To systematically synthesize existing evidence, several types of reviews are employed based on the purpose (25), such as systematic reviews or scoping reviews (25). We conducted a scoping review based on the aim of this study. This scoping review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses for Scoping Reviews (26) and followed the methodological framework of Arksey and O’Mallay (27).

The literature search was conducted in November 2024 using three electronic databases: PubMed, Scopus, and Web of Science. The “Population, Concept, and Context” approach was used to design the eligibility criteria, referring to children and adolescents (<18 years old at baseline) without diseases, any intervention of RT or exposure to any sports activity, and evaluation of muscle morphology. Database searches were conducted using a combination of terms such as “child*”, “preadolescen*”, “adolescen*”, “junior”, “resistance”, “exercise”, “sport*”, “cross-sectional area”, “muscle thickness”, “muscle hypertrophy”, and “muscle morphology”. These terms are provided as examples and do not represent the full list of search keywords used. The full search strategy for each database is shown in the Supplementary Tables.

Among studies identified in the literature search, duplicates were removed using EndNote (Endnote 20.6; Clarivate Analytics, PA, USA). In the first screening step, eligible studies were independently selected by two authors based on titles and abstracts. In the second screening step, eligible studies were independently selected by two authors based on their full text. After the completion of each step, the authors discussed discrepancies in decisions, which were resolved through consensus.

Eligibility criteria to screen studies in this study were as follows: (1) published in peer-reviewed journals, (2) included healthy or typically developing children, (3) studies on humans, (4) reported muscle morphology as an outcome, (5) included children aged <18 years old at baseline, (6) used chronic RT or combinations of RT with other training or investigated the effects of sports participation, (7) had a control group to demonstrate the effects of RT or sports participation, and (8) were written in English. No date restriction was imposed for the search. Studies were excluded if they (1) involved only children with diseases, (2) reported only body mass index or circumference or fat-free mass as indicators of muscle mass or size, and (3) were review articles.

Microsoft Excel was used to calculate the descriptive statistics of the data extracted from the included articles, which were summarized and grouped based on categories.

Two authors developed a data-charting form to address the purpose of this study. The data were extracted and then charted using Microsoft Excel. The following information was collected from all included studies: publication year, country of the first author, study design, sample size, sex, age, biological age, competition history and level of participants, targeted muscle, measurement equipment, whether an a priori power analysis was conducted, and outcomes related to muscle morphology (i.e., muscle thickness, CSA, volume, fascicle length, and pennation angle). For interventional studies, the following information was additionally collected: intensity, frequency, duration, and volume of the training program. The following information was additionally collected for observational studies: number of measurements and study duration.

The included studies were conducted in Japan, Germany, Spain, Australia, the United States, Canada, Finland, France, the United Kingdom, and Colombia (eight, six, four, three, two, two, one, one, one, and one articles, respectively) (Table 1). We included articles published in the 1990s or later, with 5, 2, 16, and 6 studies conducted in the 1990s, 2000s, 2010s, and 2020s, respectively. The study design included 20 observational studies (17 cross-sectional and 3 prospective studies) and 9 interventional studies (Table 1, Figure 2). Among the studies, 13, 7, and 8 included male, female, and both sexes as participants, respectively; one study (50) did not report the sex of the subjects (Figure 3). The age ranges were 6–11, 12–14, and 15–17 years in 12, 10, and 10 studies, respectively (Figure 3). Six studies performed a power analysis (34–36, 43, 45, 46).

Biological age was assessed in 20 articles, including one (36) that did not report results (Table 1). Assessments of maturity status included Tanner stage (15 articles), predicted peak height velocity (two articles), Tanner-Whitehouse II methods (two articles), and percentage predicted adult height (one article) (Table 1). Muscle morphology was assessed using magnetic resonance imaging (13 articles), ultrasonography (14 articles), and computed tomography (3 articles) (Figure 4). The measured variables were muscle volume, CSA, thickness, fascicle length, and pennation angle. The evaluation targets of the muscles for size are listed in Table 2.

Total sample sizes ranged from 12 to 184 (Table 1). Targeted activities included middle-distance running, weightlifting, soccer, volleyball, rowing, karate, sumo, sprinting, throwing, jumping, swimming, tennis, gymnastics, and triathlon. Four studies combined players from various sports. Four articles focused on the sex differences in muscle morphology due to participation in sports (33, 34, 38, 41). Two articles focused on age differences in muscle morphology due to participation in sports and recruited middle- and late-adolescent boys (20, 30).

