School-Based Circuit Training Intervention Improves Local Muscular Endurance in Primary School Students: A Randomized Controlled Trial

d to explore the effectiveness of a 12-week circuit training program in improving local muscular endurance in normal-weighted primary school students. Methods: The study involved a parallel-group randomized trial with 606 primary school boys assigned to an experimental or a control group. The participants underwent a 12-week circuit training program that involved multi-joint, total-body workouts using body weight, resistance bands, and medicine ball exercises. The study measured the participants' local muscular endurance during sit-ups, dynamic trunk extensions on a Roman bench (DTE), and push-ups. Results: After adjusting for

trial with 606 primary school boys assigned to an experimental or a control group. The participants underwent a 12-week circuit training program that involved multi-joint, total-body workouts using body weight, resistance bands, and medicine ball exercises. The study measured the participants' local muscular endurance during sit-ups, dynamic trunk extensions on a Roman bench (DTE), and push-ups. Results: After adjusting for the baseline, the treatment–grade interaction was signi cant for sit-ups (F= 7.74,p< 0.001, hp 2= 0.04), DTE (F= 6.49,p< 0.001,hp 2= 0.03), and push-ups (F= 9.22,p< 0.001,hp 2= 0.05), where the experimental treatment was more bene cial than the control. The treatment effect seemed to vary depending on the individual's baseline local muscle endurance capacity. As the baseline local muscular endurance values increased, the treatment and grade effects became less bene cial. Conclusion: A 12-week circuit training program involving body weight, resistance bands, and medicine ball exercises suits school-based programs and can improve local muscular endurance in normal-weighted primary school boys. The experimental treatment was more effective than the control, and the individual baseline muscular endurance should be considered when designing training programs. Keywords:circuit training; local muscular endurance; primary school students 1. Introduction Physical education is a crucial part of every school's curriculum, as it has the potential to provide students with physical tness, health, and social interaction skills [1]. However, there is uncertainty surrounding the amount of physical activity required to achieve these bene ts and whether modern physical education can ful ll these requirements. Research suggests that school-based physical activity should be extended to at least 60 min per day [2,3], which exceeds the current standard of 3 45 min physical education classes in public schools in Serbia. Moreover, limited time, a lack of resources, unsupportive settings, and con icting priorities are common obstacles that physical education teachers Children2023,10, 726.

Children2023,10, 726 2 of 17 face in delivering effective and engaging classes [4]. These organizational issues may reduce students' physical activity levels, adversely affecting their muscular tness. Thus, exploring strategies to address these challenges and promoting physical activity among students during physical education classes is essential. In addition to promoting overall physical activity, it is also essential for physical edu- cation programs to focus on developing speci c aspects of physical tness. Smith et al. [5] argue that cardiorespiratory tness, exibility, strength, local muscular endurance, and body composition are all key factors that strongly in uence health outcomes. Speci cally, building muscular endurance can result in a more resilient musculoskeletal system, re- ducing sports-related injuries, and improving overall health-related tness [5]. However, despite the existence of comprehensive school-based programs in Serbia designed to im- prove various aspects of physical tness [6], there is a concern that the current physical education curriculum may not place enough emphasis on abdominal musculature develop- ment [7,8]. Therefore, incorporating exercises involving rotational and diagonal patterns, using simple and inexpensive equipment like resistance bands and medicine balls, could be an effective strategy to enhance physical education programs in Serbia and to promote better muscular endurance in students. A possible solution is to introduce a circuit training program targeting muscular endurance within physical education classes that, considering the limited resources, could increase students' physical activity and improve overall local muscular endurance [9–12]. Faigenbaum, Kraemer, Blimkie, Jeffreys, Micheli, Nitka, and Rowland [9] suggest that bodyweight circuit training can be bene cial even if the equipment is unavailable, and in addition to enhancing muscular strength, power, and endurance, frequent involvement in a youth resistance training program can alter various other tness-related variables [13]. Likewise, resistance training may positively affect some anatomical and psychological characteristics, prevent injuries in sports and leisure activities, and enhance motor abilities and athletic performance. Therefore, circuit training emphasizing muscular endurance seems advantageous as it provides various bene ts. It is advised that the focus is placed on working sets with more than 12 repetitions when developing programs to increase local muscle endurance [14]. When recommending

