Monitoring Training Load and Responses to Load in Youth Soccer Special Reference to Biological Maturation

Abstract

Athlete monitoring is widely considered as an important part within the physical training process, which aims to maximise sports performance and health of an athlete. Within this training process the two constructs of training load and responses to load constitute the athlete monitoring framework. Given the lack of established gold standards, psycho-physiological responses are quantified by surrogate measurement instruments from multiple domains of the human body including but not limited to the cardiorespiratory, metabolic, neuromuscular, biochemical, endocrine, and musculoskeletal system. The selection of the most appropriate measurement instrument requires scientific evaluation of critical measurement properties such as reliability, validity, and responsiveness, while also considering several key aspects such as specificity to the sport, scalability and time efficiency when administered to large groups of athletes. Despite considerable research in the area of athlete monitoring over the past decade, most research has been conducted in senior male professional team sport. Given the unique set of psychological and physiological characteristics and environmental circumstances of adolescent athletes it is unclear whether the previously established knowledge about the measurement properties of the most commonly used measurement instruments can be transferred to youth athletes. Therefore, the overall aims of this thesis were to advance the field of applied research in high-level soccer by firstly comparing a new device measuring skeletal age to the established method of percentage of predicted adult height as two potential practical, non-invasive methods to assess biological maturation status. Secondly, the reliability and responsiveness of commonly used measurement instruments that aim to assess acute psycho-physiological responses to load of youth soccer players were critically evaluated. To establish a context and informed backdrop for the rest of the thesis, it was perceived as important to firstly investigate the validity of a non-invasive device which enables to enables practitioners to measure skeletal maturity within the applied setting. Results of the first study showed that the novel device to measure skeletal age based on an ultrasound-technique can be used to assess biological maturity status without the typically associated limitations of traditional standard radiographs. In addition, there was a maturity-related selection bias towards players advanced in biological maturation emerging in the U14 age group, which remained relatively constant throughout adolescence. Subsequently, reliability and responsiveness as two important measurement properties were investigated for commonly used measurement instruments within youth soccer high-performance programs. Results of the second study showed that most of the included parameters possess poor short-term between-days reliability irrespective of the maturity status. Regarding the responsiveness, study three suggested that most investigated measurement instruments to assess acute psycho-physiological responses showed trivial to small changes to a short-period of accumulated training load during the in-season. During intensified periods of increased training load such as a short pre-season training camp as investigated during study four, the athlete-reported recovery and stress scales of the SRSS and heart rate responses during the sub-maximal run showed small to moderate changes in temporal relation to fluctuations in training load and might therefore be useful measurement instruments within the athlete monitoring process for adolescent soccer players. In contrast, parameters derived from the force-time data of a CMJ might provide little insight into acute neuromuscular responses to load. Together the findings of the thesis allow practitioners to better understand the usefulness of commonly used measurement instruments. Results may also help practitioners in the selection process as to which measurement instruments are worthwhile implementing within their daily practice. In addition, findings of this thesis have important implications as they help guiding future research from a methodological and conceptual perspective to further advance the field of monitoring acute responses to load in youth soccer players. This ultimately may help practitioners in more accurately prescribing training load to elicit the desired adaptations and potentially mitigating injury risk in the long-term.