Summary of evidence
The purpose of this meta-analysis was to examine the effect of resistance training (RT) interventions on weight status in youth. In summary, there was a small, statistically significant, effect of RT interventions on body fat % and skinfolds, but no overall significant effect on body mass, BMI, fat-free mass, fat mass, lean mass or waist circumference.
The UKSCA’s  and NSCA’s  position statements on youth RT both suggest that RT may have a positive impact on body fat and the significant findings of this meta-analysis for body fat % and skinfolds support these statements.
While more studies are required to provide further understanding of the mechanism underlying a reduction in body fat due to a RT intervention, it has been reported that a possible cause could be due to an increase in skeletal muscle mass and resulting increase in basal metabolic rate , in particular, this has been noted in adolescents . However, as there were no overall significant effect sizes evident for lean mass or fat free mass in this analysis, this suggests that these changes could possibly be due to increases in overall energy expenditure that may have occurred by simply taking part in an active intervention, rather than an increase in metabolically active lean tissue. It is important to note, however, that with data from both pre-pubertal and post-pubertal participants included in the analysis, this may have had an impact on the results.
Although all of the studies that measured fat mass were in favour of the intervention, this result was not significant and the effect size was trivial. It is worth considering why the effects were not consistent across different measures of body fat. Although body scans have been used in the included studies to measure both body fat % and fat mass, studies that have used skinfolds to measure body fat % have also been included in the analysis. DEXA scans measure both subcutaneous and visceral fat; however, skinfolds measure subcutaneous fat only. As there have been variable findings with regards to the impact of training on different locations of fat [24,25,26,27,28], this could explain why there were significant findings for body fat % and skinfolds, but not for fat mass. This emphasises the importance of validity when selecting measurement tools and methods used to assess intervention outcomes indicative of body fat. An additional factor to consider in this meta-analysis is that there was more than double the number of studies that investigated body fat % as an outcome in comparison to fat mass, providing greater statistical power to the findings for body fat %.
Resistance training did not demonstrate a significant effect on body mass or BMI; however, due to growth, these results should be interpreted with caution. Since the studies included in the analysis assessed a combination of healthy weight, overweight and obese participants, changes in these measures over time are likely to be variable and this is explored further in the moderator analysis discussed below. For the purpose of this analysis, a decrease in body mass and BMI was analysed as a favourable change. However, with the interventions being resistance based, a subsequent increase in lean mass/fat-free mass (and therefore body mass) was possible, and this may have obscured the findings with an overall positive trend for RT impacting on lean mass and fat free mass in this analysis. It has been identified that there is mixed evidence with regards to whether youth may experience increases in muscle mass following RT, most likely due to inadequate levels of circulating testosterone  although it has been suggested that resistance training may develop lean body mass in adolescents . This will have been exacerbated by the inclusion of youth from 8 to 16 years of age of varying pubertal status and by varying intervention duration. It has also been suggested that periods of training in excess of 10 weeks are required for increases in lean muscle mass to occur . In this review, for lean mass, 67% of the data sets were from studies that included interventions that were > 10 weeks, and for fat-free mass, these interventions were > 10 weeks for only 50% of the studies. This suggests that the intervention duration for several of the studies may not have been long enough to invoke positive measurable changes.
For the outcome of waist circumference, there was no significant effect size evident. With a combination of healthy weight, overweight and obese participants, it might be expected that those studies including overweight/obese participants would show a larger effect on waist circumference than the studies that included healthy weight participants, and this is further explained by the moderator analysis discussed below. There were two studies that included obese participants only; one finding a large effect of the intervention on waist circumference  and the other finding no significant effect . In the 2004 study, the authors did identify that with only a 6-week intervention, this may not have been long enough to have a positive impact on the measured outcomes. Additionally, only six studies investigated waist circumference as an outcome, and this outcome therefore had less statistical power than some of the others.
Overall, these findings are similar to previous reviews. In a meta-analysis published in 2013 , there was a significant effect of interventions including a RT component on body fat % in overweight or obese youths. They also reported no significant effect sizes for body mass, BMI waist circumference, fat mass or lean mass. Out of the 40 studies included, only 9 studies were RT-only studies, and out of these 9, 6 were CTs. However, although interventions were included that also incorporated an aerobic and/or dietary component, similar effects were found for studies that included interventions that were RT alone when authors performed a sub-analysis. There were only three studies included that incorporated RT only and therefore interpreting the data should be undertaken with caution.
