Training load dose-responses in adolescent male football: the importance of biological maturation.
Authors:   , Salter Jamie 
Published: July 2023

The period surrounding the adolescent growth spurt is a turbulent but crucial stage of development for young footballers in their pursuit of becoming full-time athletes. At a time of almost constant talent (re)selection which coincides with major physical and physiological changes players experience large fluctuations in performance and a heightened injury incidence. Adding to the complexity of this period, the timing and tempo of biological maturation varies between individuals causing a diversity in physical and physiological capabilities, influencing the dose-response to training. Although differences in biological maturation and the links with injury are acknowledged in literature, little evidence exists to quantify the magnitude and extent to which these impacts perceptions of load and subsequent performance. This thesis aims to quantify the maturity-specific responses to load using ecologically valid approaches to aid the enhancement of provision offered to young academy players. To provide a context and informed backdrop for the rest of the thesis, it was deemed important to first identify the current practices of, and perceived barriers to monitoring training load and biological maturation in academies. A cross-sectional survey design was used to ascertain perceptions of staff from male (EPPP) and female (RTC) academies during the 2017/18 soccer season. In total, 49 respondents completed the survey who advocated injury prevention as highest importance for conducting training load and maturation monitoring across academy groups, with overall athletic development, load management, coach and player feedback considered important. However, there were clear differences in monitoring strategies that academies of different categories adopted, which were often associated with resources or staffing. Survey responses suggest that despite routine monitoring of biological maturation and training load being commonplace within adolescent soccer the communication and dissemination of this information is often lacking, which may ultimately impede the impact of the monitoring practices for the players. Resource and environmental constraints create natural diversity around the strategies adopted, but academies are recommended to adopt sustainable and consistent approaches to monitor key variables to inform the coaching, selection, and development process. The survey chapter identified that most clubs employ one of the various ‘non-invasive’, somatic equations to estimate biological maturation. However, the methodological differences associated with calculations often mean they provide variable estimations, even when using the same anthropometrical data. Therefore, it was deemed important to this thesis to observe the agreement of maturity estimations and compare concordance between methods when looking to estimate maturity status. Thus, anthropometric data from 57 participants was collected from a single assessment point during the 2017-18 season, with an additional 55 participants providing three repeated measurements during the 2018-19 season, resulting in 222 somatic estimations observed. Results indicated that all methods of maturity-offset (MO) produced an identical estimate of age of peak height velocity (13.3 years) with mean prediction of adult height (PAH%) providing a mean estimate of 93.6%, which also aligns closely. However, when looking to identify circa-PHV individuals there is greater concordance when using conservative thresholds (44-67%) than when using more stringent bandwidth thresholds (31-60%), with both being considered moderate concordance at best. Therefore, although overall findings indicate that there is very high to near perfect agreement between all approaches when predicting APHV, concordance of categorisation between these methods is less useful. Therefore, this chapter indicates that PAH% and MO methods are not interchangeable, and practitioners should utilise one approach routinely for all maturity-specific interventions. Academy squads are comprised of players within chronological parameters but often present significant variations in physical characteristics including body mass (~50%), stature (~17%), percentages of predicted adult height (10-15%) and fat free mass (~21%). These maturational changes likely influence performance and dose-responses to load, but limited studies using standardised activity profiles have directly observed this influence. Therefore, this thesis aimed to quantify the neuromuscular performance (CMJ, RSI absolute and relative stiffness) and psycho-physiological (d-RPE) responses to a simulated soccer-specific activity profile (Y-SAFT60) and analyse whether this dose-response was moderated by maturation in EPPP academy players. Data illustrated an interaction between perceived psycho-physiological load (RPE-T) and maturation, with absolute stiffness, relative stiffness and playerload (PL) showing slope significance across various stages of maturation (~86-96% PAH). These interactions suggest that psycho-physiological dose responses are influenced by maturation and should be considered for training prescription purposes, which is likely a result of the musculotendinous changes that occur around peak height velocity (PHV). Therefore, practitioners are urged to consider the maturational load-response variation to reduce injury incidence from inappropriate levels of physical and cognitive stress, which are likely compounded chronically with multiple weekly sessions. Typically, players experience between 3-4 acute bouts of specific training on a weekly basis, proposing that the maturity-specific load-responses observed above may be exacerbated over the course of a season. 55 male soccer players from a Category 2 EPPP academy were monitored during the 2018-19 season. Self-reported perceptions of psycho-physiological (d-RPE) intensity were collected approximately 15-minutes after each training session for a period of 40-weeks using the CR100® centi-Max scale. Analysis indicated that a 5% increase in PAH%, resulted in a reduction of ~7AU per session, with a ~14AU difference for a 10% difference in PAH%. Therefore, players less biologically mature are consistently working harder just to compete with more biologically advanced teammates of a similar chronological age. Again, these changes are mostly attributed to musculotendinous changes because of maturation and therefore a higher relative mechanical load experienced by less mature individuals. When accrued, these small inter-individual differences lead to a substantial variation in training load (~40-50%) over the 40-week season. This has the potential to undermine the whole developmental pathway, as the assumption that players of a similar chronological age are experiencing similar load-responses is precarious. Failure to act, by adopting more maturity sensitive ways of working for example, will result in a ‘survival of the fittest’ environment, rather than the systematic, considered, and individualised approach to optimal loading proposed in policy documents and literature. Bio-banding is a method to group individuals based on biological maturation rather than chronological age. Supplementing the chronological programme with bio-banded activities may offer practitioners a practical method to better control load exposure and ultimately mechanical load related injury risk. Therefore, the final thesis study explored effects of standardised chronological and bio-banded training sessions on neuromuscular performance and psycho-physiological perceptions of intensity in 55 male soccer players from a single academy. Players participated in bio-banded and chronologically categorised bouts (x5) of 5-minute 6v6 (including GK) SSG on a playing area 45 x 36 m (135m2 per player). Prior to and following this, players performed a standardised sub-maximal run using the audio controlled 30-15IFT wearing foot-mounted inertial devices. Findings indicate that the introduction of bio-banded training sessions minimises the decrement in neuromuscular and locomotor markers and psycho-physiological ratings of intensity for players across the maturation spectrum. From a load management point of view, the relatively smaller pre-post changes observed in bio-banded SSGs offer promising early indications that biologically categorising training may help to stabilise the stress-response for players across maturity groups and facilitate a load management option for practitioners. Based on this, practitioners should actively seek opportunities to integrate biologically classified training activity alongside chronologically categorised sessions within their training schedules. In doing so they may alleviate the consistent stress placed on less mature players as part of standard chronologically categorised sessions without compromising the development of those more mature and able to tolerate greater workloads.