The effects of 12 weeks of plyometric training on body composition, physical strength, and bone mineral density in obese adolescents
Article information
Abstract
This study aimed to investigate the effects of 12 weeks of plyometric training on body composition, physical fitness, and bone mineral density (BMD) in obese male adolescents. Twenty obese male adolescents were randomly assigned to a control group (n=10) or a plyometric exercise group (n=10). The plyometric exercise group performed the exercise 3 times a week for 12 weeks. Body composition, physical fitness, and BMD were measured before and after the 12-week intervention. Following results were obtained. In the plyometric exercise group, body fat mass significantly decreased before and after the intervention, but differences in body weight, body mass index, and lean body mass were not significant. Among physical fitness factors, grip strength, muscular endurance, and the 20-m shuttle run significantly improved, but flexibility did not show a significant difference. In the plyometric exercise group, BMD significantly improved before and after the intervention. These results indicate that 12 weeks of plyometric training may be effective in improving body composition, physical fitness, and bone density in obese adolescents.
INTRODUCTION
The recent rapid increase in the global prevalence of obesity has been recognized as a serious public health issue, and the various metabolic diseases associated with obesity threaten the health of modern people. Furthermore, what makes the continued rise in obesity even more concerning is the fact that the age at which obesity begins is gradually decreasing. The obesity rate among children and adolescents in Korea recently increased from 15% in 2014 to 15.6% in 2015 and 16.5% in 2016, meaning that one in six children is obese or overweight, following the global trend of obesity prevalence.
The incidence of obesity in adolescents, caused by excessive energy intake and lack of physical activity, continues to increase. Neglecting to manage obesity in adolescence can negatively impact their growth and development, potentially leading to obesity in adulthood. Obesity accelerates the accumulation of body fat and impairs the normal functioning of various organs, including the musculoskeletal system (Nilwik et al., 2013; Vincent et al., 2012), cardiovascular system (Ebbert et al., 2014), and endocrine system (Mihalopoulos et al., 2017; Witkowska-Sędek et al., 2017). This, in turn, increases the risk of noncommunicable diseases such as decreased physical strength, increased fractures, hypertension, insulin resistance, diabetes, and cardiovascular disease. Given the recent rise in obesity among Korean adolescents, coupled with the current situation in which their physiques are increasing while their physical strength is declining, various intervention strategies for preventing and improving obesity in adolescents are essential.
Numerous previous studies have reported that physical activity-related program interventions have a positive effect on preventing and improving overweight and obesity in adolescents (Kohl et al., 2012; Son et al., 2017). In particular, physical fitness levels resulting from exercise interventions are closely related to the development and improvement of obesity. Furthermore, bone health is crucial for proper physical development during adolescence. This is because bone health during adolescence is likely to continue into adulthood, and because growth hormone secretion, which is involved in bone growth, is activated during this period, appropriate stress through regular exercise is crucial (Novotny et al., 2004). Recently, research on the relationship between obesity and bone mineral density (BMD) has been actively conducted, and adolescent obesity has been reported to negatively impact BMD. Previous studies have shown that obese children have lower BMD than children of normal weight and have a fracture incidence rate more than twice as high (Coulding et al., 2005; Rocher et al., 2008). In addition, lean body mass was found to have a higher positive correlation with bone density than body fat mass, suggesting the importance of increasing lean body mass through regular exercise (Berenson et al., 2009).
Plyometric training, a form of exercise that combines movement speed and strength to produce maximum muscle strength in a short period of time, is a training method implemented in sports settings to improve specific muscle strength and power of elite athletes to achieve maximum performance. Plyometric training is not restricted by location, can be performed using relatively simple small equipment, and exercise intensity can be controlled by setting the number of repetitions, distance, and height of various movements. This study analyzed the effects of a 12-week plyometric training program intervention on body composition, physical fitness factors, and bone density in obese male middle school students, thereby providing basic data for developing an effective exercise program to improve obesity in adolescence.
MATERIALS AND METHODS
Study subjects
The subjects of this study were male students at middle school. They had no prior experience of regular exercise, had not taken any anti-obesity medication in the past 6 months, and had a body mass index (BMI) of 25 kg/m2 or higher. After receiving a thorough explanation of the study’s purpose, obtaining parental consent, and expressing voluntary participation, students were randomly assigned to an exercise group (EXE, n=10) and a control group (CON, n=10). Given the potential physical and psychological fatigue associated with participation, discontinuation was voluntary. The physical characteristics of the participants in this study are presented in Table 1. This study was approved by Eulji University (approval number, 2024-4-27).
