Background

There is a growing trend of sport specialization (i.e., repetitive training for the purpose of skill acquisition and athlete development in a single main sport at the exclusion of all other sports) across many countries around the globe [34]. Despite the broadly known beneficial effects of regular physical activity for overall health [76], the intense practice generally associated with sports specialization may increase the risk of injury [66], psychological stress [8] and performance impairment [55], due to the high sport-related physiological demand and overtraining. Therefore, it is imperative for coaches and young athletes to include complementary strategies in their routine, aiming to support enhanced motor performance and body composition, to improve markers of health and well-being, and to reduce the risk of sustaining sports-related injuries. Among such strategies, resistance training (RT) stands out as a noteworthy option.

RT is a training strategy that involves the use of a wide range of resistive loads, movement velocities, and a variety of training modalities, including weight machines, free weights, elastic bands, medicine balls, and body weight [20]. RT has been extensively studied as a tool to improve injury recovery and/or prevention in sports, as well as performance and health in adults [45, 79], especially through the development of muscle strength and mass. Over the past two decades, evidence-based reports [76, 77], meta-analyses [51] and position stands [5] have emerged, supporting the safety and efficacy of RT for children and adolescent in both clinical and non-clinical settings. This support extends to areas such as psychosocial well-being, bone mass, cardiovascular risk profile, motor performance skills, and sports performance [18] (Fig. 1). Consequently, RT has gained widespread acceptance in schools, medical and fitness centers [2]. However, data from these studies can only be partially translated to the athletic context, since the trainability, physiology, and motor performance proficiency markedly differ between non-athletic and athletic youth populations [3, 43]. Hence, in order to reduce the risk of injuries, enhance health-related indexes, and support motor performance and skill acquisition [21], coaches and instructors working with young athletes require an in-depth understanding of RT-induced adaptations, of which special attention has been given to muscle morphology (e.g., hypertrophy) and muscular fitness-related measures (e.g., muscle strength, muscle power and muscle endurance) response.

Fig. 1
figure 1

RT offers multiple benefits for children and adolescents. To date, research has shown improvements in bone mass, psychosocial well-being, motor performance skills, sports performance, and cardiovascular risk profile in this population [13]

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For instance, RT-induced strength improvements are typically related to an increase in muscle cross-sectional area [53], so that muscle hypertrophy-related strength gains may lead to sport performance enhancement and reduced injury risk [20, 79]. In turn, muscle strength consists in the ability to exert force or tension against a resistance at a given speed [2, 39]. High-velocity to overcome a resistance is usually required across several sport-specific contexts (e.g., own body mass, body mass of opponent, mass of object). Thus, maximal strength production should be an important capacity developed by conditioning programs to support sports performance, as it also enhances muscle power and muscle endurance performance [4, 39]. Muscle endurance, defined as the ability to consistently maintain force exertion against a given resistance over time [52], is an important capacity to sustain high velocities or to minimize the fatigue-related performance decrements. Hence, RT programs should promote the enhancement of this capacity to provide active muscles a higher endurance against fatigue and to maximize performance in sport modalities such as swimming, soccer, running and rowing. In addition, muscle power is defined as the amount of force/work/energy that can be produced in a given unit of time, and it is the product of the multiplication between two variables: force and velocity [15]. Consequently, muscle power is associated with explosive gestures/acts, and to be optimized, this capacity requires the training and enhancement of both force and velocity.

As more children and adolescents get specialized in sports and involved in RT throughout sport organizations and training centers, the aim of this review was to summarize the evidence regarding the risks, concerns and efficacy of RT on muscular fitness-related measures and hypertrophic responses of youth athletes, while also establishing recommendations and guidelines to assist RT application in this population. Despite the existence of distinct reviews published in the last years exploring RT impact for young athletes [10, 21, 27, 40, 41, 69], the vast majority of them were dedicated to examine the response of isolated parameters to RT programs (e.g. physical fitness; or injury risk; or muscle physiological adaptations; or health-related benefits), while none of these reviews were dedicated to compiling the main and lately findings, thereby highlighting the need to develop a more updated guideline and consensus for RT implementation in youth athletes. Considering the multiple important topics and wide scope of the current article, it was decided to concentrate the main information in the form of a narrative review.

