Reception date: 09/08/2021 / Revision Date: 10/06/2021 / Acceptance date: 10/20/2021

Resumen. El objetivo de esta revisión fue conocer la influencia que tiene el entrenamiento de fuerza en la población infantojuvenil, además de buscar los posibles riesgos o beneficios que pueda ocasionar el entrenamiento de esta aptitud física. Se ha llevado a cabo una revisión sistemática de estudios de intervención sobre el impacto que produce el entrenamiento de la fuerza en sujetos con una edad comprendida entre 6 y 18 años. Los estudios utilizados en esta revisión se identificaron a través de la base de datos PubMed, seleccionando aquellos escritos en inglés o castellano, desde el 2010 hasta la actualidad. La revisión se efectuó entre los meses de Noviembre de 2020 hasta Mayo de 2021. Se identificaron un total de 328 artículos en la búsqueda inicial, de los cuales 20 artículos fueron elegidos para esta revisión sistemática. Dentro de las principales evidencias encontradas, los beneficios derivados del entrenamiento de fuerza se imponen notablemente a los riesgos, siempre y cuando el programa de entrenamiento sea supervisado y pautado por personal cualificado, individualizando la carga a las necesidades de cada sujeto. Asimismo, el entrenamiento de fuerza en esta población contribuye a mejorar el rendimiento de distintas habilidades motoras y actividades deportivas, resultando además una estrategia eficaz ante patologías como la obesidad.

Palabras clave: Infancia, obesidad, habilidad motora, niños, ejercicio.

Introduction

The term strength training refers to a specialized method of conditioning, which involves the progressive use of resistant loads and a variety of training modalities designed to improve health, fitness, and athletic performance (Faigenbaum et al., 2009). Although strength training and weight training are often used as equal terms, the term strength training encompasses a broader range of training modalities and training goals (Faigenbaum et al., 2009).

Strength training in children and young people has been a subject that for many years has generated great controversy regarding the type of training, volume or duration, as well as numerous doubts as to its contribution to this population (Vrijens, 1978). With the passage of time, more research has been done on this subject and from the NSCA study (1985) possible benefits of this training have been found, reflecting improvements on the mastery of motor skills and generating a positive contribution on other abilities.

The physical health-related benefits of strength training for young people is another relevant issue. It has been shown that the implementation of an adequate strength program induces improvements in bone health and body composition, as well as being highly effective in preventing possible injuries, especially in sports (Faigenbaum et al., 2009).

With the right training methods, this type of training for children and adolescents can be relatively safe, leading to an improvement in overall health (Behm, 2008). In addition, another aspect that scientific evidence highlights is the need for supervision and prescription of this type of training by a professional, being key to the effectiveness of training programs, since prioritizing the physical safety of young people will be necessary for the improvement of health and performance (Loyd et al., 2014).

To this day there are still misconceptions about strength training in children that they are more prone to injury than adults. Not only do these notions appear to be incorrect, but children's response to strength training is actually quite similar to that of adults, although they do not gain as much muscle mass (Falk & Dotan, 2019). Under proper guidance and supervision, the incidence of strength training-related injuries is no higher in children than in adults. Strength training has other benefits for children beyond actual strength improvement. It can help reduce the risk of activity-related injuries in general, and especially in other sports. In overweight youth, it can also improve metabolic profile and help in the management of conditions such as diabetes (Falk & Dotan, 2019).

Regarding the prescription of the type of strength training and the development of the sessions in the infant and juvenile population, there are a variety of aspects to take into account, such as the volume of work or the intensity among other things. Given the disparity of methodologies with different types of exercises, there seems to be no ideal training model, although all agree that the training dose should be taken into consideration, this being an important aspect to produce the desired effect (Lesinski et al., 2016).

Method

For the development of this review, a search was carried out in PubMed databases between November 2020 and May 2021. A review was made of publications from 2010 to the present. The search was performed following the Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) review protocol, which consists of a 27-point checklist of the most representative sections of an original article (Liberati et al., 2009). The following keywords were used for the PubMed search: Physical activity AND Resistance training, Children AND Strength training, and Strength training AND Children AND Adolescents.