Characteristic of the participant
In this study, the plyometric training program was shown in Table 2. Considering the school curriculum, the program was flexibly operated for 45 min per day, 3 times per week, over a 12-week exercise intervention period.
Plyometric exercise training program
Measurement variable
Body composition
Body composition measurements were conducted using the InBody J10 (Biospace Co., Seoul, Korea) after fasting for at least 4 hr. Subjects stood on a metal foot-shaped plate about shoulder-width apart, holding the measuring rod in both hands and looking straight ahead. They were instructed to remain still for approximately 1 min, without moving or speaking during the measurement.
Physical fitness
The physical activity promotion system, a widely used student health physical fitness assessment tool in current school physical education settings, was used to measure physical fitness. Muscle strength, muscular endurance, flexibility, cardiorespiratory endurance, and power were measured. Power was measured by running a 50-m race according to the researcher’s instructions, with the results recorded to the nearest 0.01 sec. After sufficient rest, the participants were re-measured, and the highest score was recognized. Muscle strength was measured twice using a digital grip dynamometer. Sit-ups for muscular endurance were performed under the supervision of an assistant, with the participants’ head and back on a mat, maintaining a 90° knee angle. Flexibility was measured by sitting with both feet fully in contact with the vertical surface of the measuring device. The participant then extended their chest forward, fully flexed their upper body, and reached out to push the measuring device. The value recorded was the number of times the participant pushed the device. Cardiorespiratory endurance was measured by continuously running back and forth at 15-m intervals to the sound of a timed beep. All subjects had to completely cross the 15-m line with both feet before the audio-recorded beep sounded. If the subject was moving to the opposite side when the beep sounded, they were instructed to quickly turn around and run back at that point and were given a single warning. Applying this rule, if the subject failed to run to the other side before the beep sounded twice, the measurement was terminated and the number of times the subject had run the 15-m distance was recorded.
Bone density
Bone density of the left and right calcaneus was measured using a quantitative ultrasound device that measures the attenuation and velocity of ultrasound waves passing through bone tissue. Subjects were asked to remove their shoes and socks, seated in a comfortable chair, and the calcaneus area was disinfected with an alcohol swab. The subject was then placed on the footplate of the measuring device, ensuring the calcaneus was properly aligned with the ultrasound output before measurement. Considering the subject’s youth, an auxiliary footplate was used to ensure accurate measurements. Two measurements were taken on each calcaneus, and the average was recorded. The measured values were converted to a T-score, an index used to assess fracture risk based on World Health Organization diagnostic criteria.
Data processing
All data collected in this study were calculated using the IBM SPSS Statistics ver. 23.0 (IBM Co., USA). Mean and standard deviation were calculated for all variables. The Kolmogorov–Smirnov Test was performed on all premeasured values to confirm normality. Differences in variance between groups were examined using independent t-tests, and within-group changes were examined using paired t-tests. The statistical significance level was set at α =0.05.
RESULTS
Body composition
The pre- and posttest body composition variables and the comparison results between groups after a 12-week plyometric training intervention are shown in Table 3. Regarding changes over time within the groups, the exercise group showed increases or decreases in body weight, BMI, and lean body mass compared to the pretest values, but the differences were not statistically significant. Body fat decreased approximately 7% in the exercise group from 24.87±5.85 to 23.11±6.35, a statistically significant difference (P<0.01). The results of examining the differences in body composition between the two groups before and after the exercise intervention revealed no significant differences in any of the measured variables, including body weight, BMI, lean body mass, and body fat.
Changes of body composition in pre- vs. posttest
Physical fitness
Table 4 shows pre- and posttest measurements of physical fitness variables after 12 weeks of plyometric training intervention. Regarding changes over time within the groups, the exercise group showed statistically significant differences in all physical fitness factors except flexibility. Muscle strength increased approximately 15% from 40.22±3.75 to 46.46±6.33 (P<0.01), and muscular endurance increased approximately 40% from 62.44±12.54 to 87.77±9.07 (P<0.000). Power output improved approximately 5% from 7.91±0.25 to 7.52±0.36 (P<0.05), and cardiorespiratory endurance increased approximately 30% from 36.88±13.51 to 48.88±15.24 (P<0.01). On the other hand, the comparison group showed a decrease in explosive power and cardiopulmonary endurance from 7.98±0.24 to 8.53±0.53 and from 40.55±14.37 to 31.66±10.75, respectively, with statistically significant differences (P<0.01). As a result of verifying the difference in the amount of change before and after the exercise intervention by physical fitness factor between the groups, the exercise group showed a greater change with a statistically significant difference of 6.46± 5.72 than the comparison group with a statistically significant difference of 0.74±3.13 (P<0.01). The muscular endurance showed a higher change in the exercise group than the comparison group with a statistically significant difference of 25.33±12.61 and 3.88±11.92 (P<0.01). Both explosive power and cardiopulmonary endurance showed greater changes in the exercise group (0.39± 0.43 and 12.0±6.85) than in the comparison group (0.61±0.37 and 8.88±7.72), and statistically significant differences were observed (P<0.001).