Methodology

To counteract the subjective nature and probability of selection bias generally associated with narrative reviews, we conducted a search strategy using PubMed and Google Scholar databases without a specific set date, from February 2022 to June 2022 with an additional update in July 2023, using the following keywords combined with Boolean operators (“AND/OR”): “muscle hypertrophy”, “injury”, “sports injury”, “resistance training”, “muscle endurance”, “strength training”, “lean mass”, “muscle thickness”, “muscle power”, “youth athletes”, “young athletes”, “sports”, “sport specialisation” and “sport specialization”. Studies were considered eligible for analysis based on the following inclusion criteria: (a) published in English as a full-text manuscript or thesis; (b) involvement of youth individuals (≤ 19 years) engaged in sport modalities; (c) inclusion of RT protocols lasting a minimum of 6 weeks. Of note, this last criterion was adopted because the focus of this current review was to determine the influence of RT on muscle hypertrophy in youth individuals, and high-quality evidence [74] has demonstrated that increases in muscle cross-sectional area in response to RT lasting less than 6 weeks seem to be more related to muscle edema/swelling than actual protein accrual. Due to the difficulty in determining the total training load, studies using elastic bands or flywheels were not included in the review, as well as those failing to provide comprehensive information about RT methods. Moreover, even though plyometric training remains one of the most well-established methods to improve muscle power [46], because it differs slightly in nature and application from traditional RT, we limited our review to studies that assessed muscle power response solely through RT, without any other forms of training.

Results

Muscular Fitness-Related Measures

Muscle Strength

The literature pertaining the muscle strength response of youth athletes to RT is illustrated in Table 1, which includes data from 18 studies. In order to increase the practical applications of our observations, our review focused only on studies assessing muscle strength through the repetition maximum (RM) test. For example, Channell and Barfield [11], randomly allocated adolescent American football players to an 8-week traditional or Olympic RT program, three times a week. A control group of youth athletes, who engaged in regular training without RT, was also included. Both traditional and Olympic RT groups had similar exercise routines in terms of the number of exercises, sets, repetitions and intensity, with exception of two specific exercises in the Olympic RT group (‘power clean’ and ‘push jerk’). Importantly, a 4-week general RT period was employed before the experimental phase, in order to ensure that each participant conducted every lift with proper form. Results indicated that both traditional and Olympic RT groups experienced similar increases in their 1-RM in the squat exercise. However, only the Olympic RT group experienced improvements in the power clean exercise, probably due to the specificity principle. Interestingly, the relative increases observed by the authors were modest when compared to similar RT periods in other studies. It is possible that the initial 4-week familiarization period may have contributed to substantial strength increments from neural adaptations [4, 5, 27, 39], which tend to diminish with ongoing training, thereby limiting further gains during the experimental period.

Table 1 Studies assessing muscle strength response to RT in youth athletes
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In a subsequent investigation, Chelly et al. [12] recruited eleven junior soccer players and submitted them to an 8-week lower-body RT program, twice a week. A control group continued their soccer training, without RT. The primary exercise included in RT regimen was the back half squat, performed in four sets using in a pyramid model. Remarkably, this training protocol was sufficient to promote a ~ 35% increase in the participants' back half squat 1-RM, while the control group did not show significant differences. Similar findings were detected by Styles et al. [78], Hammani et al. [30], Harries et al. [31] and Contreras et al. [14], who employed a nearly identical RT protocol in terms of duration, intensity, and volume, as initially proposed by Chelly et al. [12]. Notably, Contreras et al. [14] made a noteworthy contribution by highlighting the importance of training and testing specificity in the context of increasing muscle strength of youth athletes through RT. The authors divided young rowers and rugby players to RT groups, performing either the hip thrust or the front squat exercise, for a duration of 6 weeks. Both training groups were assessed in both exercises before and after the RT period, despite performing just one of these exercises during the training sessions. On one hand, it was demonstrated that muscle strength increased in both exercises for both groups, suggesting some degree of transfer in the muscle strength gains between different exercises. On the other hand, the most significant improvements in muscle strength occurred in the exercises that were specifically trained during the RT period. This underscores the importance for coaches and athletes to select exercises that align with their specific goals.

