Abstract
Objective
Rapid bone development in growing children causes excessive tension in the lower extremities’ muscles and tendons, leading to reduced flexibility and increased musculoskeletal disorder risk. Further, lack of exercise causes obesity. Therefore, we created a stretching exercise protocol to prevent musculoskeletal disorders in elementary school (middle and upper grades) children during their growth period, when rapid bone development begins.
Patients and Methods
We examined the effects on pain, injury, and flexibility. Fifty-three (boys: 34, girls: 19) students in grades 3–5 (ages 9–11) performed the stretching exercises at school thrice a week for one year, and we compared the results before and a year after the intervention.
Results
A three-minute stretching exercise routine achieved an intensity of 4.6–4.9 metabolic equivalents (METs; equivalent to brisk walking). Obesity (P=1.000), flexibility problems (inability to bend forward [P=0.754] or squat problems [P=1.000]), bone/joint pain (P=1.000), and injury (P=1.000) did not significantly increase.
Conclusion
Stretching exercises during the growth period may help prevent childhood musculoskeletal disorders, obesity, and flexibility loss.
Keywords: dynamic stretching exercises, growth spurt, flexibility, musculoskeletal disorder
Introduction
During a child’s growth period, their musculoskeletal system is underdeveloped, and their epiphyses are fragile and prone to osteotomy1). During growth spurts, the bones of the lower extremities grow rapidly in the longitudinal direction. As a result, the muscles and tendons that attach to the bones become excessively tense, which may reduce flexibility and cause musculoskeletal disorders2). Since 2014, musculoskeletal examinations have been conducted on 380 elementary and junior high school students to investigate bone and joint pain, musculoskeletal disorders, and flexibility during growth3, 4). As a result, 10–20% of elementary school students and 30–40% of junior high school students had bone and joint pain. Osgood–Schlatter’s disease, which is attracting attention as a musculoskeletal disorder in the growth period, was found in 4% of elementary school students and 4.8% of junior high school students. As for flexibility, the number of children unable to squat was found in 10–20% of 4th and 5th graders. Most children unable to bend forward were 5th and 6th graders, with an incidence of 20–30%. Flexibility was lower in elementary school students than in junior high school students, consistent with other surveys in Japan5). Among children in athletic clubs, the proportion of children with low flexibility was even higher6). Considering this problem, interventions to improve flexibility from the elementary school age can prevent bone/joint pain and musculoskeletal disorders.
Stretching exercises are effective in preventing loss of flexibility and reducing the rate of musculoskeletal pain as well as disorders. In particular, for athletes, flexibility is directly related to their exercise performance; thus, stretching programs have been studied in detail according to the type of sports, such as soccer, tennis, football, and judo. Exercises involving explosive ankle strength improved the jumping power of soccer players, dynamic stretching exercises improved the serve action of tennis players, and static stretching exercises reduced the blood lactic acid level of judo players. Such exercises reportedly lead to fatigue recovery and muscle strength maintenance7,8,9,10,11). Furthermore, for children in general, static stretching exercises during warm-up reportedly affect their long-term flexibility, and dynamic stretching exercises during warm-up acutely affect their fitness performance. Several studies have reported on these findings12,13,14), but opinions are mixed. Many researchers are working on developing effective exercise protocols, but developing a muscle stretching exercise program is difficult because many exercise variation possibilities exist depending on how posture and movement are combined15, 16). Our previous research demonstrated that all children, not just athletes, are at risk of developing musculoskeletal pain and disability due to growth-related loss of flexibility during the growth period in elementary school3). Therefore, we have created a program targeted at growing children who are said to have a decline in the flexibility of their lower limbs. In recent years, obesity among children has increased because of a decrease in outdoor play, an increase in indoor playtime, and lack of exercise. Obesity has been associated with decreased flexibility in growing children and the occurrence of locomotive syndrome17, 18). Sports programs in schools were reported to reduce the incidence of musculoskeletal disorders and increase flexibility19, 20). Thus, such programs can have a good effect on preventing illness and maintaining exercise habits.
Accordingly, we made the following hypothesis: relieving excessive tension in muscles and tendons, which occur during growth, and implementing a stretching exercise program will help prevent obesity and the loss of flexibility by maintaining exercise habits. This program can contribute to preventing musculoskeletal pain and disorders.
