The demands of soccer are multifactorial, from a physical perspective, the intensity of soccer match-play has significantly increased in recent years. The training methods that aree mployed by practitioners must constantly evolve to ensure players are in a position to cope with these demands across the season. Furthermore, to perform at the very highest level, an elite soccer player must excel in physical capabilities while concurrently demonstrating excellent technical and tactical skills. However, there is little information available regarding the physical development strategies that elite soccer clubs currently employ in order for youth soccer players to develop with the required attributes to be able to cope with the demands of senior soccer. Study 1 The first study aimed to understand the coach's perspective on the importance of physical development to the success of elite youth soccer players. A qualitative interview was conducted at an English premier league (category 1) academy. Fitness coaches, technical coaches, and an academy director were asked questions in an attempt to understand the training approaches in place and the rationale behind them, specifically concerning the physical aspect of the program. An understanding of the coaches' perspectives on the relative importance of physical development within the overall development program could then be observed. The present study's findings showed that elite coaches deemed the physical development of players as a key aspect in a holistic programme of player development. More specifically the following conclusions were made; the academy program is designed to prepare players for their first team squad, the coaches were keen to stress that developing good people was also a pertinent feature of the academy syllabus and that winning matches was not important in academy soccer (although it may be used a tool to successfully reinforce ideas to players). In addition, coaches were keen to stress that physical performance improvements would allow players to spend more time training due to less injury, therefore increased opportunities to learn and develop as a player. As aforementioned an important point that was raised was that match involvement was avital part of a player's development, however, winning matches within this academy setting was not, as coaches suggested that the match was merely an extension of the training schedule within this cohort of players. This study's findings allowed us to provide new insights into the different approaches used for physical development and potential barriers and future areas of improvement too within elite youth soccer. Given the expertise of the coaches interviewed, soccer practitioners could consider the findings useful for a better understanding of the methods of physical development integration in a holistic program. Furthermore, with the findings of both chapter 3 and 4 in question, the specific structure of chapter 5 was developed to attempt to understand if what the coaches perceived was important (i.e match involvement) was then reflected when it was analysed in practice. Study 2 The aims of the second study (chapter 4) were as follows; (i) to understand which physical tests are sensitive to identify changes across respective periods (ii) to understand the respective relationship of these tests with measures of internal and external training load. To investigate these aims, a systematic review was conducted. Five electronic databases were searched (PubMed Medline, SPORTDiscus, Web of Science, CINAHL, and Scopus), with three additional studies identified from Science and Medicine in Football Journal. A total of 5683studies were identified after removing duplicates, with only 30 meeting the search criteria. The findings reported within this review, show that a maximal aerobic speed (MAS) test, YoYo Intermittent test derivatives and laboratory-based incremental running tests seem to be sensitive enough to detect aerobic and anaerobic capacity changes across respective training periods. The use of countermovement jumps (CMJ) to detect lower body power changes maybe useful, however, ensuring the correct variables such as jump height via impulse and flight time:contraction time are measured is imperative. Also, maximal sprint testing (>30 m) maybe used for speed-based changes. Findings from this study were then applied to study 3(chapter 5) of the thesis by providing invaluable information to design and implement the longitudinal tracking of training load and fitness change study. Following this review, it was clear that there is limited evidence in youth soccer that accurately monitors TL in relation to physical development. Therefore, further research is required to understand the best methods to track and monitor physical development allowing for a consensus of best practices to use within professional youth soccer to be made (chapter 5). Study 3 The study's primary aim was to quantify the accumulative training and match load across an annual season in elite academy soccer. In doing so, its effectiveness in relation to internal physical performance development across this time frame could be observed. Furthermore, this research aimed to understand how training influences athlete performance. Finally, an analysis of the key parameters which may provide correlations to allow for an understanding between TL and changes in fitness with the submaximal YoYo IE2 fitness test across an elite youth soccer season. In addition, an understanding of how much of the physical stimulus comes from the training prescription and how much comes from matches in periods two, four, and six weeks before the physical testing period. The key finding of this study was that when the starting status was accounted for concerning the examination of aerobic fitness; Starters had a consistently lower heart rate (HR) in the submaximal YoYo IE2 across the season (apart from Evaluation point 1; E1). Using Pearson's correlation, it was evident that there were moderate correlations with number of matches played, total duration of training, total distance (TD) performed and total high-speed running (HSD) between tests, with the strength of the correlation increasing the further from the test. At six weeks the correlations were strongest; Number of Matches Played (-0.33), Total Duration of Training (-0.45), TDPerformed (-0.51), and HSD (-0.42), Sprint SPR (-0.32) and total a/ds (-0.4) were found. This suggests that the more training and matches a player completes the lower the players maximum HR in the test. The findings of our study also demonstrate that there are significant differences in physical fitness for players who are consistently involved in match play in comparison with those who are not. Study 4 In order to provide information as to how to close the gap between players who play regularly in matches with those who do not in terms of physical stimuli, it was important to provide recommendations to do so. Therefore, the aim of study 4 was to provide a commentary which summarises the scientific rationale, current literature and practical recommendations to maintain and develop physical fitness in professional soccer. This was divided into six sections: 1) Evidence-based training load methodologies for maintaining and developing squad fitness; 2) How to keep players 'ready to play' who are not involved regularly within competitive match play; 3) How should interventions be implemented to close the gap between players who receive varied accumulative physical loads?; 4) What other methods could fitness coaches implement to allow for development/maintenance of fitness of players who are regular non-starters?; 5) Recommendations for effective implementation of top ups and post-match conditioning to prepare all players from a physical standpoint in applied soccer settings; 6) Limitations and future directions. The contemporary literature provides preliminary methodological guidelines for coaches and practitioners to use. The recommendations include; Individual periodization of training should be based on starting status (e.g. starters or non-starters) and match position demands in order to optimize recovery, match-day performance and overall physical development. Practitioners should understand how manipulating SSG variables such as pitch size and duration affect mechanical, metabolic and acute physiological responses to allow for appropriate top up protocols to be implemented. In relation to designing top up sessions, medium and large SSG can be used to ensure HSR and SPR exposure demands are met, with smaller SSG allowing for mechanical based adaptations. Isolated running drills may also be used within conditioning sessions to provide appropriate conditioning stimuli. Summary In summary, this research program provides novel information about the training loads and changes in physical fitness in elite youth soccer players. In addition, this work systematically examined the training load across a competitive youth soccer season to understand how significant the involvement in match play is for physical performance markers. Furthermore, it was the first study to conduct this type of qualitative interview with elite youth soccer oaches to further understand the relative importance of physical development in elite youth soccer. It is hoped that the findings from this research project can help applied staff understand the most appropriate practices surrounding match play in long term development programs for elite youth players.