The most comprehensive study was conducted by Sander et al. [70], who conducted a 80-week follow-up in 134 elite youth soccer players to examine the effect of RT on muscle strength performance while they continued with regular soccer training. Participants were divided in 3 categories according to age (under-13, under-15 and under-17 years old); and within each category, athletes were further divided into those who performed RT and those who did not (i.e. control group). The RT sessions were conducted twice a week and were focused on hypertrophy and intramuscular coordination throughout the study period. Participants were tested for their 1-RM in both the back and front squat exercises, which were part of the RT program along with other lower-body and upper-body exercises. The authors demonstrated a substantial relative increase in 1-RM strength among those who participated in the RT regimen, compared to those in the control group, with improvements ranging from ~ 100% in the oldest category to ~ 300% in the youngest category. These results align with those reported by Rodríguez-Rosell et al. [65] and are consistent with a recent quantitative review using the meta-analysis [41], both of which showed that younger age groups experienced more significant muscle strength gains with RT. While it remains difficult to explain these differences, bone plausible explanation could be the increased neural plasticity observed in children as compared to adolescent athletes [60].

Muscle Endurance

There are fewer studies examining the impact of RT on the muscle endurance of youth athletes compared to those examining its effects on muscle strength (N = 6; Table 2). To our knowledge, DeRenne et al. [16] were the first to assess muscle endurance in response to RT in youth athletes. The authors employed a 12-week RT program for young baseball players, dividing them into experimental groups that either trained one or two times a week. Even though both trained groups improved their muscle endurance compared to the control group during the pull-ups test, the group that trained twice a week achieved greater improvements. Practical assessments conducted by Christou et al. [13], Klusemann et al. [37] and Weston et al. [87] revealed nearly identical enhancements in the lower-body, upper-body and core muscle endurance of young soccer, basketball and swimmers, respectively, in response to systematic RT programs, despite their variations in training duration, intensity and volume.

Table 2 Studies assessing muscle endurance response to RT in youth athletes
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It is noteworthy that, following the principle of specificity, improvements in muscle endurance seem to most pronounced in the muscle groups that have been specifically trained and evaluated. In this direction, Moore et al. [54] recruited adolescent baseball players with 8–10 years of experience in the sport, and submitted them to a 20-week RT focused on strengthening the shoulder muscles, occurring three times a week. For this purpose, the authors employed a stair-step progression that prioritized endurance over strength, by increasing repetitions with proper technique before increasing resistive load through elastic bands and weight room exercises. A posterior shoulder endurance test was conducted before and after the intervention to assess the effects of RT. The authors observed a 166.5% improvement in posterior shoulder muscle endurance after the specific training.

The limited number of published investigations indicate RT as an effective method to improve muscle endurance in youth athletes. Additionally, it emphasizes the significance of two key factors: (1) training specificity, to improve muscle endurance in a given muscle group; and (2) testing specificity, to accurately confirm such improvement. Notwithstanding, the scarcity of examinations on this topic cast the need for additional studies involving various sports modalities and the utilization of RT protocols with distinct characteristics, to provide a clearer understanding of the true effect of RT on muscle endurance of child and adolescent athletes. Moreover, it's crucial to acknowledge the lack of information regarding well-conducted familiarization sessions for the exercise tests, which makes it difficult to differentiate the genuine effects of RT from the inherent learning process associated with the tests.