Therefore, we conducted a one-year stretching exercise program designed to suit the characteristics of the growth period for children in the middle and upper grades of elementary school, where bone development begins rapidly. The purpose of this study was to examine the safety of exercise and the possibility of its future use by observing the status of pain, disability, flexibility, and obesity.
Patients and Methods
Creating a stretching exercise protocol for the growth period
We discussed stretching programs for athletes with physical therapists and sports trainers7,8,9,10,11). To avoid focusing on specific exercise, we referred to programs developed for school children19, 20). Finally, we developed exercises that involve flexion, adduction, and abduction of the hip, knee, ankle, shoulder, and elbow joints. In this study, we wanted to develop an exercise program that could contribute to preventing obesity and musculoskeletal disorders while maintaining exercise habits in growing children. We created a dynamic stretching exercise routine based on the opinion of a physical education teacher who teaches direct stretching exercises to children. The teacher stated that static stretching exercises can lengthen muscles but are not enjoyable. Music was added to the routine, and the time was set to approximately three minutes so the children could have fun while stretching. The following 13 movements were involved: 1. Arm lifting (shoulder stretching), 2. Up and down movement of the arm (stretching around the shoulder blade), 3. Forward and backward arm movement (stretching around the shoulder blades), 4. Bending and stretching of the elbow (upper arm stretching), 5. Rotation of the shoulder (stretching around the shoulder blade), 6. Raising the leg (stretching the back of the thigh), 7. Lifting the thigh (stretching the glutes), 8. Lifting the heel (stretching the front of the thigh), 9. Squatting (lower body trunk exercise), 10. Single leg lifting (hip stretching and single leg balance), 11. Twisting or rotating of the waist (coordinated movement of the whole body), 12. High five and skipping (coordinated movement of the entire body), and 13. Moving forward and backward while holding hands with the child next to you (coordinated movement, contact). Figure 1 shows an excerpt of the stretching exercises.
Figure 1.
Stretching exercises (excerpt). A: Stretching around the scapula, B: Elbow flexion and extension, C: Stretching the posterior thigh, D: Stretching the gluteal muscles, E: Coordinated exercise.
Survey before and after the stretching exercises
Participants. A longitudinal survey was conducted from October 2018 to October 2019 on Japanese elementary school students in grades 3–5 (9–11 years old at the start of the survey). The peak age of growth for Japanese children is 11–13 years for boys and 9–11 years for girls21), which are the ages we set for this study. We consulted with the organization overseeing elementary schools and selected schools with physical education teachers who could continue instructing children on stretching exercises. We excluded children who did not want to participate and children undergoing bone/joint treatment. Children and parents were informed verbally and in writing about the voluntary nature of participation, protection of privacy, and publication of results; consent was obtained from those who agreed to participate. After conducting preliminary research on the musculoskeletal system in elementary school students, we compared the results of previous studies with those of the elementary school students who performed the stretching exercises. Because obesity is included in Japan’s School Health Statistics report21) every year, we included it in our comparison.
Instrumentation. The children performed stretching exercises thrice a week just before the start of physical education classes and continued doing them for one year. Body size (height, weight) was measured in school every three months, and data were used with permission from the participants, their parents, and the school. Based on the data, height increase, weight gain, and body mass index (BMI) were calculated. In addition, muscle mass, body fat percentage, and estimated bone mass were measured using a body composition analyzer (MC-190® multi-frequency body composition analyzer manufactured by TANITA, Tokyo, Japan). When assessing body composition, children’s height was measured in a standing position, and the results were entered. Additionally, flexibility (whether the children could bend forward and squat) was assessed. A standard checklist used in Japanese schools was used to assess flexibility, and the results were evaluated by two researchers. Forward bending is evaluated in two ways: standing forward bending and sitting forward bending. Standing forward bending is widely used as an evaluation item in athletic examinations in Japanese schools. Therefore, standing forward bending was used for evaluating forward bending, and the assessment was based on whether the child’s fingertips or palms touched the floor. For squatting, the ability to squat with arms crossed in front and heels firmly on the ground was evaluated. For flexibility assessment, in addition to the author of this paper, another evaluator was assigned, and each result was confirmed to be the same. The assessment was performed by the same person each time. The intensity of the stretching exercises was measured by having the children wear an activity meter (Activity meter Calorhythm AM-142® manufactured by TANITA) when performing the exercise. Finally, the following questions were asked to assess children’s awareness of exercise: “Do you like moving your body?” and “Do you find it easy to move your body?”