Regarding prospective studies, the total sample sizes ranged from 32 to 38. Two studies recruited males, and one study recruited participants of both sexes. The durations of study were 6 (37), 9 (28), and 12 (46) months. One article assessed the outcome measures more than three times, at three-month intervals (46).

The designs of interventional studies included eight quasi-experimental parallel-group trials and a quasi-experimental crossover trial (Table 1, Figure 2). No study followed the CONSORT guidelines for conducting randomized controlled trials (RCTs). The sample size per group ranged from 6 to 58. The age ranges were 6–11, 12–14, and 15–17 years in 4, 3, and 2 studies, respectively. Among the nine studies, five involved participants with no athletic background (18, 19, 31, 32, 35), two involved participants engaged in extracurricular activities at a regional competitive level (51, 52), one involved surfing athletes (50), and one involved gymnasts (43). The intervention periods were from 5 weeks to 10 months. The activities of the intervention group included RT, multiple exercise and sports, gymnastics, plyometrics, and neuromuscular exercise (Table 3). The frequency of intervention was two to six times per week. One article focused on the differences in training effects between sexes or grades in school (19). Volume was only evaluated for the total thigh muscle group (Table 2).

This study included interventional and observational studies. Nine studies examined the effects of RT or sports activities compared with a control group. The designs of these studies included eight quasi-experimental parallel-group trials and a quasi-experimental crossover trial. However, none of these studies followed the CONSORT guidelines for RCTs, which are the primary standard for determining relationships between interventions and outcomes. In studies involving adults, numerous RCTs have been conducted on the effects of RT on muscle hypertrophy (53), and systematic reviews synthesizing these findings have contributed to a growing body of high-level evidence on the effects of RT on muscle morphology (54). Our findings not only emphasize the scarcity of studies on children and adolescents but also highlight the low quality of existing evidence compared with studies on adults. In contrast to research involving adult populations, interventions are often implemented at the class level during regular classes of children and adolescents, making it difficult to randomly assign individual participants. In this review, most of the interventional studies stated that they conducted interventions in school settings (31, 32, 35, 51, 52). Moreover, the studies included in this review were non-RCTs with relatively small sample sizes, precluding the implementation of intention-to-treat analysis. By comparison, both RCTs (55) and cluster RCTs (56) have been implemented as part of efforts to strengthen the evidence in other research fields. Future studies should consider adopting designs such as that of cluster RCTs, for example by utilizing after-school programs or community-based settings as intervention platforms.

Additionally, 20 studies examined the effects of sports participation, including 17 cross-sectional studies and three prospective observational studies. Time-course data on muscle morphological adaptations provide valuable insights into the timing and induction period of adaptations. Specifically, Pentidis et al. (46) assessed the muscle morphology in preadolescent gymnasts and untrained peers at three-month intervals over a year, providing time-course data on muscle morphological adaptations during this age group. However, due to the design limitations of other studies, a time-course analysis was not feasible.

Among all studies included herein, only 20.7% determined their sample size using an a priori power analysis [three cross-sectional studies (34, 36, 45), one longitudinal study (46), and two interventional studies (35, 43)]. An a priori power analysis is a critical procedure for sample size determination (57). This scoping review highlights the importance of building evidence in this field using appropriately determined sample sizes.

In addition to study design and methodological rigor, the selection of the study population is also essential. The following distribution was obtained after categorizing the included studies according to participant age: aged 6–11 years, 12 articles; aged 12–14 years, 10 articles; and aged 15–17 years, 10 articles. Additionally, 34.5% of the studies did not report biological age. Testosterone (58) or insulin-like growth factor-1 (59), which influences muscle morphological adaptations, fluctuates across developmental stages. Therefore, understanding how muscles adapt to RT and sports participation at different ages and developmental stages is essential. A suitable approach for investigating age- or growth-related differences in adaptation involves designing experiments that include multiple age groups or biological ages as independent variables. This approach enables the examination of how muscle morphology adapts to the same training or sports participation across different ages or biological age groups within the same study. Our findings revealed that only two observational studies (20, 30) and one interventional study (19) adopted this experimental design. Furthermore, studies that target groups subdivided by biological maturation (e.g., early, mid, and late adolescence) would be valuable. Such study designs could provide more detailed information on muscle morphological adaptations across biological maturation.

