Obesity among children is a growing public health concern in many countries. For example, in Australia the prevalence of overweight and obesity in children ages 2-18 years has increased by approximately 20-25% (1). Even more disturbing is that childhood obesity is strongly predictive of obesity in early adulthood (15). Obesity in children has been associated with health problems such as hypertension and Type II diabetes (3). A major concern is developing and investigating effective and practical interventions for the management of children who are already overweight and who are at risk of developing health problems and of suffering from reductions in quality of life as a result of further weight gain; this is a research priority.
It is clear that, along with nutrition and lifestyle, exercise plays a significant role in overcoming obesity in children. Many research studies have shown the benefits of exercise for children, and resistance training is gaining in popularity for use with children (7,8,14,18). Resistance exercise, which includes the use of body weight and external resistance, offers an alternative to other modes of exercise that may not be tolerated as well by children who are overweight. Because of their larger body mass, these children are typically stronger than their peers and, therefore, receive positive feedback from this type of exercise, which helps to improve their self-esteem (14). This provides a relative psychological advantage over other types of activity that require aerobic ability or agility and that, therefore, put children who are obese at a distinct disadvantage.
Although most research has suggested that increased levels of physical activity combined with improved nutrition can improve body composition and the health of children who are obese or overweight, the majority of these programs use aerobic exercise interventions that may not be well tolerated by children who are overweight and obese (17). One reason for the failure of aerobic-based interventions, such as brisk walking, is that excess weight increases the intensity of weight-bearing activity, resulting in higher perceived exertion among children who are overweight or obese compared with children of normal weight performing the same activity. Further, there is increased risk of musculoskeletal overuse injury, particularly in ground-based activities, and thermal stress when children who are overweight and obese perform continuous exercise modalities (17).
Resistance exercise has been demonstrated to have good efficacy and acceptance by adults who are overweight and obese, reducing body fat and increasing self-esteem and body image. Nevertheless, it remains to be determined whether resistance training results in similar positive outcomes for children who are overweight and obese. It has previously been shown that participation in a resistance training program can produce significant improvements in body composition, including decreased body fat and increased muscle and bone mass, in normal-weight boys ages 13-17 years (16). Sothern et al. (13) examined the effects of a moderate-intensity, progressive resistance training program for 10 weeks for 15 preadolescent children who were obese and between the ages of 7 and 12 years. They reported improved strength, power, muscular endurance, bone density, motor performance, self-satisfaction, self-esteem, and body image. Similarly, Yu et al. (18) have shown that only 6 weeks of resistance training and dietary control improved lean body mass in 41 children who were obese. It seems that the intermittent nature of resistance exercise and the increased lean body mass that results from this form of training can have positive impacts on overall body composition.
Because of the growing popularity of youth resistance training, further studies are needed to evaluate the effects of varying the combination of sets and repetitions on these and other health and performance measures in children who are obese. Therefore, the purpose of this study was to determine the efficacy of an 8-week resistance training program on body composition in children who were overweight and obese. It is important to know the answer to this question because it can assist strength and conditioning professionals and exercise physiologists in designing the most effective programs in terms of both physical outcomes and compliance.
Experimental Approach to the Problem
The study involved an 8-week training intervention for 48 children who were overweight and obese. All subjects participated in a supervised resistance training program (3 d·wk−1) for 8 weeks. The subjects were measured at baseline and after the training program for body composition, body mass index (BMI), strength, power, and activity levels. For all performance tests and training, the subjects were familiarized with the technique in a separate session.
The participants were between the ages of 7 and 12 years. Each subject's BMI (kg·m−2) was used as an index of relative weight. Children with a BMI ≥ 95th percentile were classified as obese, and those with a BMI ≥ 85th percentile were classified as overweight (9). On the basis of statistical power analysis for n size determinations, approximately 25 participants were needed to attain a statistical power of 0.80 at an alpha level of p = 0.05. Maturity status was self-reported (Tanner stage) by participants with the help of their parent(s). The study was approved by the university human research ethics committee, and all children and their legal guardians were informed of the experimental risks and signed a consent document before the start of the study.
Nutritional intake was assessed on 3 days (2 “normal” school days, and 1 “other/weekend” entry) at weeks 0 and 8. The children and their parents were provided with specific verbal and written instructions and procedures for recording all foods and beverages consumed during the 3-day period. Dietary records were checked for any significant changes in consumption and eating habits over the course of the study.
Subjects were required to record any significant physical activity that they performed during any 3 days of their choice at baseline. They were asked to include specific information regarding the type of exercise, duration, and intensity (using the rating of perceived exertion [RPE] scale). Activity records were checked for any significant changes in activity levels at weeks 0 and 8.
Anthropometric measurements of height and weight were determined using standard procedures at weeks 0 and 8. Body mass was measured on an electronic scale (HW200, A&D Mercury Pty Ltd, Thebarton, SA) to the nearest 100 g, and height was determined with a wall-mounted stadiometer (Model 220, SECA, Hamburg, Germany) to the nearest millimeter, with children wearing lightweight clothing and no shoes. All anthropometric measures were carried out by the same investigator.