Procedure. The children hung an activity meter on their upper chest using a neck strap to measure their energy expenditure. They wore T-shirts and gymnastics jerseys over them. To minimize the vibration of the activity meter, it was worn inside the gymnastics jersey jacket and sandwiched between the jacket and T-shirt. The activity meter was worn immediately before the stretching exercises and removed immediately after the exercise. Two researchers confirmed, read, and recorded the amount of energy consumed displayed on the activity meter. Energy expenditure was measured by inputting the age, sex, height, and weight of each child. After calculating each child’s metabolic equivalents (METs) based on energy expenditure, the total and average expenditures were tabulated. The following formula was used to calculate METs: energy expenditure (kcal) = 1.05 × exercise (MET × time) × body weight. Medians were calculated because they did not follow a normal distribution. In addition, a survey was conducted on attributes (grade, age, sex), exercise habits, bone/joint pain and injury, and willingness to exercise. Bone and joint pain and injuries were reported within the past month.
Data analysis
The data at the start of the stretching exercises were compared with the data obtained one year later. Age, weight, BMI, estimated bone mass, body fat percentage, amount of physical activity per week, and METs were not normally distributed. Therefore, the Wilcoxon signed-rank test was performed. A paired t-test was used because height and muscle mass were found to have a normal distribution. The number of children and their percentages regarding whether they belonged to a sports club, their flexibility, and the presence or absence of bone/joint pain or injury were also analyzed using McNemar’s test. The effect sizes were also calculated. SPSS (Ver. 29, IBM Japan, Tokyo) was used for statistical analysis. The level of significance was set at P<0.05.
Results
Students who transferred to or from school or dropped out because of illness were excluded. The remaining 53 students (34 boys, 19 girls) performed stretching exercises just before their physical education classes three times a week every week for approximately 10 months when school was in session.
Participant characteristics
Table 1 presents a comparison of the participants’ attributes, physiques, and body compositions at the start of the stretching exercise program and one year later. Height, weight, BMI, muscle mass, and estimated bone mass increased with growth in boys and girls. Body fat percentage increased in boys but decreased in girls. Approximately 70% of boys and girls belonged to a sports club when they started the program, but after one year, the proportion decreased for boys and girls. Exercise time per week also decreased after one year, but the difference between individuals was considerable. No discrepancy was observed in the physiques and sports club affiliations of the participants in this study when compared with aggregate data for Japan as a whole21), confirming consistency with the characteristics of Japanese elementary school students. Figure 2 shows changes in height and weight by sex and grade, as well as the total amount of increases in height and weight, that is, annual growth. For boys, the cumulative amount of height increased as the grade progressed, but for girls, the cumulative amount of height increase was greater in the 4th grade than in the 5th grade.
Table 1. Participant characteristics (comparison at the start of the stretching exercise program and one year later).