In a further systematic review , only six studies were included that investigated RT only interventions in overweight/obese youth. Three out of the six studies showed a significant decrease in percentage fat and a significant increase in fat-free mass, although none of the studies found a decrease in total fat mass, which is in support of the findings of this review. Four studies reported significant changes in body composition, with an increase in fat-free mass and BMI and additionally, all studies reported an increase in body weight. This is conflicting with the current review, although their review only included overweight and obese youth. Unlike the studies in the current analysis, all studies in their review included RT of moderate to submaximal intensity during treatment, rather than high intensity work and while acknowledging the limitations of using percentage of 1RM to prescribe intensity, higher intensity work (however calculated) will provide a greater stimulus for overload than low-medium intensity work. This may have had an impact on their overall findings with regards to weight status, as high intensity work has been reported as a key component to elicit changes in body composition . Supporting the variable findings with regards to the impact of RT on obese adolescents, inconclusive results were found from just seven studies included that focused on RT alone .
In the only systematic review to date that included both healthy weight and overweight/obese participants, 12 studies were included . It was reported that for CTs, no studies found a significant change in BMI body fat, two studies reported a change in lean body mass and one study that reported a change in waist circumference. In their review, there were only five RCTs and three of these included a dietary and/or aerobic component. The interventions were also different to the current review with all but two of the RT interventions being circuit based.
Overall, with just one of the reviews described above including a meta-analysis, it makes comparisons with the current meta-analysis somewhat challenging.
To investigate the findings further, a moderator analysis was completed on all outcomes to identify if any effects could be explained by specific moderator variables. It was found that weight status was a moderator for BMI and waist circumference, and these outcomes showed moderate and high heterogeneity respectively and therefore the variance between studies could be explained by weight status. All other outcomes did not display significant heterogeneity, and no significant findings were apparent from the moderator analysis (Table 2).
There was a significant effect of the intervention on the BMI and waist circumference of obese participants (Table 2) but not on other weight categories, indicating that RT could be an effective intervention on these outcomes in obese individuals. It would seem plausible that BMI and waist circumference outcomes would vary significantly across studies due to the inclusion of both healthy weight and overweight/obese participants. It has been reported that obese youth are more sedentary than their healthy weight peers  and require more energy to move . Therefore, an increase in physical activity might have a larger relative increase in energy expenditure reflected in reduced BMI and waist circumference. It should be noted, however, that for waist circumference, there were only six studies included in the analysis. Three were mixed weight with one study each for obese, overweight and healthy weight participants. This small number of studies may explain the high heterogeneity, and therefore interpreting the results should be undertaken cautiously. Additionally, a longer term follow up study would be beneficial to investigate resistance training as an obesity prevention method.
These findings conflict with previous findings  that suggested that there was a very small moderation effect of age and sex on various weight status outcomes. It was reported that for youth 12 years or older, there was an intervention effect on body mass, BMI, waist circumference, body fat %, fat mass and lean mass, and for males, there was an intervention effect on body mass, BMI, body fat % and fat mass. However, these were small influences on intervention effects, and their analysis included studies that incorporated an aerobic or dietary component which is different to the analysis in the current review and therefore it is difficult to make conclusions regarding RT alone based on their findings.
Strengths and limitations
There were a number of strengths to this review. There should be strong confidence in the main findings given the rigorous review process. A strict inclusion/exclusion criteria resulted in an analysis of 24 data sets that examined the effects of RT on weight status in 554 youths from 8 countries.
This is also the first meta-analysis to include healthy weight and overweight/obese participants taking part in RT only interventions, which is important to identify the impact of RT not only as a treatment for obesity but also as a prevention.
There was high compliance reported in the included studies. For the studies who reported it, compliance was 88%. As well as a strength of the current meta-analysis, high compliance adds substance to the potential for RT as a viable mode of intervention to improve weight status.
There are however limitations that need to be considered when interpreting the results. There was large variability within the study interventions with regards to participant numbers (ranging from 5 to 129 participants), frequency, duration and programme content. The frequency ranged from 2 to 6 times a week and duration ranged from 8 to 20 weeks. Programmes also involved a mixture of sets and reps with a range of intensities and some being circuit based. The forest plots also indicate large variation with the individual studies’ results.
For certain outcomes there were a variety of different methods of measurement. For example, body fat % was measured by DEXA, BodPod, bioelectrical impedance scales, skinfolds and MRI scanning. While reporting standardised mean differences allowed us to pool the data for the purposes of the meta-analysis, differences between measurement tools should be acknowledged.
A limitation of the moderator analysis was not all of the studies reported data to enable a thorough investigation, so limited conclusions can be made based on this additional, incomplete level of analysis.
Finally, there was a mixture of quality of the studies included, with only 44% of the studies classified as ‘strong’. When moderating for quality of studies, it is difficult to make conclusions despite there being significant findings for waist circumference, as there was only one ‘strong’ study for this outcome. This had a large effect size in comparison to the ‘moderate’ and ‘weak’ studies.