Children and youth of the same chronological age may vary considerably in biological maturity status, particularly during adolescence, and individual differences in maturity status influence growth and performance during this period [13]. The structural, functional, and performance advantages of early-maturing boys in sports requiring size, strength, and power are well known. Similarly, late-maturing girls tend to excel in sports like gymnastics where small stature is beneficial [27]. Bone age, which can be determined using standardized radiographs of the wrist, is one way to assess biological age. Bone age reflects the degree of maturity of the child, but the appearance of bone may differ between various ethnic groups.

Chronological age may add yet another dimension of individual variation, as most pediatric and adolescent sports are categorized by chronological age. Within a single age group (e.g., 13 years of age), for example, the child who is 13.9 is likely taller, heavier, and stronger than the child who is 13.0 years of age, even though both are classified as 13 years of age [18]. Not surprisingly, investigations into a variety of chronologically grouped team sports have reported that elite young athletes were more likely born in the early months of the selection year, a phenomenon known at the relative age effect [28]. Thus, when children are grouped by age, variation is associated with chronological age per se and also with differences in biological maturity [13].

The fear is that an unbalanced competition between early- and late-maturing and/or older and younger boys in contact sports such as football and wrestling contributes to at least some of the serious injuries in these sports. For example, in a study of injury incidence in elite French youth football (soccer) participants, late-maturing boys sustained a significantly greater incidence rate of major injuries than early-maturing boys [29]. There were also differences between maturity groups when patterns of injury location, type, severity and reinjury were analyzed [29]. In contrast, Malina et al. [30] reported that injured and non-injured youth football players did not differ significantly in maturity status. Notably, a noninvasive method for estimating maturity status as a basis for grouping young athletes has recently been proposed [31].

The adolescent growth spurt is believed to be associated with an increased risk of SRE injury [14, 15, 32]. Height and weight increase during the preadolescent and adolescent years [13]. Girls tend to reach peak height and weight earlier than boys, at about age 15 compared to age 18 or older in boys. The adolescent growth spurt appears to be a time of increased risk for sports injury, including both acute and overuse injury. Some SRE injuries indicate an increased occurrence of injury during pubescence [33, 34]. However, prospective studies are needed to evaluate this relationship further [32]. The results of recent research suggest that increased quadriceps strength, combined with increased knee laxity and no accompanying hamstring strength development during the adolescent growth spurt in girls, might contribute to a decrease in their knee joint stability during landing tasks. These musculoskeletal changes could potentially increase anterior cruciate ligament (ACL) injury risk at a time of rapid height and lower limb growth [35]. In addition, sensorimotor function is not fully mature as children reach adolescence and some mechanisms may actually regress during this period [36]. Deficits in a variety of these sensorimotor mechanisms have been correlated with increased ACL injury risk [37–39]. Notably, three studies reported that neuromuscular control of knee motion and landing forces is significantly worse in females than in males during the transition from prepubertal to pubertal stages, with females showing regressions in control abilities [40–42].The growth spurt also appears to be a significant factor in the development of overuse injuries [34]. Overuse or repetitive microtrauma can strain the musculotendinous units which may occur more frequently during growth spurts [43, 44]. For example, chronic wrist pain in young non-elite gymnasts is significantly more likely to occur during 10–14 years of age (the expected age of peak height velocity) than either before or after this period [45]. It has been suggested that an explanation for the increased risk of overuse injury during the growth spurt was increased muscle-tendon tightness and accompanying loss of flexibility during the growth spurt [14, 15]. However, the results of several studies have not supported this concept [42, 46–48].

The adolescent growth spurt is also believed to be associated with an increased risk of epiphyseal growth plate injury due to decreased physeal strength [49–51]. During this time, structural changes in growth plate cartilage occur that result in a thicker and more fragile epiphyseal plate [52]. In addition, bone mineralization may lag behind bone linear growth during the pubescent growth spurt, rendering the bone temporarily more porous and more subject to injury [17]. Studies of the incidence of acute physeal injuries in humans indicate an increased occurrence of fractures during pubescence [17, 53–55] and a noteworthy association between peak height velocity and peak fracture rate [17]. Peak adolescent fracture incidence at the distal end of the radius coincides with a decline in size-corrected bone mineral density (BMD) in both boys and girls. Peak gains in bone area preceded peak gains in BMD in a longitudinal sample of boys and girls, supporting the theory that the dissociation between skeletal expansion and skeletal mineralization results in a period of relative bone weakness [56].