| At the start of the stretching exercises October 2018 |
One year later October 2019 |
P-value | ||
|---|---|---|---|---|
| Boys (n=34) | ||||
| Age (years)*1 | 9.0 (9.0, 10.0) | 10.0 (10.0, 11.0) | <0.001* | |
| Height (cm)*2 | 134.0 (7.2) | 140.1 (8.0) | <0.001* | |
| Weight (kg)*1 | 29.9 (27.8, 34.3) | 33.4 (31.5, 38.7) | <0.001* | |
| BMI (kg/m2)*1 | 16.9 (16.1, 18.7) | 17.5 (16.5, 19.2) | <0.001* | |
| Muscle mass (kg)*2 | 24.9 (3.7) | 27.8 (4.6) | <0.001* | |
| Estimated bone mass (kg)*2 | 1.3 (0.2) | 1.5 (0.3) | <0.001* | |
| Body fat percentage (%)*1 | 14.4 (11.1, 22.1) | 15.1 (11.0, 23.5) | 0.043* | |
| Belonging to a sports club*3 | 24/34 (70.6) | 23/34 (67.6) | 1.000 | |
| Exercise time per week (hours/week)*1 | 2.0 (0.8, 12.0) | 1.5 (0.0, 15.0) | 0.408 | |
| Girls (n=19) | ||||
| Age (years)*1 | 9.0 (9.0, 10.0) | 11.0 (10.0, 11.0) | <0.001* | |
| Height (cm)*2 | 139.5 (7.8) | 145.8 (6.8) | <0.001* | |
| Weight (kg)*1 | 32.5 (29.4, 45.3) | 39.9 (33.1, 55.4) | <0.001* | |
| BMI (kg/m2)*1 | 17.5 (16.2, 21.1) | 18.7 (16.8, 23.2) | <0.001* | |
| Muscle mass (kg)*2 | 26.8 (5.1) | 29.7 (5.2) | <0.001* | |
| Estimated bone mass (kg)*2 | 1.5 (0.4) | 1.8 (0.4) | <0.001* | |
| Body fat percentage (%)*1 | 20.8 (16.1, 26.1) | 24.8 (17.0, 28.6) | 0.006* | |
| Belonging to a sports club*3 | 13/19 (68.4) | 10/19 (52.6) | 0.250 | |
| Exercise time per week (hours/week)*1 | 3.0 (0.0, 3.0) | 2.0 (0.0, 6.0) | 0.127 | |
(*P<0.05)
*1Numbers represent medians (1st quartile, 3rd quartile), Wilcoxon signed-rank test
*2Numbers represent averages (standard deviations), paired t-test
*3Numbers represent the number of people (%), McNemar’s test
BMI: body mass index.
Figure 2.
Changes in height and weight by sex and grade and total height and weight gain. Numbers represent average values aggregated by grade at the start of the stretching exercise program.
Exercise intensity
Table 2 presents a comparison of the intensity of the stretching exercises and children’s obesity, flexibility, pain, and injury of musculoskeletal organs (bones and joints), as well as exercise awareness at the start of the stretching exercise program and one year later. We calculated the intensity of the stretching exercises using METs and found that it was 4.6 at the beginning and 4.9 after one year, reflecting an insignificant change and equivalence to brisk walking.
Table 2. Exercise intensity and children’s obesity, flexibility, pain and injury of musculoskeletal organs, and exercise awareness at the start of the stretching exercise program and one year later.
| (n=53) | At the start of the stretching exercisesOctober 2018 | One year laterOctober 2019 | P-value | Effect size | |
|---|---|---|---|---|---|
| Exercise intensity of the stretching exercises | |||||
| METs*1 | 4.6 (3.7, 5.3) | 4.9 (3.9, 5.8) | 0.177 | −0.19 | |
| Obesity | |||||
| BMI (kg/m2) ≥25*2 | 3/53 (5.7) | 4/53 (7.5) | 1.000 | 0.55 | |
| Flexibility | |||||
| Unable to bend forward*2 | 10/53 (18.9) | 12/53 (22.6) | 0.754 | 0.43 | |
| Unable to squat*2 | 1/53 (1.9) | 2/53 (3.8) | 1.000 | 0.80 | |
| Locomotive status within 1 month | |||||
| Bone/joint pain*2 | 2/53 (3.8) | 1/53 (1.9) | 1.000 | 0.03 | |
| Bone/joint injury*2 | 1/53 (1.9) | 1/53 (1.9) | 1.000 | 0.02 | |
| Awareness of exercise | |||||
| I like to move my body*2 | 51/53 (96.2) | 49/53 (92.5) | 0.625 | 0.32 | |
| I feel that I can move my body easily*2 | 46/53 (86.8) | 49/53 (92.5) | 0.375 | 0.52 | |
*1Numbers represent median values (1st quartile, 3rd quartile), Wilcoxon signed-rank test
*2Numbers represent the number of people (%), McNemar’s test
METs: metabolic equivalents; BMI: body mass index.
Obesity status
Our calculation of the percentage of people with a BMI of 25 or higher to assess their obesity status yielded 5.7% at the start and 7.5% after one year, showing no significant increase.