Body Composition and Bone Density
Body composition analysis was performed using dual-energy X-ray absorptiometry (DXA, Hologic). Percent body fat from the DXA testing was calculated as fat tissue mass divided by the total soft tissue mass plus the estimated bone mineral content (BMC). Coefficients of variation (CVs) in our laboratory (duplicate scans with repositioning) for body composition components are less than 1.0% (10).
Maximal strength of the lower body was determined using a machine squat exercise. Warm-up trials were performed using 30% (8-10 repetitions), 50% (4-6 repetitions), 75% (2-4 repetitions), and 90% (1 repetition) of each subject's estimated 1-repetition maximum (1RM). The load was then increased to a point at which the child had 3-4 maximal efforts to determine the 1RM. Adequate rest was allowed between trials (3-5 minutes). Each participant was asked to lower the bar to a knee angle of 90° (marked by an audible cue and adjustable stoppers) and immediately move the resistance upward in a controlled but forceful fashion to the starting position. Previous research has demonstrated that children can safely perform 1RM strength tests, provided that appropriate procedures are followed (5). The reliability of the test was high, with CV = 6.9% (determined in 10 children using repeat testing).
Power and vertical jump height were measured using the Ballistic Measurement System (Performance Plate, Fitness Technology, Adelaide, Australia). Countermovement jump (CMJ) and static jump (SJ) squats were performed. The CMJ involved lowering to a self-selected depth and explosively jumping upward as quickly as possible with the feet leaving the floor. The SJ used the same procedure except that the child was asked to pause for 2-3 seconds at the bottom position before explosively jumping upward. Vertical ground reaction forces were recorded using a force plate (Fitness Technologies, Adelaide, Australia). Measures of relative peak power and vertical jump height were recorded. Each subject performed 3 trials, and the best result was used for analysis. The reliability of the test was high, with CV = 5.1% for power and 9.8% for jump height (determined in 10 children using repeat testing).
To assess muscular endurance, the children were asked to perform as many push-ups as possible. These were performed using full push-ups with a shoulder-width grip. They were also required to maintain a constant cadence (steady pace) as determined by the tester.
The resistance training sessions consisted of total body workouts using a combination of different body weight and power exercises, as well as a variety of equipment (Table 1). The families of the participants were encouraged to attend all the sessions in order for them to actively support their children and to be directly involved in the study. The primary aim was to incorporate exercises that required minimal equipment including dumbbells, elastic bands, medicine balls, and weighted bags. The program consisted of varying training loads within each week of training (i.e., undulating variation) as well as increasing intensity during the 8 weeks. This type of program has been found to be effective for improving body composition in normal-weight boys (16).
The first workout of the week consisted of 3 sets of 8-10RM. Exercises used included squats, bench presses, lunges, rows, shoulder presses, push-ups, and sit-ups. The second workout involved high-volume, moderate-intensity training sessions, with three sets each of squats, straight leg deadlifts, flys, front raises, triceps extensions, bicep curls, and heel raises, performed using 10-12RM. The third workout involved moderate- to high-intensity training sessions including explosive power exercises; three sets each of repetitive-body weight squat jumps, CMJs, explosive hang pulls, bench presses, rows, shoulder presses, and sit-ups were performed using 3- to 5-repetition training loads. Such periodized training has been shown to be effective for normal-weight children (4,16).
Each session was supervised by qualified exercise instructors who kept detailed training logs of all the sets, repetitions, and exercises performed by each subject, which were later used to measure overall training compliance. Measures of the children's RPE were also recorded after each exercise and at the end of each training session. They were also used as a monitoring tool. The OMNI RPE scale was used because this scale has been previously validated in children (12).
Data were reported as mean ± SD. The criterion alpha level for significance was set at p ≤ 0.05. One-way analysis of variance was used to compare pre and post variables. Reliability of the criterion measures was determined using CV.
There were no reported training injuries or excessive muscle soreness at any stage of the training program. Training compliance was 89 ± 7% (mean ± SD). We originally had 63 children enroll in the study, but 15 withdrew before the completion of 8 weeks. When reasons for withdrawal were given, they included lack of time, other commitments, and moving interstate. Thirty-eight children were in Tanner stage 1, and 12 were in stage 2.
Changes in percent body fat over time are presented in Figure 1. There was a significant decrease in absolute percent body fat of 2.6% (p = 0.003). Pre and post values for the various body composition measures are presented in Table 2. There was also a significant increase in lean body mass of 5.3% (p = 0.07). There were no significant changes in height, weight, BMI, total fat mass, or BMC.
Strength and Power Performance
The pre- and posttraining strength and power measures are provided in Table 3. There were significant increases in 1RM squat (74%), number of push-ups (85%), CMJ height (8%), SJ height (4%), and power (16%).
Nutritional Intake and Activity Levels
No significant changes of nutritional intake or activity levels outside of the extra participation in the resistance training program were seen over the course of the study.
These results clearly demonstrate that the resistance training program was able to produce significant changes in body composition and strength and power measures, as well as being well tolerated by the participants. Although numerous studies have investigated the effects of exercise on children, relatively few have used resistance training models (8,13,14,18). This study adds to the body of literature by showing that a short-term resistance training program can effectively benefit body composition and physical performance in children who are overweight.