some anatomical and psychological characteristics, prevent injuries in sports and leisure activities, and enhance motor abilities and athletic performance. Therefore, circuit training emphasizing muscular endurance seems advantageous as it provides various bene ts. It is advised that the focus is placed on working sets with more than 12 repetitions when developing programs to increase local muscle endurance [14]. When recommending an appropriate training intensity for children and adolescents, setting a repetition range between 10 and 15 and controlling the training load to maintain the intended intensity seems reasonable [8]. However, it is essential to note that resistance training should not be the sole focus of a physical education program. Faigenbaum, Kraemer, Blimkie, Jeffreys, Micheli, Nitka, and Rowland [9] recommend integrating team sports, swimming, distance running, cycling, and strength training in children's and adolescents' training programs. Therefore, local muscular endurance training programs should be time ef cient to provide enough time for other physical education curriculum contents. Strength training interventions for children and adolescents have been the subject of several studies to determine their effectiveness in improving muscular tness and perceived physical competence. Faigenbaum et al. [15] compared the effects of low-repetition heavy- load and high-repetition moderate-load resistance training on children's muscular strength and endurance development. The study showed that both interventions improved leg extension strength and endurance compared to the control group, but the high-repetition moderate-load training program was more effective in improving leg extension muscular endurance and chest press strength and endurance. Recently, Villa-Gonz¡lez et al. [16] con- ducted a systematic review and meta-analysis to examine the effectiveness of school-based exercise interventions for enhancing children's muscular tness. The ndings indicated that exercise interventions, predominantly combined interventions targeting selected domains of muscular tness, were associated with signi cant moderate increases in local muscular endurance, muscular strength, and muscular power, with more signi cant effects observed with three sessions per week. Lastly, Zhao et al. [17] implemented a comprehensive strength training program for Chinese male adolescents to examine its effects on muscular tness and perceived physical competence. The comprehensive strength training intervention

in local muscular endurance, muscular strength, and muscular power, with more signi cant effects observed with three sessions per week. Lastly, Zhao et al. [17] implemented a comprehensive strength training program for Chinese male adolescents to examine its effects on muscular tness and perceived physical competence. The comprehensive strength training intervention

Children2023,10, 726 3 of 17 improved their muscular tness, notably lowered their extremity muscle power and ab- dominal core endurance, and increased their perceived physical competence, according to the study. According to the studies mentioned above, strength training programs, especially those used in educational settings, can enhance children's and teenagers' perceived physical competence, as well as their muscle tness. The studies also emphasize how crucial it is to take into account the type of resistance training intervention being utilized, as higher- repetition moderate-load regimens may be more successful in enhancing muscle endurance. We adhered to these recommendations while designing speci c circuit training programs to improve local muscular endurance. The bene t of circuit training programs involving body weight, resistance bands, and medicine ball exercises in enhancing local muscular endurance in elementary school pupils, however, is not supported by suf cient research. While previous studies incorporated school-based circuit training to improve local muscular endurance, there is insuf cient evidence on how individuals' baseline local mus- cular endurance tness may affect the effectiveness of the applied treatment. Therefore, this study considered the individual baseline local muscular endurance capacity when exploring the effects of the applied treatment, which could provide valuable information for practitioners and educators designing school-based physical education programs. Ad- ditionally, this study may contribute to the literature by investigating the effectiveness of a circuit training program that uses body weight, resistance bands, and medicine ball exercises, speci cally for school-based programs, in improving local muscular endurance in primary school boys, which does not interfere with the other contents of the PE cur- riculum. The main idea was to include local muscular endurance training in the rst 15 min of a PE class to produce signi cant effects, which aligns with the previous study byFaigenbaum et al. [18] . Therefore, creating a strength training program that relied on limited resources and was space and time ef cient, without interfering with the prescribed contents of the physical education curriculum, was desirable. Based on the aforementioned, the purpose of this study was to determine whether a 12-week circuit training program that uses