Changes of physical fitness in pre- vs. posttest
Bone density
The pre- and posttest values for bone density variables and the results of the intergroup analysis after 12 weeks of plyometric training intervention are shown in Table 5. Regarding changes over time within the groups, the left-sided bone density tended to increase slightly in the exercise group compared to the pretest, but the difference was not statistically significant. In contrast, the comparison group decreased from −0.78±0.89 to −1.50±0.61, showing a statistically significant difference (P<0.05). The right-sided bone density increased in the exercise group from −1.24±0.71 to 0.01±1.11, showing a statistically significant difference (P<0.05). The comparison group showed a trend toward an increase compared to the pretest, but the difference was not statistically significant. As a result of verifying the difference in bone density change before and after exercise intervention between groups, the exercise group showed a greater change in left bone density (−0.48±0.87) than the control group (0.71±0.70), showing a statistically significant difference (P<0.01). The exercise group showed a greater change in right bone density (−1.23±1.28) than the control group (−0.46±1.23), but there was no statistically significant difference between the two groups.
Changes of bone mineral density in pre- vs. posttest
DISCUSSION
The positive effects of various lifestyle interventions for managing and improving obesity are widely recognized. Exercise, in particular, is considered the most economical and effective obesity management method among lifestyle interventions. In this study, 12 weeks of plyometric training was shown to have a positive effect on improving waist circumference, muscle strength, power, cardiorespiratory endurance, and bone density in obese male middle school students.
In this study, which analyzed changes in body composition following a 12-week plyometric training intervention, body weight, BMI, and lean body mass tended to improve somewhat in the exercise group, but there was no statistically significant difference. On the other hand, body fat was significantly reduced in the exercise group after the 12-week exercise intervention. Despite the long-term exercise intervention, the minimal improvement in body weight and BMI is thought to be due to the imbalance of energy intake and expenditure due to excessive nutrient intake in growing middle school boys, and at the same time, it is thought that this result means that exercise alone may have a somewhat limited weight loss effect for the treatment of adolescent obesity (Birch and Fisher, 1998).
In the results of this study on muscle strength, muscular endurance, cardiopulmonary endurance, flexibility, and explosiveness, the exercise group after 12 weeks of plyometric training showed a higher tendency in overall physical fitness factors than the control group, and in particular, showed significant differences and improvements in muscle strength, muscular endurance, cardiopulmonary endurance, and explosiveness.
Although bone growth is determined by genetics, continuous physical activity and training can significantly affect bone density. Regular exercise at an appropriate intensity has been reported to result in higher bone density, and weight-bearing exercise has been closely associated with higher total body bone density and bone mass (Ribom et al., 2004). Furthermore, bone density has been shown to be highly correlated with factors such as body weight, lean body mass, and body fat mass. In particular, increased lean body mass has been reported to have a positive effect on bone density by increasing blood flow to skeletal muscles (Colleran et al., 2000). Furthermore, a study reported that the high-impact load generated by jumping exercise can be an effective strategy for maintaining or increasing bone density along with weight loss (Hunter et al., 2014). Forero-Bogotá et al. (2017) reported that improvements in body composition and muscular fitness can have a positive effect on overall bone health in school-age children.
Therefore, it is thought that plyometric training intervention with various and repetitive jumping movements in middle school boys with obesity during the growth period can contribute to improving bone density by transmitting an appropriate load shock to the lower extremities in addition to improving fat metabolism due to changes in body composition.
Twelve weeks of plyometric training can contribute to improving body composition and health-related physical fitness in obese male middle school students, and it also showed positive effects on lower extremity bone density. Therefore, the application and development of a plyometric exercise program that considers stability and subject characteristics is expected to be a valuable exercise strategy for improving the overall health of obese students in schools and promoting physical development.
Notes
CONFLICT OF INTEREST
No potential conflict of interest relevant to this article was reported.
ACKNOWLEDGMENTS
This work was supported by the University Innovation Support Project of Eulji University.