For the selection of the articles a series of inclusion criteria were taken into account: a) Intervention studies that evaluated the effect of a physical training program; b) Studies carried out in children over 6 years old up to 18 years old; c) Randomized and Non-Randomized samples; d) Studies that evaluate the positive effects of strength training; e) The languages selected for the search were English and Spanish.

On the other hand, the criteria chosen for exclusion were the following: a) Studies that did not evaluate the effects of regular physical exercise; b) Studies with adults; c) Clinical trials carrying out strength training programs in children with special needs.

In order to assess the scientific quality of the studies, the PEDro scale was used. This scale consists of 11 items, although the evaluation is given out of 10, since the first item is not taken into account in the rating. Scores between 9 and 10 are considered to be of excellent quality; between 6 and 8, of good quality; between 4 and 5, of fair quality and, finally, values lower than 4 mean poor quality.

Results

A total of 328 articles were identified in the initial search. After the first screening, 190 articles were eliminated, leaving 138 articles selected for full-text analysis. Finally, after exclusion of articles that did not meet the inclusion/exclusion criteria, 20 articles were eligible for this systematic review (Figure 1).

Figure 1. Flowchart

In total, 1461 participants (517 girls and 944 boys) were analyzed in the studies presented in this review. Table 1 describes the studies carried out, describing the type of training performed, the duration of the training programs, the weekly training sessions, and the main findings after the completion of the interventions. Of these, eight studies investigated the effect of strength training in the child-adolescent, athletic population (Rodriguez-Rosell et al., 2017; Panagoulis et al., 2020, Moran et al., 2018; Hopper et al., 2017; Piazza et al., 2014; Amaro et al., 2017; Negra et al., 2016; Parsons et al., 2017), five studies investigated the effect of strength training in obese subjects (Alberga et al., 2016; Goldfield et al., 2015; Monteiro et al., 2015; Schranz et al., 2014; Davis et al., 2011), and seven studies investigated the effect of strength training in the general infant and juvenile population (Pichardo et al., 2019, Meinhard et al., 2013; Granacher et al., 2011; Lee et al., 2012; Lloyd et al., 2016; Bernardoni et al., 2014; Deldin et al., 2019). In all studies, the strength training sessions were supervised by a professional strength expert, controlling in situ the appropriate training load for each subject in order to carry out progressive overload and avoid any type of poor execution or excessive load. The training methods varied among the studies, being able to observe great heterogeneity in the exercises carried out.

Table 2 shows the methodological quality of the articles reviewed with a rating range from 6/10 to 9/10 on the PEDro scale.

Discussion and conclusions

Over the last few years there has been controversy about strength training and children, about the possible effects on both performance and development. Strength training could have an impact on improving the level of performance in various motor skills and sporting activities in physical education, as well as being a promising strategy in schools to counteract decreasing levels of physical activity (Meinhard et al., 2013).

Comparisons have been made between strength training and aerobic training (Pichardo et al., 2019; Alberga et al., 2016; Lee et al., 2012; Goldfield et al., 2015), where the benefits of the different trainings are tested. Both types of training seem to have significant improvements in terms of power (Pichardo et al., 2019), being the aerobic training where greater benefits were found at cardiorespiratory level (Alberga et al., 2016). Other authors (Goldfield et al., 2015) mention that where they find greater benefits in fat loss is in a type of training that combines both types of skills. However, other authors find benefits related to insulin sensitivity in strength training (Lee et al., 2012).

Strength training can also be performed as an anti-obesity strategy (Alberga et al., 2016; Goldfield et al., 2015; Monteiro et al., 2015; Schranz et al., 2014; Lee et al., 2012; Davis et al., 2011). In comparing different types of training, Goldfield et al. (2015) observed how both aerobic, strength, and combined training reduced total body fat and waist circumference in obese adolescents. Following this same line, it has been seen that metabolic risk profiles can be achieved by performing any type of training in obese adolescents (Monteiro et al., 2015). Another research by Schranz et al. (2014) showed that training can positively affect self-concept in addition to strength in overweight and obese adolescents. Finally, it is highlighted that programmed training can become an excellent tool for the improvement of cardiorespiratory health (Alberga et al., 2016).