with the previous study byFaigenbaum et al. [18] . Therefore, creating a strength training program that relied on limited resources and was space and time ef cient, without interfering with the prescribed contents of the physical education curriculum, was desirable. Based on the aforementioned, the purpose of this study was to determine whether a 12-week circuit training program that uses body weight, resistance bands, and medicine ball exercises, designed especially for school-based programs, can increase local muscular endurance in primary school boys who are of normal weight, while taking into account each participant's individual baseline local muscular endurance capacity. Additionally, the study aims to provide valuable information for practitioners and educators designing school-based physical education programs and contribute to the literature on the effec- tiveness of school-based exercise interventions for enhancing primary school students' muscular tness. 2. Materials and Methods 2.1. Study Design This parallel-group study compared the effects of circuit training with the conventional physical education curriculum (allocation ratio, 1:1). For our study in urban school settings, we selected four elementary schools from Belgrade and performed both the experimental and control treatments, as well as the outcome measurements, in school gymnasiums. Normal-weighted boys aged 11–14 followed one of the treatments, during the rst school semester from October to December. After 12 weeks of treatment, the participants un- derwent a nal measurement to assess whether any improvements had emerged to the response variables (sit-ups, DTE, and push-ups). Our study was approved by the Faculty of Sport and Physical Education's ethical council and was carried out using experimental methods that adhered to the Helsinki Declaration's ethical norms. Before conducting the baseline measurements, parental/legal guardian written consent forms were collected. No further modi cations to the design emerged following the study's commencement.

Children2023,10, 726 4 of 17 2.2. Participants Six hundred and fty primary school boys participated in this parallel-group ran- domized trial. The study inclusion criteria were: (1) elementary school boys aged 11–14, (2) BMI values within acceptable normal ranges for the age indicated in the classi cation proposed by Onis et al. [19], (3) they did not suffer from chronic diseases, and (4) they were not involved in any organized form of physical activity for at least three months prior to the start of the trial. Participants with BMI values outside of acceptable normal ranges, respiratory and cardiovascular diseases, developmental disabilities, students who were recovering from an injury or illness based on a recent medical diagnosis and receiving prescribed medical treatment, and participants who were engaged in organized physical activity at the time of the randomization were all excluded. We excluded participants who did not adhere to the experimental and control protocols 85%. A purpose-designed questionnaire evaluated the eligibility criteria for inclusion in the study, while Figure shows the student selection process.Children 2023, 10, x FOR PEER REVIEW 4 of 18 collected. No further modifications to the design emerged following the study’s com- mencement. 2.2. Participants Six hundred and fifty primary school boys participated in this parallel-group ran- domized trial. The study inclusion criteria were: (1) elementary school boys aged 11–14, (2) BMI values within acceptable normal ranges for the age indicated in the classification proposed by Onis et al. [19], (3) they did not suffer from chronic diseases, and (4) they were not involved in any organized form of physical activity for at least three months prior to the start of the trial. Participants with BMI values outside of acceptable normal ranges, respiratory and cardiovascular diseases, developmental disabilities, students who were recovering from an injury or illness based on a recent medical diagnosis and receiv- ing prescribed medical treatment, and participants who were engaged in organized phys- ical activity at the time of the randomization were all excluded. We excluded participants who did not adhere to the experimental and control protocols ≥ 85%. A purpose-designed questionnaire evaluated the eligibility

who were recovering from an injury or illness based on a recent medical diagnosis and receiv- ing prescribed medical treatment, and participants who were engaged in organized phys- ical activity at the time of the randomization were all excluded. We excluded participants who did not adhere to the experimental and control protocols ≥ 85%. A purpose-designed questionnaire evaluated the eligibility criteria for inclusion in the study, while Figure 1 shows the student selection process. Figure 1. Flow chart diagram of subjects included in the study, randomization, and analysis. 2.3. Physical Education Group Interventions The total duration of the program was 12 weeks, with a frequency of 3 45 min phys- ical education classes (total number of sessions = 36). The structure of the physical educa- tion classes consisted of the preparatory (15 min), main (25 min), and cooldown phase (5 min). Each physical education class began with a 10 min low-intensity warm-up period with brisk walking and light running alternations, followed by 5 min of whole-body mo- bility exercises. The main part of the classes contained elements of various sports Figure 1.Flow chart diagram of subjects included in the study, randomization, and analysis.