Flexibility
The percentages of children who could not bend forward owing to the lack of flexibility were 18.5% at the start and 22.6% after one year, and those who could not squat comprised 3.7% at the start and 3.8% after one year, all of which showed no significant increase.
Musculoskeletal condition
As for the musculoskeletal condition within one month, those with bone/joint pain comprised 3.8% at the start and 1.9% after one year, and those with bone/joint injuries comprised 1.9% at the start and 1.9% after one year, neither of which showed a significant increase or decrease.
Exercise awareness
The difference between exercise awareness at the start of the stretching exercises and one year later was not significant.
Discussion
The peak of growth in Japanese children reportedly occurs at 11–13 years for boys and 9–11 years for girls, and the peak growth rate is 9–11 cm/year for boys and 7–9 cm/year for girls22).
In this study, the participants’ ages at the start of the stretching exercise program ranged from 8 to 12 years, covering both growth peaks. The height increase per year was 5.5–6.6 cm/year for boys and 5.8–7.1 cm/year for girls. Some participants were believed to have passed their growth peak.
We developed a three-minute dynamic stretching exercise program for growing children. The exercise intensity was 4.6 METs at the start of the program and 4.9 METs one year later, indicating no significant change, and the intensity was equivalent to brisk walking. During the study, the children performed the stretching exercises thrice a week at the beginning of physical education classes under the guidance of a physical education teacher. The involvement of specialists is essential for performing stretching exercises correctly, and correct guidance in school can ensure the appropriate exercise intensity23).
Table 3 shows national data and results of previous studies on the prevalence of childhood obesity, pain, trauma, and flexibility in the bone and joint system as reference data3,4,5,6, 21). Obesity rates (BMI over 25) increase with age among elementary school students24), but in this study, no significant difference was observed after one year. As children grow older, they become less active25), resulting in a lack of exercise and, potentially, childhood obesity. Childhood obesity not only carries the risk of decreased flexibility and occurrence of locomotive syndrome17, 18), but also the risk of transitioning into adult obesity26). For children to continue exercising, finding ways to increase their motivation is important. Therefore, we believe that having fun while exercising with friends in school is highly meaningful.
Table 3. Reference: data from previous studies on children’s obesity, flexibility, pain and injury of musculoskeletal organs.
| Kindergarten children | Elementary school students | Junior high school students | |||||
|---|---|---|---|---|---|---|---|
| 3rd grade | 4th grade | 5th grade | 6th grade | ||||
| Obesity22) | |||||||
| Boys | 2.60% | 8.20% | 10.60% | 10.60% | 11.60% | 9.0–11.2% | |
| Girls | 2.90% | 6.90% | 7.90% | 8.20% | 8.80% | 7.4–8.5% | |
| Flexibility | - | - | - | 15.4%3) | 26.9%3) | - | |
| Unable to bend forward | 6.9%5) | - | - | 22.2%5) | 31.5%5) | 44.2%5) | |
| Unable to squat | - | 6.7%6) | 20.0%6) | 66.7%6) | 31.3%6) | 30.3%6) | |
| - | - | - | 15.4%3) | 15.4%3) | - | ||
| 5.9%5) | - | - | 21.0%5) | 19.2%5) | 16.3%5) | ||
| - | 20.0%6) | 50.0%6) | 16.4%6) | 21.7%6) | 32.3%6) | ||
| Locomotive status | - | - | - | 15.4%3) | 19.2%3) | - | |
| Bone/joint pain | - | - | - | - | - | 29.9%4) | |
| Bone/joint injury | 2.0%5) | - | - | 9.9%5) | 9.6%5) | 19.0%5) | |
| - | 13.3%6) | 20.0%6) | 0.0%6) | - | 48.5%6) | ||
| - | - | - | 3.8%3) | 15.4%3) | - | ||
| - | - | - | - | - | 14.7%4) | ||
| 7.9%5) | - | - | 14.8%5) | 24.7%5) | 43.5%5) | ||
Obesity22): Japanese school health statistics use standard body weight to calculate obesity levels. We observed obesity levels using BMI, which is widely used outside Japan, so a simple comparison cannot be made, but please refer to the percentage distribution.