Some clinical trials wanted to observe the effects of training in different sports such as football (Rodriguez-Rosell et al., 2017; Panagoulis et al. 2020; Negra et al., 2016), swimming (Moran et al., 2018; Amaro et al., 2017), rhythmic gymnastics (Piazza et al., 2014), or netball (Hopper et al., 2017). The contribution of Rodriguez-Rosell et al. (2017) related to strength and football showed benefits in maximal strength, vertical jump, and sprint time compared to the control group. On the other hand, Negra et al. (2016) showed benefits in the half squat and, as in the previous study, in jumping and sprinting. Meanwhile, Panagoulis et al. (2020) established that neuromuscular training can induce positive adaptations in the performance of early adolescent soccer players during regular season. Regarding swimming, Moran et al. (2018) seem to find improvements in body composition, vertical jump, and anthropometric measurements, while the study by Amaro et al. (2017) mentions that they found improvements in swimming crawl style especially in 50 meters as a result of strength training. Likewise, the relationship of strength training and rhythmic gymnastics was shown with increased jumping performance and an increase in lower extremity explosive strength of 6-7%, with no side effects (Piazza et al., 2014). In a less common sport such as netball, we also wanted to evaluate the influence of strength training, observing an improvement in sprinting, change of direction, and high jump as a result of strength training (Hopper et al., 2017).

Regarding strength training as a function of gender, Meinhard et al. (2013) wanted to make a comparison of strength training in boys (10-14 years) and observed that girls showed a similar increase in strength, but not in energy expenditure, which they do not attribute all the credit to this type of training but may also be explained by their earlier pubertal development. Another research that compared strength training as a function of gender was that of Deldin et al. (2019), who allude that strength training produced significantly greater increases in total and regional skeletal muscle in boys than in girls.

Within physical education in the school setting, strength training could also have a place, showing that it could have a positive impact on improving the level of performance in various motor skills and sporting activities (Granacher et al., 2011). Lloyd et al. (2016) did a study where they evaluated strength training, plyometrics training, and a combined training in school-aged boys and girls pre and post their peak performance velocity (PHV). The authors mention that it was the plyometrics training where the pre-PHV children made the greatest gains in all performance variables, while the combined training was the most effective in eliciting changes in all performance variables for the post-PHV cohort, in both cases speaking over a short period of time (Lloyd et al., 2016). Other authors such as Davis et al., (2011) also relied on school girls for their intervention, where they showed a decrease in waist circumference, subcutaneous adipose tissue, visceral adipose tissue, fasting insulin, and insulin resistance, thus significantly improving their fitness. In addition, strength training may be a promising strategy in schools to counteract decreasing levels of physical activity (Meinhard et al., 2013).

In terms of improvement in landing jumping skills, there was no difference between groups on improvement following an intervention comparing lower body focused strength training and upper body focused strength training in female athletes (Parsons et al., 2017).

In this review we have found some limitations that are set out below: studies with a short duration of time. The time period of the interventions is different and generally short, with interventions lasting 7 months and interventions lasting only 4 weeks. In addition, the authors use different training methods, which makes it difficult to make a proper comparison and to know which methodology offers the best results. On the other hand, most of the investigations (n=8) analyzed strength training in comparison to other types of training and as many (n=8) to see the performance of this type of training within a specific sport, and very few (n=4) chose to analyze strength directly. The age of the children is very different from one study to another, there being significant differences in their maturation, many of the improvements obtained could be due to the maturational development. Similarly, the sample of more than half of the studies included (n=12) is less than 50, which may be too small to extract results.

After reviewing the studies analyzed, it can be concluded that strength training in the infant and juvenile population is an effective and safe strategy to contribute to improving strength parameters and general physical health, thus clarifying the possible doubts of the population as to whether or not strength training is beneficial. Regarding the method and strength exercises proposed, heterogeneity has been found among the different works analyzed, with no clear consensus among them and showing that some types of exercises are not better than others, being the key to progress the individualized progressive overload. In addition, another important part of strength training in children and adolescents is that it does not generate negative impacts on health, both physical and psychological; being a great ally to combat pathologies such as obesity, generating a good physical preparation for young athletes, and can contribute positively to school physical education. Future interventions in this type of population are recommended, carrying out more programs of only regular strength training in order to clearly demonstrate the benefits of strength training, since many studies combine it with aerobic training, which can make it difficult to clarify that the improvements obtained are mainly caused by strength training.

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