It is known that a child is NOT an adult, with the main differences in the musculoskeletal system being immature bone in terms of both of its composition (rich in organic matter and water and a relative mineral deficiency) and its architectural structure, elastic joints (in terms of both joint cartilage and capsuloligamentous elements) and a lesser likelihood of muscle hypertrophy, for which at this particular juncture the system is still fragile while being simultaneously influenced by hormonal, nutritional and mechanical factors which give it even greater structural variability [10, 14, 21, 27, 39, 53–58].

The growth plate is only present at children’s bones. The child grows thanks to the physis or growth cartilage found in long bones and which appears at different points in time depending on the location [3, 14].

Being aware of huge variability on physis fusion appearance, we could consider that until the age of 10, the growth process occurs largely in parallel in both boys and girls. By contrast, girls undergo a growth spurt between the ages of 11 and 13, while this occurs in boys between 13 and 19, coinciding approximately with the hormonal changes of puberty and the physeal closure which is no longer active at 16–17 in girls and at 17–18 in boys [2, 3, 14, 53].

The structural development of bone originates in the growth plates of long bones, and this growth plate is avascular and aneural and consists of cells (chondrocytes) embedded in an abundant extracellular matrix. These cells are contained in the growth plate at different stages of differentiation, which are organised and arranged into several zones [22, 29, 58].

The growth plate (bone, periosteum and cartilage) is less resistant to stress than adult articular cartilage [22], and also, it is less resistant than adjacent bone to shear and tension forces. The physis may be 2–5 times weaker than the surrounding fibrous tissue [29]. Therefore, when disruptive forces are applied to an extremity, failure could occur through the physis. So physeal injuries may produce irreversible damage to the growing cells, leading to growth disturbance [15, 27].

There is no doubt that microtraumas caused by sport are repetitive by nature and that when they act on nonvascularised or poorly vascularised structures, such as elements of the joints, certain layers of growth cartilage, epiphyseal ossification centres or apophyses with tendinous, aponeurotic insertions become accumulative and thus acquire the ability to trigger the overload injury.

When practising sport, the growing osteoarticular system and, more specifically, the musculoskeletal system are subject to exogenous and endogenous mechanical stresses of pull/pressure, flexion and torsion, namely, self-trauma and microtrauma, which have repercussions on those same structures by triggering inflammatory processes that result in endochondral ossification and subsequent growth [57].

Repetitive altered or asymmetrical loading over growth plate cartilage could produce many skeletal deformities [15, 27]. Premature fusion may create a shortening of the limbs. If this shortening is in the upper limbs, the functional consequences tend to be minimal or insignificant. Load bearing could produce more significant effects on lower limbs, like leg-length inequalities, torsions and deformities resulting from malunion of fractures.

Several authors [8, 15, 27] have described that certain sports, like gymnasts, have altered relative growth of the radius and ulna which could be associated with repetitive growth plate injuries and/or altered loadings selectively retarding radial or ulnar growth, while in asymmetry sports like tennis [27], professional players had wider bones on their playing-side arms. They have also described that bone length differences between dominant and nondominant arms on the playing-side ulnar and 2nd metacarpal were, respectively, 3 % and 3.7 % longer than on the nondominant side, while control subjects presented no measurable differences.

As the joint, and in particular the growth cartilage, is the weak link in this assembly, it is thought that the risk of injury could be increased at this site during the growth spurt [2, 3, 34, 38] (Fig. 6.3). The muscle-tendon elongates in response to longitudinal growth in the long bones of the extremities, which could produce a temporary disparity between muscle-tendon and bone lengths. Therefore, it has been suggested that the growth spurt may also increase susceptibility to growth plate injury by causing an increase in muscle-tendon tightness about the joints and an accompanying loss of flexibility [35], and if excessive muscular stress is applied, a muscle-tendon imbalance is created which may predispose to injury [11, 13]. On the other hand, it has been questioned whether a reduction in flexibility happens during the adolescent growth spurt [21].

There are many anatomical differences between children and adolescents if we compare them with adults, which is why practising adult-level physical activity as a youth can result in injury and disease. The ongoing change to their anthropometric and organic components produces significant differences in the different stages of development between childhood and adulthood.

The most notable differences are at the osteoarticular level, and as this is still a developing system, it is physically vulnerable to loads, which may not seem excessive in other circumstances. The existence of other areas of growth, which are fragile under the mechanical stresses they receive, as well as the accelerated and often irregular growth of musculotendinous structures, can cause injuries.