Referring to previous studies3,4,5,6), the incidence of bone and joint pain and trauma increases from the upper grades of elementary school to junior high school. The rate of decline in flexibility also increases from the middle to upper elementary school grades. Yamaguchi et al. report that 7.1% of Japanese elementary and junior high school students are unable to squat27). However, in our experience, no significant changes were observed after one year. Compared with previous studies, the original proportion was lower in this study, but the proportion was maintained at a similar level after one year. Muscle and tendon development cannot keep up with the rapid development of bones during the growth period, which causes the flexibility of muscles and tendons to decrease28). In addition, during the growth period, there is an interval in which the trunk stretches remarkably, following a period in which the lower extremities stretch remarkably, and it is important to perform stretching exercises according to this growth phase29). Furthermore, 60% of sports injuries during the growth period are disorders of the knee extension mechanism, indicating the importance of preventing tightness in the quadriceps and triceps surae muscles30, 31).
No significant change was observed in exercise awareness owing to the stretching exercises, but this was influenced by the fact that more than 90% of the participants had already said, “I like moving my body” at the start of the exercise. As for the frequency of stretching exercises, a previous report observed no difference in flexibility between children who did stretching exercises three days a week and those who did them every day32). In this study, we conducted stretching exercises for elementary school students for one year and confirmed their safety. Moreover, there were no significant increases in musculoskeletal pain, disability, flexibility, or obesity before and after exercise, suggesting the possibility of its use. In the future, it will be necessary to establish a control group and examine the effectiveness of exercise.
Implications
Stretching exercises suitable for the growth period help prevent loss of flexibility, bone/joint pain and injury, and obesity in children, thereby contributing to the locomotory health of all children, not just athletes. Such exercises can improve children’s quality of life in the long term.
Limitations
The children who participated in this study comprised a combination of those who had not yet entered the growth spurt period and those who were considered to have passed it. Furthermore, the growth spurt situation was estimated from the increase in height, but by objectively evaluating bone development from images, the timing of the growth spurt could be judged more accurately, and the effect of stretching exercises could be examined. Because the timing of the growth spurt differs between boys and girls, we should expand the age range of the participants and conduct a longitudinal survey to examine changes in body composition before, during, and after the growth spurt. Analyzing and examining the characteristics of each growth stage and considering more effective stretching exercises for the respective stages are necessary.
Conclusion
The results of continuing stretching exercises tailored to the children’s growth period for one year showed no increase in obesity, flexibility, and bone/articular pain or trauma. The exercises showed no adverse effects, suggesting that stretching exercises during children’s growth spurt period may contribute to preventing musculoskeletal disorders, decreased flexibility, and obesity.
Conflict of interest
The authors have no conflicts of interest to declare.
Funding information
This study was supported by JSPS KAKENHI Grant Number JP18K02431.
Ethics approval and consent to participate
This observational epidemiology study followed the STROBE (strengthening the reporting of observational studies in epidemiology) statement. This study was reviewed and approved by the Akita University Graduate School of Medicine Ethics Committee (October 1, 2018; approval number 1648). The study was conducted in accordance with the Declaration of Helsinki and its later amendments. Children and parents were informed verbally and in writing about the voluntary nature of participation, protection of privacy, and publication of results; consent was obtained from those who agreed to participate.
Data availability statement
The data underlying this article will be shared upon reasonable request to the corresponding author.
Author contributions
M.O.: Conceptualization, Methodology, Formal analysis, Data Curation, Writing – Original Draft, Project administration, Funding acquisition, Resources, Software, Validation, Visualization, and Writing – review & editing. M.S. and Y.I.: Methodology, Formal analysis, Data Curation, and Writing – Original Draft.
Acknowledgment
We would like to thank the children, their families, and everyone in the school who was involved for their cooperation in this research.
