Professional soccer clubs invest significantly into the development of their academy prospects with the hopes of producing elite players. Professional soccer players have become faster over time (Haugen et al., 2013) and the evolution of the modern game has seen increasing tactical requirements predicated on high-intensity pressing, counter-pressing, and counter-attacking, all of which require co-ordinated multi-player sprinting that increase the physical demands placed on players (Harper et al., 2021; Nassis et al., 2020; Yi et al., 2020). These physical characteristics are forecast to further increase in the future (Nassis et al., 2020) and those tasked with identifying and developing the next generation of ‘future stars’ will consider these physical attributes as critical factors in determining the retaining or release of players in talent development systems (Saward et al., 2020). Resultantly, talented youngsters in elite development systems are exposed to high levels of intensive training with the aims of developing the foundational skills underpinning the capabilities needed to excel in the modern game. However, these same high-intensity qualities (i.e., acceleration, high-speed running, cutting, pivoting, etc.) we may strive to develop are also factors that have been linked to an increased injury risk through a range of mechanisms, such as hazardous movement patterns, deficient physical capacity, or inappropriate training regimes. Furthermore, large disparities in maturation status, growth-related problems, as well as highly-specialised sport practise, may compound these issues if the youth soccer player is inappropriately managed. Thus, a conflict may exist in talent development systems whereby the accumulated training exposures necessary to exceed at the highest level may be ‘at-odds’ with an increased injury risk.
With that being said, the following discussion will highlight how regular, appropriate and well-monitored multi-directional speed (MDS) training can form part of a two-fold solution in this performance-injury risk conflict. This article will be the first of a two-part series, where we will discuss how the sports science practitioner may navigate their youth athletes through these turbulent pubertal years. Firstly, we will highlight the injury characteristics inherent in rapidly growing youth soccer players, but also demonstrate that the practitioner can provide individualised and stage-specific training methods that may simultaneously progress athleticism while reducing injury risk. These articles will be in reference to our recent publications that have extensively discussed MDS development (McBurnie, Parr, et al., 2021; McBurnie & Dos’Santos, 2021) and training management (McBurnie, Dos’Santos, et al., 2021) in the context of elite youth soccer players.
Injury Risk Characteristics for the Growing Soccer Player
Unique to the youth athlete population are the constructs of growth (i.e., changes in an individual’s physical dimension over a specific time) and maturation (i.e., the process toward a mature state). These concepts are critical to appreciate, as they have each been identified as dynamic moderators for injury risk in youth athlete populations. As such, periods of rapid growth and the period around peak-height velocity have been suggested as key elements to consistently monitor and review in academy environments (Kemper et al., 2015; Rommers et al., 2020; Wik et al., 2020). In addition, the large inter-individual variation in physical and neuro-development between players performing in the same age group will potentially present stark contrasts in physical capabilities, neuromuscular control and psycho-social development (Malina et al., 2004; McKay et al., 2016; Quatman-Yates et al., 2012). In effect, the same way an individual with advanced maturity may demonstrate a competitive ‘advantage’, their underdeveloped counterpart may demonstrate disadvantages which may predispose them to increased risk of sustaining an injury (Johnson et al., 2019).
Sport Specialisation & Overuse Injuries
Overuse injuries (i.e., Osgood-Schlatter disease) are prevalent in youth soccer populations (Price et al., 2004) and may be explained by a reduced structural tolerance of growth plates and developing bone during rapid growth (Wang et al., 2010). By definition, these injuries may occur in the absence of a singular, identifiable traumatic cause (Adirim & Cheng, 2003) and may be explained by the failure of the musculoskeletal system to withstand repetitive, submaximal forces over chronic time frames. Training volumes are thus key to monitor in this regard, and links are now beginning to materialise between, growth, excessive training exposures, and injury incidence (Johnson et al., 2022). In addition, the high exposures to repetitive patterning of sport-specific activities (i.e., kicking) typically observed in highly specialised academy soccer players can lead to other morphological maladaptations (e.g., Cam-type deformities and femoral acetabular impingement). The sensitivity to these issues can be exacerbated during the stage of skeletal maturation when the proximal femoral growth plates are open, which typically occurs in boys between 12 and 14 years of age (Agricola et al., 2012). Due to increases in circulating growth hormones during this period, the bone’s osteogenic responsiveness to joint loading may be heightened. Crucially, we believe that promoting movement variability in training during this phase will enable a more even stress distribution to a greater range of anatomical structures by altering the point of force attenuation or production. This is a fundamental principle within our MDS development framework in adolescent athlete populations (Figure 3).