Muscle Power

Compared to muscle strength and muscle endurance, there is a greater number of studies that have investigated the effects of RT on muscle power of youth athletes (Table 3), in which 22 studies were identified. The vast majority was conducted on adolescents, and it was possible to detect some degree of heterogeneity in their RT protocols. To illustrate this argument, Gorostiaga et al. [26], Channell and Barfield [11] and Tran et al. [80] detected positive, but small improvements in muscle power, ranging from ~ 2.5% to 5.5% derived from RT protocols of similar characteristics. However, it's worth noting that some studies with similar durations and characteristics [12, 30, 42] detected 2 to 3 times greater improvements in muscle power.

Table 3 Studies assessing muscle power response to RT in youth athletes
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It is challenging to reconcile the discrepancies among these findings, but as described in the 'muscle strength' topic, it's crucial to consider the potential interference of an individual's training level on the improvements induced by RT. For example, after a 26-week high-velocity squat training, González-Badillo et al. [25] observed that the most substantial improvements in the countermovement jump (CMJ) test performance were observed in the ‘under-15’ category compared to the ‘under-21’. Similarly, Rodríguez-Rosell et al. [65] showed that the ‘under-13’ category demonstrated a 12% improvement in CMJ performance after 6 weeks of high-velocity squat training, while the ‘under-17’ category showed less than half of that improvement. Hence, superior gains in muscle power were more evident in the less experienced categories when compared to the more trained ones. On one hand, increased neural plasticity in child compared to adolescents is highlighted as a possible explanation for these results. On the other hand, the lack of adequate familiarization sessions for the exercise tests was a key feature among the different studies, hampering interpretations on the true extent of the effects of RT on muscle power.

Despite the heterogeneity underpinning their experimental designs, many studies reported enhancements in muscle powerin response to RT. In fact, with the exception of Prieske et al. [62], all other authors reported an improvement in muscle power, regardless of the magnitude. It might be speculated that the lack of change in muscle power in the study of Prieske et al. [62] is related to the use of unspecific testing (i.e., CMJ), especially considering that exercises targeting core muscles strengthening were employed. Therefore, RT is indeed an effective method to improve indicators of muscle power in youth athletes across different sport modalities. However, the current diversity in experimental designs still hampers the establishment of precise instructions and guidelines for optimizing muscle power improvement through RT in youth athletes.

Muscle Hypertrophy

There has been a long-standing paradigm supported by longitudinal studies that muscle hypertrophy either does not occur or occurs minimally in children and preadolescents in response to RT [42, 58, 68]. It is speculated that the low amount of circulating anabolic hormones [82] may contribute to morphological or architectural changes in this public, despite existing evidence showing that acute exercise-induced elevations in endogenous anabolic hormones does not enhance muscle hypertrophy [86].

Alternatively, the lack of gold-standard methods to assess and detect small but important changes in muscle hypertrophy could have had increased the chances of measurements-associated variation/error in most of the previous findings, thus hampering their interpretation. For example, no morphological changes were detected by Ozmun et al. [58] after an 8-week RT training program for elbow flexors in 16 male and female children aged between 9 and 12 years. Such changes were assessed through skinfolds, known for their poor accuracy and reliability [83]. Similar findings from anthropometric measures were demonstrated by Sadres et al. [68] and Lillegard et al. [42] after applying progressive RT over a 2-year and 12-week period, respectively, to prepubescent boys and girls.

Some of the studies conducted with more accurate, reliable and sensitive methods (i.e., magnetic resonance imaging; ultrasound), have shown results in the opposite direction (Table 4). For example, Mersch and Stoboy [49], with a small sample size (i.e., two sets of twins), demonstrated an increase in quadriceps cross-sectional area through magnetic resonance imaging in pre-adolescent boys after an RT program for the lower body. In another study [24], 1st–3rd grade Japanese boys and girls were assigned to a control or a RT group, with the latter being submitted to 12 weeks of RT for the elbow flexors. The RT group showed significant increases in muscle cross-sectional area using ultrasound technique, and such increment was significantly correlated with the skeletal age. However, contrary findings also exist; in the study by Ramsay et al. [63], no significant increases in the cross-sectional area of the elbow flexors and knee extensors muscles were observed (through computerized tomography) among prepubescent boys, who participated in a progressive RT program three times a week for 20 weeks. Similarly. using the magnetic resonance imaging technique, Granacher et al. [27] did not demonstrate significant differences in the quadriceps cross-sectional area among prepubertal boys and girls after a well-controlled 10-week progressive RT program. Of note, none of these studies [24, 27, 49, 63] was conducted with young athletes. In fact, to the best of our knowledge, there are no longitudinal studies that have assessed muscle hypertrophy among young athletes in response to a systematic and supervised RT program.