References
- 1.Adirim TA, Cheng TL. Overview of injuries in the young athlete. Sports Med 2003; 33: 75–81. doi: 10.2165/00007256-200333010-00006 [DOI] [PubMed] [Google Scholar]
- 2.Nakase J, Aiba T, Goshima K, et al. Relationship between the skeletal maturation of the distal attachment of the patellar tendon and physical features in preadolescent male football players. Knee Surg Sports Traumatol Arthrosc 2014; 22: 195–199. doi: 10.1007/s00167-012-2353-3 [DOI] [PubMed] [Google Scholar]
- 3.Ohtaka M, Hiramoto I, Minagawa H, et al. Screening of the maturity status of the tibial tuberosity by ultrasonography in higher elementary school grade schoolchildren. Int J Environ Res Public Health 2019; 16: 2138. doi: 10.3390/ijerph16122138 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 4.Ohtaka M, Hiramoto I, Minagawa H, et al. Musculoskeletal examination of junior high school students using a diagnostic ultrasound device: investigation of Osgood–Schlatter disease and its related factors. J Phys Fit Sports Med 2009; 9: 105–113. doi: 10.7600/jpfsm.9.105 [DOI] [Google Scholar]
- 5.Shibata T. [Sports injuries in children] Prevention of sports injuries through athletic organ examinations. Jpn J Pediatrics 2015; 68: 205–212(in Japanese). [Google Scholar]
- 6.Ohtaka M, Hiramoto I, Minagawa H, et al. An initiative on musculoskeletal examinations for elementary and junior high school students: assessment of musculoskeletal disorders and flexibility. Bulletin of Graduate School of Health Sciences Akita University 2017; 25: 151–159(in Japanese). [Google Scholar]
- 7.Hammami M, Gaamouri N, Suzuki K, et al. Effects of unloaded vs. ankle-loaded plyometric training on the physical fitness of U-17 male soccer players. Int J Environ Res Public Health 2020; 17: 7877. doi: 10.3390/ijerph17217877 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 8.Gelen E, Dede M, Bingul BM, et al. Acute effects of static stretching, dynamic exercises, and high volume upper extremity plyometric activity on tennis serve performance. J Sports Sci Med 2012; 11: 600–605. [PMC free article] [PubMed] [Google Scholar]
- 9.Fernandez-Fernandez J, Ellenbecker T, Sanz-Rivas D, et al. Effects of a 6-week junior tennis conditioning program on service velocity. J Sports Sci Med 2013; 12: 232–239. [PMC free article] [PubMed] [Google Scholar]
- 10.César EP, Júnior CSR, Francisco RN. Effects of 2 intersection strategies for physical recovery in Jiu-Jitsu athletes. Int J Sports Physiol Perform 2021; 16: 585–590. doi: 10.1123/ijspp.2019-0701 [DOI] [PubMed] [Google Scholar]
- 11.Peacock JCA, Ball K. The influence of joint rigidity on impact efficiency and ball velocity in football kicking. J Biomech 2018; 71: 245–250. doi: 10.1016/j.jbiomech.2018.02.015 [DOI] [PubMed] [Google Scholar]
- 12.Coledam DHC, Arruda GA, Oliveira AR. Chronic effect of static stretching performed during warm-up on flexibility in children. Rev Bras Cineantropom Desempenho Hum 2011; 14. [Google Scholar]
- 13.Kamandulis S, Emeljanovas A, Skurvydas A. Stretching exercise volume for flexibility enhancement in secondary school children. J Sports Med Phys Fitness 2013; 53: 687–692. [PubMed] [Google Scholar]
- 14.Faigenbaum AD, Bellucci M, Bernieri A, et al. Acute effects of different warm-up protocols on fitness performance in children. J Strength Cond Res 2005; 19: 376–381. [DOI] [PubMed] [Google Scholar]
- 15.Hall PT, Crouch DL. Effect of continuous, mechanically passive, anti-gravity assistance on kinematics and muscle activity during dynamic shoulder elevation. J Biomech 2020; 103: 109685. doi: 10.1016/j.jbiomech.2020.109685 [DOI] [PubMed] [Google Scholar]
- 16.Siegel KL, Kepple TM, Stanhope SJ. Using induced accelerations to understand knee stability during gait of individuals with muscle weakness. Gait Posture 2006; 23: 435–440. doi: 10.1016/j.gaitpost.2005.05.007 [DOI] [PubMed] [Google Scholar]
- 17.Nakayama A, Nagazumi T. Relationships between lower limb flexibility and physical characteristics of growing children. Rigakuryoho Kagaku 2011; 26: 19–22 (in Japanese). [Google Scholar]
- 18.Goda A, Abiko T, Murata S, et al. Relationship between locomotive syndrome development and motor imagery ability in children. Jpn J Health Promotion and Phys Ther 2021; 10: 183–188 (in Japanese). [Google Scholar]
- 19.Foss KDB, Thomas S, Khoury JC, et al. A school-based neuromuscular training program and sport-related injury incidence: a prospective randomized controlled clinical trial. J Athl Train 2018; 53: 20–28. doi: 10.4085/1062-6050-173-16 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 20.Lo KY, Wu MC, Tung SC, et al. Association of school environment and after-school physical activity with health-related physical fitness among junior high school students in Taiwan. Int J Environ Res Public Health 2017; 14: 83. doi: 10.3390/ijerph14010083 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 21.Ministry of Health, Labour and Welfare: School health statistics for 2019. (school health statistics survey report). Available at: https://www.mext.go.jp/content/20200325-mxt_chousa01-20200325104819_1-1-1.pdf (Accessed November 2023).