Neuromuscular Control & Ligament Injuries
The delays in musculoskeletal growth, relative to the rapid growth in bone, subsequently creates a change in inertial parameters in the body segments (i.e., mass, position of centre of mass, moments of inertia, and radii of gyration), which can lead to compromised neuromuscular control during dynamic activities (e.g., running, cutting, and landing). This is suggested to be a key mechanism for lower-limb ligament injuries, with a high proportion (20%) of injuries in male youth soccer players being acute traumatic ligament sprains at the ankle and knee (Hewett et al., 2004, 2005; Price et al., 2004). This may be explained by deficiencies in active muscular protective mechanisms that are unable to adequately support joint torques during dynamic movements involving high force impacts and deceleration. It may be the case that the ‘lag’ in growth between muscle length and the cross-sectional area may affect the sensorimotor function processes responsible for maintaining optimal alignment during rapid adjustments of the centre of mass, which typically occur when performing these high-impact or high-speed cutting, decelerating and sprinting actions. This is crucial, as actions, such as cutting, have been identified as key actions that amplify multiplanar knee joint loading (i.e., knee flexion, rotational and abduction loading) while the foot is planted (Jones et al., 2015, 2016), which can induce strain on the anterior cruciate ligament. Furthermore, chronic exposure to hazardous movement patterns may lead to the development of patellofemoral pain, another common injury observed in youth soccer (Myer et al., 2015). Resultantly, another core theme within our training philosophy for youth athletes, particularly during phases of ‘adolescent awkwardness’ coinciding with periods of heightened growth, is movement quality. Developing an athlete’s technical competency, using a controlled and progressive approach, can reinforce desirable movement mechanics during this phase, as previously attained movement patterns may need to be re-learned or modified during the adolescent growth spurt.
High-Speed Running and Hamstring Strain Injury
A highly popular training method in soccer is the use of small-sided games (SSGs; e.g., 4 vs 4, 5 vs 5) which can be used to embed coaching principles to improve youth athlete’s specific technical and tactical skills. SSGs are here to stay, being an essential part of soccer training and offering great benefits in terms of providing simultaneous development of physical, technical, and tactical performance qualities. Yet academy systems may fall guilty of an over-emphasis on SSGs when striving to develop technical skill, and consequently, youth soccer players may perform a high volume and density of repetitive movements, sports-specific skills and highly demanding mechanical activity (i.e., accelerations and decelerations) during training. These physical demands are in contrast to what occurs during match play (e.g., 11 vs 11), where players may be required to sprint over much greater distances and at far higher intensities than they are accustomed to from training. In addition, the highly chaotic nature of SSG training also falls far outside the controlled and progressive approach we advocate during periods of heightened growth. Resultantly, a training programme that fails to incorporate the necessary physical overloads may predispose youth soccer players to being unprepared for the physical demands of match play, if compensatory programming isn’t accounted for.
Further to this point, as a result of the muscle loading patterns induced by an over-exposure to SSGs, the reciprocal balance of activation and force outputs between the hamstrings and the quadriceps may be developed sub-optimally, thus reducing the strength and dynamic stabilisation around the knee joint. Quadriceps dominance relative to the hamstrings has been identified as an injury risk factor in youth soccer, in this regard (Iga et al., 2009). The functional hamstrings: quadriceps ratio, however, has been shown to improve with maturity, as youth athletes experience positive shifts in muscle architecture and develop coactivation of the hamstrings (Ritsche et al., 2020). As we have discussed already, sprinting exposure can elicit more favourable muscle architecture for hamstring injury risk mitigation (Mendiguchia et al., 2020). During high-speed running, well-developed hamstrings will serve to tolerate the high torque requirements to both eccentrically reduce the kinetic energy of the lower limb during the late swing phase (Dorn et al., 2012) and produce high rates of force during the early stance phase of ground contact (Clark & Weyand, 2014). Furthermore, the hamstrings may reduce anterior tibial translation and high shear forces generated by the quadriceps. It seems, much like their senior peers, the need for the youth soccer player to be frequently exposed to high movement speeds and greater sprinting distances remains an essential conditioning emphasis during the training week to prepare them for the demands of match play.
Shaping Training Through the Lens of the Rapidly Growing Athlete
With a recognition of maturity- and sport-specific injury risk patterns, the practitioner can begin to appreciate how these may interact with the elite youth soccer player’s training programme. We have proposed key MDS training themes intended to support the youth soccer player’s athleticism and resilience throughout their adolescent training years. These revolve around movement variability, movement quality and frequent and well-monitored exposure to high movement speeds. The use of individual growth rates alongside this approach may further individualise the delivery of a programme and practitioners should be aware of how the consequences of rapid growth, disparities in maturity and sports-specific technical practise relate to key physical performance indicators (stay tuned for Part 2 where this is discussed in more detail). Players can often experience spikes in growth rates throughout various periods of their adolescent journey, and not just ‟circa-PHV” (Figure 2). In this regard, a player may sporadically go through periods of reduced co-ordination, display exacerbated asymmetries, or demonstrate reduced loading tolerance that will need to be accommodated for in training. Alongside periodic assessments (pre-, mid-, end-season), more consistent monitoring strategies (monthly/bi-monthly) should be used in conjunction to provide regular updates on an individual’s growth and maturity profile, underpinning strength characteristics, symmetries, and movement quality and capacity, as a means of identifying facets of their programme that may need adjusting at different instances of the season (McBurnie, Dos’Santos, et al., 2021).