Children2023,10, 726 5 of 17 2.3. Physical Education Group Interventions The total duration of the program was 12 weeks, with a frequency of 3 45 min physical education classes (total number of sessions = 36). The structure of the physical education classes consisted of the preparatory (15 min), main (25 min), and cooldown phase (5 min). Each physical education class began with a 10 min low-intensity warm-up period with brisk walking and light running alternations, followed by 5 min of whole-body mobility exercises. The main part of the classes contained elements of various sports (athletics, gymnastics, swimming, futsal, basketball, volleyball, and handball) and exercises designed to improve strength, muscular and cardiovascular endurance, speed, agility, stability, exibility, and coordination. We should note that common strength-based strategies in regular PE curriculums in Serbia primarily consist of traditional exercises like push-ups, sit-ups, trunk extensions, and squats, which could ultimately contribute to the overall effectiveness of the PE curriculum on local muscular endurance. The only difference was that higher grades had more complex technical elements than lower grades. In general, we adopted the program of the Ministry of Education of the Republic of Serbia. 2.4. Circuit Training Group Interventions We integrated the circuit training program into the main part of the physical education class and implemented three weekly resistance workouts according to the recommendation for novices, since the rest days between the sessions allowed for adequate recovery [10–12]. We adopted circuit training due to the limited time and the substantial number of par- ticipants in physical education classes, to keep the class active. Therefore, we implemented circuit training where the order of exercises topographically alternated between upper- and lower-body exercises, which allowed the individuals to recuperate more adequately between sets. This approach proved advantageous for untrained individuals who may have found it too taxing to perform multiple upper- or lower-body exercises consecutively. Additionally, this method of exercise arrangement decreased the duration of the rest periods between the exercises and maximized the rest between body areas [14]. This study's local muscular endurance training program had brief rest periods, no more than 45 s, followed by

for untrained individuals who may have found it too taxing to perform multiple upper- or lower-body exercises consecutively. Additionally, this method of exercise arrangement decreased the duration of the rest periods between the exercises and maximized the rest between body areas [14]. This study's local muscular endurance training program had brief rest periods, no more than 45 s, followed by an optimal number of repetitions performed with the participant's own body weight or light loads. In addition, due to the lack of sophisticated equipment, we used the rate of perceived exertion for children scale to control the targeted session intensity [20]. We were fully aware of the advantages and disadvantages of the circuit training ap- proach before determining the exercise plan. For instance, since bodyweight resistance training is limited to the individual's body weight, the intensity might alter when increasing the number of repetitions or changing the movement pattern. Therefore, we implemented effort intervals of no less than 30 s. The advantage of such a speci c approach is that the teacher might manipulate the load by adjusting the effort interval and exercise tempo. How- ever, from a practical standpoint, we were aware that some individuals could not perform exercises in a given repetition range due to a lack of tness or other common restrictions, like a range of motion issues, that could impair correct exercise posture. Therefore, if the individual could not perform at least 12 repetitions of a given exercise while maintaining the correct exercise posture, a more accessible alternative (modi ed exercise) or assistance was implemented. Each exercise contained three sets, and the adopted exercise tempo was 1-0-1-0 with some exceptions (weeks 9–11), where the exercise tempo was 2-0-2-0. The reason for such alteration was to maintain an optimal number of repetitions per set as the work interval rises and to employ a slightly different stimulus. Additionally, the participants were allowed to rest brie y before continuing the set to maintain the proper exercise posture. To ef ciently stress the skeletal muscle system, the participant was required to maintain a stable body position that permitted a safe

an optimal number of repetitions per set as the work interval rises and to employ a slightly different stimulus. Additionally, the participants were allowed to rest brie y before continuing the set to maintain the proper exercise posture. To ef ciently stress the skeletal muscle system, the participant was required to maintain a stable body position that permitted a safe and correct body alignment throughout the activity. During unsupported ground-based workouts, the feet were broader than shoulder- width apart. The experimental program primarily consisted of exercises to isolate speci c abdominal muscles and large muscle groups. For example, students performed prone,

A 12-week circuit training program improved local muscular endurance in primary school boys.