Strength Training and Athletic Performance
The improvement of athletic performance in youth athletes is a complex task, and to achieve high-levels of athleticism requires a robust long-term plan. Sports participation alone, in many cases, does not offer the sufficient stimulus to achieve this. Resistance training in all forms (e.g. strength, power or speed training) can help to attenuate these issues by protecting against injuries and positively affecting youth athlete’s physical literacy, thus, diminishing the impact of low physical activity and early sport specialisation among youths .
Stronger young athletes will be better prepared to learn complex movements, master sport tactics, and sustain the demands of training and competition . Thus, resistance training prescription should be based on an appropriate progression according to training age, motor skill competency, technical proficiency and existing strength levels. Another factor to consider is the biological age and psychosocial maturity level of the child or adolescent [1, 6].
A high-level of muscular strength contributes to enhancing performance ability in young athletes. Moreover, it is also important to build a good base of fundamental movements during childhood and adolescence, as this will help the youths develop more efficient motor skills whilst simultaneously reducing their risk of injury due to improved body control and/or technique .
The ability to produce high levels of force is important for sports performance at all levels . Good parameters of maximal muscular strength influence performance due to increases in muscular power and muscular endurance .
Resistance training has been found to be an effective method to promote muscular strength and jump performance in youth athletes . Moreover, it has been shown that muscular strength has a direct impact on running speed, muscular power, change of direction speed, plyometric ability, and endurance . In accordance to this, it seems that muscular strength is critical for an efficient development of fundamental movement skills (FMS) .
The development of muscle strength depends on multiple factors, such as muscular, neural, mechanical, psychological and hormonal [13, 14]. Moreover, strength develops in a non-linear way throughout childhood and adolescent. Nevertheless, strength tends to increase similarly both in girls and boys until the age of 14, where a plateau begins in girls and a spurt is evident in boys .
It is important to acknowledge the fact that growth and maturation will affect strength gains both before, during and after puberty . In this sense, it has been found that relative strength gains in prepubescents are equal, or greater, to those shown by adolescents. In general, adolescent absolute strength gains appear to be greater than prepubescent gains, but less than adult gains .
The development of speed throughout childhood will be influenced by multiple changes in the muscle, such as growth in cross-sectional area and length, biological and metabolic changes, neuromuscular development, and also changes in biomechanical factors and coordination . As well as with other physical traits, speed development occurs in a non-linear way throughout childhood .
In terms of speed development throughout childhood and adolescence, it has been shown that the weight gains during puberty can negatively affect a young athlete’s speed. Strength training can, therefore, be an effective way to overcome the negative influence of this increase in mass by enhancing force production. Simultaneously, it would also positively impact favourable changes in body composition, thus maximising relative maximal force (i.e. the amount of force an athlete can apply in comparison to their body weight) .
Finally, recent findings have shown that the variance in sprint performance in adolescent boys may be the result of varying degrees of strength and power. This implies the importance of early introduction into resistance training for boys wishing to enhance their maximal speed .
Increases in muscular power occur around the time of peak height velocity among youngsters. Moreover, the time where peak muscular powers become noticeable tends to coincide with peak weight velocity. This phenomenon suggests that increases in both muscle mass and motor unit activation are closely linked to the development of muscular power .
Evidence in the literature has shown that plyometric training  and strength training [3, 18] both have a positive impact on enhancing muscular power in young athletes; even when used in isolation  and in combination . As such, strength training can have a significant impact on the power production abilities of young athletes, and considering power is a vital aspect of many sports , there is plenty of justification for the inclusion of strength training within the young development programme.
Participation in sport involves some inherent risk of injury, and although the total elimination of sport-related and physical activity-related injuries is an unrealistic goal, it appears that an all-round programme which focusses on increasing muscle strength, enhancing movement mechanics and improving functional abilities may be the most effective strategy for reducing sports-related injuries in young athletes [2, 6, 10]. The strengthening of muscles and connective tissues through strength training makes young athletes capable of sustaining higher external forces, which therefore makes them less susceptible to soft-tissue injury [6, 21].
Additionally, the effectiveness of these injury prevention programmes – a good example of which is the FIFA 11+ – is greater if implemented in younger age groups prior to the commencement of neuromuscular deficits and biomechanical alterations seen during growth spurts . In female athletes, for example, early engagement in neuromuscular training is likely to result in a reduced risk of anterior cruciate ligament injury later in life [6, 21]. Furthermore, specific resistance training exercises can help to prevent the development of bone injuries (e.g. Sever’s disease) .
Moreover, as growth and maturation is a period of rapid development, young athletes are at a greater risk of sustaining injuries, whether they participate or not in competitive sports or non-competitive recreational physical activity . In many cases, it is also well-acknowledged that strength training sessions carry a lower risk of injury in comparison to the sport themselves . This simply means that children are more likely to get injured playing their respective sport than they are during strength training (providing appropriate supervision is in place).