## 1 Introduction  
The ability to perform high-intensity runs during football matches has been shown to be an important determinant in players performance [1]. Overlapping, supporting moves, recovery runs or covering actions are some of the crucial actions carried out through high-intensity runs [2]. Therefore, a developed running performance seems to be a requirement for young football players for reaching professional levels [3]. On-topic investigations reported that running ability discriminates between competitive levels in young football players, suggesting that, from a talent selection perspective, running performance may play a major role in players' early performance [4]. The efficiency of aerobic and anaerobic capacity is the pathway for performing more and more intense high-intensity efforts [5]. In practice, coaches employ different training approaches to improve players' running capacities. A substantial body of the scientific literature supports the use of high-intensity interval training (HIIT) for improving running performance in young football players [6,7]. The HIIT method is based on intense bouts of short-duration exercise (performed at intensities that typically range from near to above lactate threshold or critical speed/power), with recovery intervals at low intensity or complete rest [8]. In youth football, players are categorised by chronological age and grouped into 1-or 2-year cohorts, accordingly [9]. Players from the same age group may be in quite differing stages of maturational development. Maturation refers to the developmental process by which individuals transition into their adult state, while the concept of maturity status pertains to a distinct point within the progression of an individual's maturation, expressed as age at peak height velocity (PHV) [10]. Evidence suggests that maturity status has a great influence on young players physical performance. Players with an advanced maturity status are generally taller and heavier and surpass less mature players in strength, jumping or change of direction performance [11,12]. The influence of biological maturation on running performance is not clearly defined yet. The latest literature evaluating young players running performance by intermittent running tests has established maturity development as a primary contributor to the outperforming running capacities of maturity-advanced players, although these associations may also be influenced by contextual factors such as playing position or category [4]. The maturation process brings with it physical and physiological adaptations such as fibre type composition, tendon size and stiffness, and hormonal or anthropometrical development (muscle mass and size increases) [13]. These maturity-related adaptations may influence the individual response to training stimulus. An extensive range of the literature has analysed the variation in adolescents adaptations to resistance training according to their maturity status, reporting differences in the adaptation pathways between maturity groups. Players advanced in maturation present accelerated gains in strength supported by gains in muscle mass due to rising concentrations of anabolic hormones, which enhances force production [14]. Adaptations for players less advanced in maturation may primarily come from neuromuscular adaptations and intra- and intermuscular coordination [15,16]. As mentioned, the literature concerning responses to endurance conditioning in youths, which encompass running performance capacity, presents inconsistent findings. There is not solid evidence for the existence of a window of opportunity for young athletes around their PHV, which has been advocated in the earlier literature [17]. Despite these concerns, itis reasonable to expect that growth-related changes in central and peripheral cardiovascular systems, neuromuscular function and metabolic capacities influence the adaptations of endurance and metabolic fitness throughout the maturity process [18].