Sex differences exist in the hormonal regulation of muscle morphology and hypertrophy (59). Extensive research has been conducted on how sex differences influence the effects of RT on muscle morphology in adults (60, 61). Among the included studies, the sex distribution was as follows: male, 13 articles; female, seven articles; both, eight articles; not listed, one article. Notably, only four observational studies (33, 34, 38, 41) and one interventional study (19) designed experiments using sex as an independent variable. For example, Gomez-Bruton et al. (34) compared the muscle CSA between young male and female swimmers. To gain a deeper understanding of muscle morphological adaptations to exercise stimuli and develop appropriate training programs, future studies should more thoroughly assess sex differences in these adaptations. Moreover, it should be noted that the number of studies involving female participants was approximately half that of those involving males (Figure 3).

Several studies have investigated the effects of training parameters (intensity, frequency, duration, and volume) of RT on hypertrophic outcomes in adults (16, 62). Among the nine interventional studies included herein, the intervention duration ranged from 5 weeks to 10 months, with 77.8% implementing interventions lasting 12 weeks or less. The total number of interventions ranged from 14 to 60. Regarding load, various methods were employed; for example, body-mass based squat and isometric training of elbow flexion. However, none of the included interventional studies examined the influence of these factors on muscle hypertrophy. While training intensity, frequency, duration, and volume are critical components in the designing of RT programs, current evidence on muscle hypertrophy in children and adolescents remains insufficient to evaluate their specific effects.

Across the included studies, 14 different sports were analyzed for their effects on muscle morphology. Four studies combined players from various sports (Table 1). Although participants in each study had engaged in the target sports for a certain period, there were variations in the participants’ reported competitive levels. Both Hoshikawa et al. (39) and Giraldo García et al. (33) required junior soccer players as participants. In the study by Hoshikawa et al. (39), participants took part in regional and national junior competitive meets during the research period. In contrast, Giraldo García et al. (33) did not report the competitive level of their participants. Competitive level may be associated with the nature of training, which can, in turn, influence muscle morphological adaptations. Therefore, future studies should provide as much detail as possible regarding participants’ competitive level to better understand how sports participation affects muscle morphology in children and adolescents.

Not only the type of activity but also the targeted muscles and measurement methods can influence the morphological outcomes of RT and sports participation. The included studies examined the size of 47 different muscles or muscle groups (Table 2). Our results identified unexplored muscles and measurement methods. For example, the included studies did not examine the volume of lower leg muscles, which play a crucial role in human locomotion—such as walking and running—as well as in sports activities. Previous studies on adults have reported that muscle adaptations to RT (63) and sports participation (64) vary according to muscle. Abe et al. (63) reported that upper-body muscle thickness increased more rapidly and to a greater extent than lower-extremity muscle thickness. Although these findings have not been corroborated in children and adolescents, if similar heterogeneity exists in muscle adaptations, the choice of the target muscle could alter the results of muscle size adaptations. Future studies should determine whether morphological adaptations vary across the muscles.

Regarding the number of studies classified according to the measurement method, those assessing muscle volume were the fewest (Figure 4). Measuring muscle volume requires a relatively greater number of slices compared with measuring CSA or thickness, which may have limited growth in the number of such studies. However, for example, muscle volume has been shown to be more appropriate than anatomical CSA for evaluating the size–strength relationship (65), highlighting the importance of measuring muscle volume. Future research should therefore assess muscle volume to better understand the effects of RT and sports participation in children and adolescents.

This review has practical implications. First, we identified relatively fewer interventional studies than observational studies, indicating that information on the effects of RT on muscle morphology in children and adolescents remains limited for coaches, physical education teachers, and strength and conditioning professionals. Second, as discussed above, the relationships between RT and sports participation and muscle morphological adaptation in children and adolescents can be influenced by various factors such as developmental stage, sex, and the specific muscles targeted. These factors varied greatly across the studies included in this review. Moreover, in the included interventional studies, training parameters, such as intensity, frequency, duration, and volume, also varied considerably. Therefore, when those engaged in coaching and physical education attempt to apply existing evidence in practice, careful attention should be paid not only to the results and conclusions, but also to how these influential factors were defined in the original studies, as they may affect muscle morphological outcomes.

This review only included studies that directly measured muscle morphology, excluding articles that reported body mass index or circumference or fat-free mass as indicators of muscle mass or size. This should be considered when interpreting the results of the present study. Moreover, we only included studies published in English. Consequently, the scope of the collected studies and regions in which the research was conducted may have been biased.