Table 4 Studies assessing muscle hypertrophy response to RT in children and adolescents through gold-standard methods
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It is difficult to understand the nature of the limited availability of research on this topic, even though one potential reason could be the difficulty of accessing sophisticated and expensive gold-standard methods. Likewise, from an ethical standpoint, subjecting children and adolescents to other invasive methods, such as muscle biopsy, for the investigation of physiological adaptations raises concerns. In view of the mentioned, while some of the above mentioned studies [49, 63] do support for the possibility of muscle hypertrophy among children and adolescents, the contrary results of others [24, 27], associated with the scarcity of qualified investigations, make it premature to conclude that this adaptation indeed occurs in this population. This is especially important for the purposes of this short narrative report, the response of youth athletes to RT. Future investigations should involve large sample sizes, longer durations, and the utilization of accurate and reliable techniques to elucidate the actual effect of RT on muscle hypertrophy in young athletes.

Risks and Concerns

There has been a traditional/cultural concern that RT during childhood and adolescence may induce potential injuries to the epiphyseal plate or growth cartilage. This preconception was originated from several retrospective studies in the 70s and 80s, which reported damage to the growth cartilage in youth undertaking RT [6, 29, 35, 67, 85]. Data from the National Electronic Injury Surveillance System (NEISS) have indicated increasing trends of epiphyseal injuries in youth lifters, further reinforcing such concern. NEISS data also suggest that many of the reported injuries are muscle strains, with the hand, lower back, and upper trunk being the most commonly affected areas. Recent NEISS data even suggest that hand injuries are particularly common in children < 12 years old [36]. Indeed, caution merits to be exercised on this matter if we consider that (1) injuries to these structures could result in lost of training time, significant discomfort, and growth disturbances (in the case of epiphyseal plate or growth cartilage injuries) [9]; and (2) the growth plate may be less resistant to shear and tension forces [75]. However, an in-depth analysis of these retrospective studies reveals that most of the reported injuries were linked to improper lifting technique, poorly designed RT programs, and lack of qualified supervision, instruction or equipment [20].

In fact, the number of prospective studies reporting RT-related injuries in young lifters is scarce. For example, one study reported a participant who required 1 week of rest due to anterior shoulder pain [64]. Another participant experienced had a shoulder strain that led to missing a single training session [42]. However, other studies reported high rates of lower back/lumbar spine pain; with 29 out of 43 adolescents experiencing RT-related injuries in this region. While most of these injuries were minor, 4 were severe enough to necessitate surgery [7]. Although these data may initially raise concern, the relative high incidence of lower back injuries could be a result of insufficient focus on strengthening the trunk or posterior chain musculature, and once again, suboptimal program design. Additional factors such as inappropriate RT progression or incorrect technique could also increase the risk of soft-tissue injury. Supporting this notion, some authors found that there was no increased risk of injury when children were adequately supervised and submitted to one repetition maximum training with weight machines [19]. This finding is supported by other investigations employing similar RT designs but with free weights [32]. Of note, a review [20] of the above-mentioned findings revealed estimated injury rates of 0.176 [64] and 0.053 [42] per 100 participant hours, respectively. Importantly, these injury rates are lower than those exhibited by heavier contact sports such as rugby, which has reported injury rates approaching 0.800 per 100 participant hours [47], suggesting that well-designed and supervised RT protocols are relatively safe for youth.