- 22.Yoshii K, Tanaka T. Establishment of a longitudinal growth chart corresponding to pubertal timing. Clin Pediatr Endocrinol 2018; 27: 215–224. doi: 10.1297/cpe.27.215 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 23.Cross KM, Worrell TW. Effects of a static stretching program on the incidence of lower extremity musculotendinous strains. J Athl Train 1999; 34: 11–14. [PMC free article] [PubMed] [Google Scholar]
- 24.Namikawa K. Meta-analytical summary to identify obesity risks in schoolchildren. Bull Yamaguchi Med Sch 2004; 9: 15. [Google Scholar]
- 25.Janssen I, Leblanc AG. Systematic review of the health benefits of physical activity and fitness in school-aged children and youth. Int J Behav Nutr Phys Act 2010; 7: 40. doi: 10.1186/1479-5868-7-40 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 26.Huang TT, Nansel TR, Belsheim AR, et al. Sensitivity, specificity, and predictive values of pediatric metabolic syndrome components in relation to adult metabolic syndrome: the Princeton LRC follow-up study. J Pediatr 2008; 152: 185–190. doi: 10.1016/j.jpeds.2007.08.007 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 27.Yamaguchi N, Chosa E, Yamamoto K, et al. Screening for musculoskeletal problems in Japanese schoolchildren: a cross-sectional study nested in a cohort. Public Health 2016; 139: 189–197. doi: 10.1016/j.puhe.2016.08.010 [DOI] [PubMed] [Google Scholar]
- 28.Micheli LJ. Overuse injuries in children’s sports: the growth factor. Orthop Clin North Am 1983; 14: 337–360. doi: 10.1016/S0030-5898(20)31352-3 [DOI] [PubMed] [Google Scholar]
- 29.Ashizawa K, Kawabata M. Daily measurements of the heights of two children from June 1984 to May 1985. Ann Hum Biol 1990; 17: 437–443. doi: 10.1080/03014469000001212 [DOI] [PubMed] [Google Scholar]
- 30.Donatelli R, Wooden M, Ekedahl SR, et al. Relationship between static and dynamic foot postures in professional baseball players. J Orthop Sports Phys Ther 1999; 29: 316–325, discussion 326–330. doi: 10.2519/jospt.1999.29.6.316 [DOI] [PubMed] [Google Scholar]
- 31.de Lucena GL, dos Santos Gomes C, Guerra RO. Prevalence and associated factors of Osgood–Schlatter syndrome in a population-based sample of Brazilian adolescents. Am J Sports Med 2011; 39: 415–420. doi: 10.1177/0363546510383835 [DOI] [PubMed] [Google Scholar]
- 32.Cipriani DJ, Terry ME, Haines MA, et al. Effect of stretch frequency and sex on the rate of gain and rate of loss in muscle flexibility during a hamstring-stretching program: a randomized single-blind longitudinal study. J Strength Cond Res 2012; 26: 2119–2129. doi: 10.1519/JSC.0b013e31823b862a [DOI] [PubMed] [Google Scholar]
Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
The data underlying this article will be shared upon reasonable request to the corresponding author.