesting the NHE may be used to reduce the higher injury risk in female players. Further, relatively more women’s coaches (63%) utilised rating of perceived exertion-based load prescriptions than men’s coaches (37%, p = 0.002). The subjective methods for training prescription may underload strength training exercises, limiting physical development in female players. Thus, coaches in women’s soccer may wish to increase weekly frequency of S&C sessions and use objective methods to prescribe load, thereby optimising performance and minimising injury risk. Building on the findings in chapter five, the aim of chapter six was to investigate current S&C practice in first team and academy level soccer. Scientific guidelines exist regarding S&C best practice, for both first team and academy level soccer. However, it is not known if these research-informed guidelines are followed in such applied settings. A greater proportion of academy compared to first team coaches assessed acceleration/sprint (92% vs. 83%, p=0.026), jump (95% vs. 83%, p=0.023) and change of direction performance (77% vs. 61%, p=0.031). Therefore, the testing approach taken by academy S&C coaches appears to align with the suggestions from the scientific literature. However, the RT approach employed does not appear to align with scientific guidelines. A greater proportion of academy (54%) versus first team (35%) coaches prioritised bodyweight training (p=0.031), despite a similar distribution of movement patterns trained. Overall, 44% S&C coaches reported using training intensities below strength training guidelines (≥80% 1RM). This disparity between strength training guidelines and applied practice of S&C coaches in first team and academy may be due to perceived time restrictions (50% and 49%) and concerns of muscle soreness (70% and 37%). The purpose of the final experimental study (chapter seven) was to investigate the efficacy of high (HRT) versus moderate intensity RT (MRT) (the latter was the approach generally reported in chapters four, five and six) on changes in strength, power, and speed, and to compare delayed onset muscle soreness (DOMS) between HRT and MRT. Intervention groups completed one session per week of parallel back squat for six weeks in-season alongside regular soccer training. Participants performed either 2 4 at 90% single repetition maximum (1RM) (HRT) or 3 8 at 80% 1RM (MRT). Both training groups experienced similar increases in absolute and relative back-squat strength and vertical jump following the intervention. Further, HRT improved horizontal jump more so than MRT (p = 0.011). Importantly, the increases seen following HRT were achieved with 58% less training volume than MRT (p < 0.001), and with similar DOMS compared to soccer alone. These findings suggest that HRT may be a more efficient training method to improve physical performance in academy soccer players in-season than the most common training prescription (MRT) currently used by S&C coaches in soccer. In summary, this thesis observed variation in S&C practice in soccer between global regions (chapter four), between coaches working with male and female soccer players (chapter five), and between coaches working in first team and academy settings (chapter six). The application of scientific research-based RT principles varies widely, with a large proportion of S&C coaches in soccer not following guidance for maximal strength development or maintenance. This may be due to the perceived restrictions of limited time and the potential for DOMS following RT. However, as seen in other sports and shown in chapter seven, when employing scientific research-based strength training principles, a high-intensity, low-volume RT programme is not only feasible in-season but more effective than current practice and helps manage perceived restrictions