Any type of sport carries some degree of injury. Although RT might present a risk of injury, this method does not seem to add any injury risk to the sports that youth athletes are already engaged. Furthermore, the risk of injury resulting from RT can be minimized with a number of procedures (see Table 5), which include safe exercise equipments, effective supervision, lifting form education, appropriate overload, gradual progression, careful selection of exercises, and adequate recovery between training sessions. It is important to remember that children and adolescents of the same age may differentially tolerate a given physical and mental stress, and therefore, an individual approach should be prioritized.

Table 5 Potential procedures that can be employed to minimize RT-related injury factors in youth athletes
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Recommendations

Adult training guidelines and philosophies should not be imposed on youth, since they are physically and psychologically less mature than adults. However, as confirmed throughout this report, the studies conducted with young athletes are reduced, and their characteristics differ substantially from each other in terms of sample size, participants-related sports modality, participants’ training status, and RT duration and protocol (volume, intensity, frequency), making it difficult to establish accurate guidelines for RT prescription to this population. Therefore, suggestions regarding exercise intensity and volume, inter-set resting interval and frequency will be provided for young athletes engaged in RT to help optimize results and reduce the risk of injury (Table 6). Naturally, the first consideration should be working with qualified instructors having appropriate certifications, who understand youth RT principles and pediatric exercise science, enabling them to provide real-time feedback and ensure safe and correct development of movement.

Table 6 Suggested recommendations for youth athletes-based RT prescription
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RT Frequency

The training frequency refers to the number of workouts performed per week, and more specifically, how often a muscle group is worked in a weekly manner. It is an important variable to ensure sufficient recovery, avoid overtraining, and achieve maximal benefits of RT. Most previous research (Tables 1, 2, 3) employed 2 or 3 weekly RT sessions. This frequency is in agreement with established position statements [18] and recent meta-analysis [56], although information from these reviews was mainly based on physically active children and adolescent, but not athletes. The only meta-analysis published to date encompassing youth athletes did not uncover statistically significant differences in muscle strength or power between training with 2 or 3 sessions per week [41]. Hence, one might speculate that training twice per week might be sufficient and even preferable to achieve gains in muscle strength and power in youth athletes, while also minimizing the physical stress from higher exposures. It is worth mentioning that, to the best of the author's knowldege, there are no available studies directly comparing the effects of distinct RT frequencies (e.g., 1 time/week vs. 3 times/week vs. 5 times/week) for youth athletes, reinforcing the necessity of studies to address this issue.

RT Volume and Intensity

The relationship between volume and intensity is inverse in nature, and both of them require special consideration when prescribing RT to induce muscular fitness gains and reduce injury risk. While intensity most commonly refers to the magnitude of resistance that is required to be overcome during a repetition, volume refers to the total number of within a training session, the number of sets and repetitions within each set [4]. To prescribe training intensity, coaches typically specify a percentage of an athlete’s 1 RM. While this approach is routinely used within a research environment or elite level sport, equipment constraints and time (for the test itself and for familiarization) may lead to the use of repetition-maximum ranges (e.g., 8–12 RM) or predictive equations that estimate 1RM values based on sub-maximal loads in youth populations.

A recent meta-analysis showed that conventional RT programs with average RT intensities of 80%–89% 1RM were most beneficial in terms of improving muscle strength in youth athletes [41]. These results are in accordance with previously published position stands [1] and meta-analysis [59] demonstrating that the most substantial muscle strength gains in adults, trained individuals and athletes were achieved when training at 80%–85% 1RM. Regarding the number of sets per exercise, the aforementioned meta-analysis [41] showed similar effects between single-set and multiple-set RT programs. Despite the time-efficiency of single-set programs, this result was extracted from only one study, thereby requiring precaution when interpreting this result. To date, there are no studies directly comparing the impact of RT volume in youth athletes. Evidence from adult athletes demonstrated that single-set RT programs may be appropriate during the initial phase of RT [88], whereas multiple-set should be used to promote additional gains in muscle strength [39]. Therefore, multiple-set RT may be necessary to elicit adequate stimuli during long-term youth athlete development.