Significant body composition changes were seen in percent body fat and lean body mass. The changes in percent fat (decreases of up to 7% in some children) that were observed indicate that an 8-week resistance training program is sufficient time to produce significant and meaningful improvements in body composition in overweight children. Previous studies have shown small (but not significant) effects on body composition in children who were overweight across 6 weeks (18), and larger, significant effects across 10 weeks (13). The present study has shown that 8 weeks is adequate to achieve significant favorable changes in body composition. The significant increase in lean body mass, in combination with a relatively small (−1.2 kg) decrease in total fat, resulted in improved body composition. Specifically targeting skeletal muscle with resistance exercise seems to bring about significant changes in lean body mass in children who are overweight/obese.
No significant changes were observed for BMI during the course of this study. The most likely explanation for this is that significant increases in lean body mass observed would alter body composition and negatively affect BMI, albeit a positive change in terms of health and function. Although other studies have shown the efficacy of using BMI (9,18), it has to be questioned whether this is the most accurate method of measuring obesity in children. This is supported in the present study, where BMI baseline values ranged from 20.5 to 32.8, whereas percent body fat baseline values ranged from 25.3 to 46.3%. Body mass index alone does have limitations for assessing overweight/and obesity, and it is suggested that other measures of assessing body composition in children, such as weight, skinfolds, or hydrostatic weighing, should be considered (17). Dual-energy X-ray absorptiometry as used in our study is the most valid and reliable technique for assessing body composition in children and adults; however, the equipment cost is acknowledged. Hussey et al. (11) recently have suggested that relatively simple measures such as waist circumference, and perhaps fitness, may also give a clearer picture of health risk compared with BMI. Nonetheless, in the present study, the measure of BMI was used because it was necessary to categorize subjects as overweight or obese for inclusion into the study. Among adults, BMI is highly correlated with adiposity (as assessed by DEXA and other laboratory measures) and can account for up to 75% of the variability in body fatness (9). Associations among children, according to Freedman et al. (9), have been more variable, and associations are relatively weak in some subgroups. Watts et al. (17) reviewed many studies examining the effects of resistance training on body composition in children who are overweight and obese and found that although exercise training alone does not always seem to decrease body weight or BMI, it may still be associated with beneficial changes in percent body fat and lean body mass. Regular exercise can, thus, have significant benefits for health that are independent of changes in BMI (2). The present study supports these results because it was found that BMI outcomes can fail to reflect significant improvements in body composition.
There were significant increases in strength and power after only 8 weeks of training. Resistance training has been previously demonstrated to significantly increase strength in children (5,6). However, data pertaining to the changes in muscular strength and power after exercise in children who are obese are limited. Some studies with this population have not included measures of strength (13,18), presumably because of concerns with maximal strength testing in this population. Previous research has shown that strength testing can be performed safely in children (5). Our results show that resistance exercise can significantly improve both strength and power, and this can be safely assessed in this population with no injuries or excessive exercise-induced muscle soreness at any stage of the testing or training. The periodized model that was used in the present study seemed to provide good variation for the children and potentially contributed to the high compliance to the program. Further research is required to compare different types of training models and to determine the optimal program design for children who are overweight and obese.
It is possible that the lower physical fitness associated with excess body weight may be a barrier impeding physical activity among children who are overweight and obese. For example, children who are obese may be less likely to engage in physical activity because of fear of poor performance. Such a barrier to participation in sports and physical activity is likely to perpetuate overweight/obese status. This underlies the need for specific programs and social support to encourage the participation of youth who are overweight or obese in physical activity. It was interesting to note that several of the parents of children involved in this study stated that their children became involved in organized sports after the resistance training program. It is possible that the quite large increases in strength and power of the children gave them the confidence to engage in other activities, which, if confirmed in future studies, provides strong efficacy for resistance training in this population.
In conclusion, this study supports the participation of children who are overweight/obese in a resistance training exercise program Significant improvements in body composition, strength, and power were observed, indicating that resistance exercise programs could provide a preferable alternative to more traditional aerobic-based programs in this population. Further research is required to determine whether these changes can be maintained over a longer period of time (i.e., greater than 8 weeks) and to determine the effects of long-term resistance training programs including a control group, along with dietary interventions, in this population.
Short-term resistance training can significantly improve body composition and increase strength and power in children who are overweight or obese. It seems that this mode of training can be well tolerated and enjoyed by participants. An undulating periodized program provides variation and results in significant increases in lean body mass, decreased percent body fat, and increased strength and power.
There is considerable scope for strength and conditioning specialists to contribute to reducing the problem of childhood overweight and obesity because they have the greatest understanding and skills for implementing periodized resistance training programs. Program design and equipment such as dumbbells, weighted bags, and bands can be used effectively to improve body composition in children.
This study was funded by the Telstra Foundation. The authors wish to thank the dedicated group of trainers and the children and parents for being involved in the study.
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Keywords:© 2009 National Strength and Conditioning Association
strength training; obesity; exercise