Forget Windows of Opportunity: Speed Should Be Trained During all Phases of Development
Previous models have suggested that sensitive periods exist, in which the training of targeted physical attributes in the expense of others (e.g., flexibility or balance training) may enable greater performance gains to be made. We disagree with this approach and feel that limiting speed training exposure based around theoretical windows of opportunity may amount to a missed opportunity for the youth athlete (McBurnie, Parr, et al., 2021; Van-Hooren et al., 2020). Young individuals have greater plasticity and ability to learn new skills. Furthermore, it has been highlighted in cross-sectional analyses that speed development increases in youth and may in fact plateau early on in a professional player’s senior career (e.g., before 20–22 years old) (Haugen et al., 2013). Resultantly, if movement capabilities are not well developed at late-junior or early-senior age, it may become more challenging to improve these capabilities to the standard required when performing at the senior level. Given the aerobic/metabolic conditioning focus already provided to youth soccer players during practice, matches and even during PE/free play in school, youth athletes may be better off developing MDS and underpinning muscle strength qualities as early as possible, which are often lacking within club and school sport environments.
One principle remains ever-present throughout all stages of development, that is, the effective application of large relative ground-reaction forces in the intended direction of travel is fundamental to performance of any MDS action (McBurnie & Dos’Santos, 2021). Not only is this concept conducive to optimal performance, but efficient movement, underpinned by enhanced mechanical capabilities, may also serve to provide youngsters with the resiliency to cope with the intensive demands of their sport. We believe that MDS training should remain ever-present in a youth soccer player’s training programme. With an informed understanding, the practitioner can be smart when applying targeted MDS training methods, which can be performed concurrently, albeit at different volumes, densities, and intensities, depending on the individual’s needs, which is the fundamental basis of periodisation.
The Multi-Directional Speed Training Framework in Youth Soccer
It is now widely recognised that previous training experience should be a primary factor in the level of technical and physical requirements of a given training task, irrespective of age or maturity level (Lloyd et al., 2016). Our proposed framework for the development of MDS in youth soccer players uses ‟training age” as a key mediator in the planning and progression of MDS training. We recommend a 3-phase structure for developing MDS (Figure 2), where, from a motor skill learning perspective, pre-planned drills are required to learn correct techniques, with gradual progression in intensity (i.e., velocity and angle) and complexity (i.e., introduction to stimuli) as the athlete develops in movement competency and capacity (DosʼSantos et al., 2019; McBurnie, Parr, et al., 2021).
This decision process should operate on a fluid continuum, where different volumes and intensities are targeted depending on the individual (Figure 3). For example, an individual with a heightened growth rate may require a movement quality emphasis and a reduction in training volume, so lower-intensity technique-focused MDS training will be preferential. In contrast, an early-developing individual who is post-peak height velocity may benefit from more physically demanding training methods (e.g., increased volumes and intensities) that harness the late-pubertal individual’s heightened androgenic responsiveness. This individual may possess a greater training age and training load tolerance, thus allowing for an emphasis to be placed on building movement capacity, developing specialised movement capabilities, and providing supplementary resistance-based strategies that develop explosive strength qualities. In essence, the training contribution of each phase will vary depending on the individual’s training age, chronic training load, maturation status, rate of growth, technical competency, and physical capabilities (Figure 3). Using the framework provide here, coaches & athletes can dictate the journey through the continuum and optimise the acquisition and retention of new skills.
Those working in academy soccer environments will appreciate that the process of development for their youth athletes rarely follows a linear trajectory. The key thing for practitioners to possess is an understanding of the youth athletic performance, epidemiology, and fundamental training principles, ensuring that their training programme accommodates for the individual’s needs during their adolescent training years. In part one of this article, we have briefly outlined some of our training principles and have also provided a reference framework to assist in the delivery of MDS training in a youth soccer context. Although this discussion has been with reference to soccer, in a general sense, this information can be applied to any multi-directional, locomotive sport, as long as the nuances of each sports’ specific context is appreciated. Young soccer players can and should be exposed to all aspects of the MDS continuum and we hope this article can serve as a practical reference for which overarching MDS foci can be integrated into a youth athlete’s programme. In part-two, we will take a deeper dive into this framework, also offering an applied perspective on how to monitor and progress MDS development in an academy setting.
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