At the organic level, there are other differences which also facilitate the occurrence of injuries and overloads during development, including thermoregulatory, cardiorespiratory, endocrine, nervous and adipose tissue (BMI) systems, and so on. Not to mention biomechanical and postural aspects typical of these ages and deriving from factors occurring prior to axis alignment.

It is vital to identify intrinsic factors of the child, namely, flexibility, elasticity, leg-length differences, axes of the limbs, nutritional state, systematic health problems, or poor prior rehabilitation, as these will be associated with extrinsic factors, namely, ground surface (natural grass, artificial turf or outdoors hard track) (Fig. 6.2), footwear, climate, trauma, loads, and poor technique, which are the main causes of injury.

The inverse relationship between the intensity of the microtrauma and the capacity of absorption on the surface of contact (i.e. hardness of the ground) must be taken into account [1, 26].

Probably, the risk is increased at adolescent growth spurt due to injuries attributable to such factors as decreased physeal strength and considerable augmentation of muscle-tendon tightness [13, 35]. The reduction of physeal strength is promoted by a lag of bone mineralisation behind bone growth during pubescent spurt, thus rendering the bone temporally more porous and more prone to injury during this stage [2, 57]. It has been observed that vigorous activity at early ages can cause a greater risk of bone deformities, although the veracity of this hypothesis is still being studied to determine whether intense activity during adolescence can abnormally alter the growth plate in the epiphyseal extension before and/or after closure of the physis [50].

In children, both tendon and ligament structures are strong and elastic, enabling them to withstand mechanical loads better than their areas of insertion, resulting in bone deterioration in the event of intense trauma or repeated microtrauma. For this reason, injuries to the growth cartilage and epiphysis are often observed alongside a low incidence of ligament injuries [35].

Moreover, dietary and nutrient recommendations for adolescents are needed in order to ensure correct nutrition to optimise bone growth and consolidation during this important life stage [58]. An adequate dietary intake of calcium, vitamin D, proteins and other minerals, as well as iron for girls, shall largely determine the correct osseous, muscular and organic growth in adolescents [10, 18, 33, 34].

Body mass index (BMI; kg/m²) is probably the most commonly used indicator for anthropometry assessments, as it is easy to use in measurements and shows a reasonable relation to body fatness in the general population. Heavier weight causes greater forces, which are absorbed through soft tissue and joints, thus perhaps related to an increased risk of injury. To this respect, several authors have reported an increased rate of injury among heavy players or players with high BMI [1, 10].

Therefore, it is of critical importance that the total caloric intake and quantities of macro- and micronutrients are adapted according to the needs of the child or adolescent.

Heat shocks and heat exhaustion, particularly in hot climates, are more likely to occur in children than in adults, because children produce more heat relative to body mass, their sweating capacity is low, and they also tend not to drink enough compared to adults [11]. For this reason, to be able to establish a correct guideline of hydration during and after physical activity (drink 1.5 L of water for each kilogram of weight lost during activity) is important to reduce the occurrence or recurrence of injury. This practice should be followed in training as well as in competition

Strength is the amount of tension that can be produced by a muscle in a voluntary contraction. During pubescence, young boys and girls commence to increase muscle strength and demonstrate a high capacity and responsiveness to an increased training load, but it is important to note that in this stage it is easier to create an exercise overload that may induce overtraining; however, their bone structure is less stable and can be injured easier than in previous phases due to excessive loads or training volume [5].

It is important to underline that resistance training exercise, involved at multifaceted programme of physical activities, should commence at prepubertal ages and should be maintained throughout the pubertal development, to obtain a well-developed bone structure activity before pubertal growth spurt. This resistance programme training stimulates both bone and skeletal muscle hypertrophy to a greater degree than observed with normal growth in nonphysically active or sedentary children [48, 54, 56].

Regardless of the chosen sport, the main concern of parents, trainers and health professionals has to be the health and wellbeing of the child. It is essential to plan childhood sporting activity carefully, as a poorly guided or practised sport can have harmful physical and psychological consequences, in some cases irreversible.

All people in the child’s environment (family, coach, physiotherapists, doctors, etc.) and who have a more or less significant involvement in the planning of the child’s physical activity must at least be aware of all aspects which could, at any given time, expose the child or adolescent to injury or any other negative effect deriving from the practice of sport (risk factors, injury mechanisms, etc.). Ideally, they should also be familiar with the positive effects of physical activity on the body of a child or adolescent, both physiologically and psychologically.