Young athletes incur many of the same injuries as their adult counterparts; however, they are at risk of incurring unique injuries not seen in adults because of the different structure of growing bone compared to mature bone [18]. Examples of injuries unique to the young athlete include epiphyseal plate fractures, stress-related epiphyseal plate injury, apophysitis, apophyseal avulsion fractures, and incomplete fractures of the greenstick type. The differences between adult and growing bone are summarized below [14, 15, 18]:

Because of these differences, children and adolescents are more likely to injure bone or avulse an apophysis than to sprain a ligament or tear a muscle or tendon. They are also more likely to injure the articular surfaces of joints, especially during periods of rapid growth. However, it is also possible that the injury mechanism may be of sufficient magnitude and orientation to sprain a ligament or tear a muscle or tendon. Additionally, due to the slower rate of apophyseal growth, athletes in their late teens or early twenties may occasionally incur apophyseal injuries.

Heat injury occurs when excessive thermal energy is generated or absorbed by the human body [86, 87]. Between 1995 and 2008, 29 high school football players in the US died from heat stroke [88]; in autumn, 2008 alone, there were four heat-related deaths in US high school football [8]. In the US, more than 9,000 high school athletes are treated each year for heat-related injury [89]. Sports and recreation heat illnesses are most common among males (72.5 %) aged 10–19 years and occur most often during July–September [90].

Reliable data on the incidence of nonfatal heat-related injuries in youth sport are lacking. However, Kerr et al. [91] analyzed the rates and circumstances of exertional heat illness from 2005/2006–2010/2011 and reported a rate of 1.20 per 100,000 athlete exposures. Exertional heat illness occurred mostly in August (60 %) and almost one third (32 %) occurred more than 2 h into the practice session. The rate in football was 11.4 times higher than in all other sports combined.

Compared with adults, exercising children were formerly believed to be inefficient when it comes to thermoregulation [13]. However, more recent studies, in which both groups were exposed to equal relative intensity exercise workloads and environmental conditions while minimizing dehydration, have compared 9–12 year-old boys and girls to similarly fit and heat-acclimatized adults [92]. These newer findings indicate that children and adults have similar rectal and skin temperatures, cardiovascular responses and exercise-tolerance time during exercise in the heat [93–96]. Thus, it may be that children are at an increased risk simply because they are more likely to be exposed to vigorous physical exercise during the warm summer months [86]. It has also been shown that during exercise, children may fail to ingest sufficient fluid to prevent dehydration, because they often do not feel the urge to drink enough to replenish the fluid loss before or following exercise [97].

Participating in organized sports can be enjoyable physical activity for many children and adolescents, if the activity is developmentally appropriate. Putting children into sports that are beyond their developmental ability can be frustrating and cause them to drop out of sports altogether [98]. Deciding on an appropriate sport requires knowledge of a child’s sport readiness.

Sport readiness means that a child’s cognitive, social and motor abilities enable him/her to meet the demands of a particular sport [98–100]. If a young athlete is expected to learn and perform skills that exceed their ability and level of development (motor, sensory, cognitive, socio-emotional), there will be little motivation to learn new skills [101]. Young children do not respond to coaching, understand strategy and tactics, or interact with teammates the same way as adults because they lack the social and cognitive skills necessary for competition, appropriate positioning, rapid decision-making and teamwork [98]. To envisage the likely development of a particular motor skill or to suppress one’s personal desires for the interests of the team as a whole would require a level of intellectual and psychosocial maturity unavailable to most pre-operational and egocentric children [102].

Motor development is also important for sport participation. Acquiring fundamental motor skills, including throwing, hopping, jumping, kicking and running, is an innate process not dependent on stage of physical maturity or gender [98, 100]. Each fundamental skill is composed of a sequence of stages of development which children progress through at various rates. A child who does not progress through all the stages may be less proficient in sports than a child who has fully developed motor skills. Many children have acquired some motor skills by preschool age; however, most children do not acquire the majority of fundamental motor skills until the age of 6 years. Therefore, organized sports that require performance of motor skills in combination are not recommended until children reach age 6 [98, 100].

Sport activities should be modified to the developmental level of the child by focusing on fun, having shorter games and practices, using smaller equipment and changing positions frequently to increase a child’s likelihood of enjoying the activity and achieving success [98, 100]. Readiness to learn certain skills cannot be determined by chronological age, body size or biological maturation alone, but rather can be assessed by determining whether the requisite antecedent skills are sufficient to provide the basis for mastering the new activity [103]. Choosing appropriate sport activities for children can be guided by appreciation of developmental skills and limitations of certain age groups, described in Table 1.1.