Additionally, based on the current scientific knowledge, it seems that there is no significant difference in muscle strength and/or hypertrophy gains between non-failure training and training to failure [28]. Therefore, it would be unnecessary to systematically train to failure (i.e. on each set) to increase a youth athlete’s strength to a greater extent than a workout where sets conclude with a few repetitions left in reserve. It is therefore quite possible, depending on individual preferences, to choose one or the other method, although minimizing failure might also be interesting to prevent overtraining.

In view of the mentioned, when introducing RT to a youth athlete's routine, especially given the already high demands associated with their respective sport modalities, it seems reasonable to prescribe an appropriate repetition range initially. This approach allows for the development of technical competency and the acquisition of a base level of adaptation. Over time, the external load can be increased as technique proficiency improves. In this sense, a beginner may be prescribed 1–2 sets of 15–20 repetitions with a light or moderate load (50%–70% 1RM or equivalent). As exposure to RT increases, the prescription may be augmented to 2–3 sets of 8–12 repetitions with a heavier load (70%–85% 1RM). When technical expertise is appropriate, lower volumes (3–5 sets of 3–8 repetitions) and higher loads (> 85% 1RM) can be introduced to optimize training adaptations.

RT Inter-set Resting Interval

Due to the limited number of studies reporting the duration of rest between sets, this author has chosen not to include this information in Tables 1, 2 and 3. Nevertheless, the inter-set resting interval is another variable commonly manipulated in RT programs, it is worth briefly discussing what the literature may indicate regarding the most appropriate duration of rest between sets, which should be influenced by parameters like RT intensity and volume.

Although limited evidence regarding the optimal rest periods for youth-based RT, available research indicates that children can recover more rapidly from fatigue-inducing resistance exercise [89]. It has been suggested that children are less likely to suffer muscle damage following such exercise, owing to the increased pliability of their muscle tissue [17], thus rest periods of approximately 1 min may be sufficient for inexperienced children. However, a recent meta-analytic review revealed that long inter-set resting periods (i.e. 3–4 min) were most effective for improving muscle strength following RT in youth athletes. It is likely that longer resting periods allow athletes to maintain higher volumes and intensities during each set, thereby maximizing long-term muscle strength gains [44]. Hence, while beginners may cope with RT demands using shorter rest intervals (e.g. 1 min), it is reasonable to assume that these intervals should be extended as children enter adolescence and become more experienced, especially when exercises require high levels of skill, force or power production.

Final Considerations

There is a growing body of studies investigating the effects of RT programs on muscle strength, endurance and power of youth athletes. The present report attests the consistent positive effects of this method for this population, with most findings demonstrating improvements in muscle strength and power, and few in muscle endurance. Despite the promising prospects for in this field, the existing investigations suffer from the overlapping of diversified experimental designs, demonstrating that a need for further exploration. Presumably, the improvement in muscular fitness should be largely explained by the RT-induced muscle hypertrophy. The few and heterogeneous examinations up to date employing gold-standard methods for determining muscle hypertrophy, however, prevent definitive conclusions from being drawn. Future studies should aim to clarify the underpinning mechanisms contributing to the improvement in muscular fitness observed in youth athletes. Based on years of research, it appears that RT injury rates among youth participants are low and less concerning in well-designed, progressed, supervised and technique-oriented programs, which should be safely achieved through the guidance of experienced and certified professionals. Finally, to enhance RT-induced muscular fitness gains without increasing risk of injury, care must be taken to provide appropriate instruction and prescription for child and adolescent athletes. Although international consortia have disseminated RT recommendations for young participants, these guidelines do not necessarily apply to the athletic community. The suggestions on RT frequency, intensity volume and inter-set resting interval presented in this brief report are based on scientific evidence and represent an important starting point. However, it is imperative to establish more specific guidelines for youth athletes, encompassing other less investigated parameters, such as ideal warm-up, movement velocity, exercise selection, and order.