Summary of evidence
The UKSCA [13] and NSCA’s [12] position statements on youth resistance training both suggest that resistance training may have a positive impact on fundamental movement skills. This is the first meta-analysis reported that focusses solely on resistance training, with the aim of examining the impact of resistance training on fundamental movement skills in youth. Analysis indicated that resistance training has a positive impact on a number of fundamental movement skills as assessed by product-oriented measurement outcomes. Statistically significant effect sizes were found for all of the FMS outcomes included a medium effect of resistance training interventions on squat jump and a small effect on all other outcomes (vertical jump, standing long jump, sprint and throw).
It has been identified that muscular strength is an essential component of motor skill development [15] and both functional (e.g., changes in motor unit coordination) and structural (e.g., muscular hypertrophy) adaptations as a result of resistance training might bring about changes in motor competency [16] which therefore may be linked to the development of FMS. In particular, neural adaptations as a result of resistance training include changes in motor unit coordination, firing and recruitment, which are factors that are known to be essential for optimal movement, and likely to play a major role in reported changes, especially in younger children for whom hypertrophy is less likely [16].
Reinforcing this, it has been reported that increases in sprint performance due to resistance training are most likely caused by increases in neuromuscular activation of the trained muscles [28, 29]. Thus, there appears to be strong evidence from this meta-analysis to support the role of resistance training to enhance outcomes commonly associated with FMS in youth, which might be a logical assumption when strength is reported to be an essential component of motor skill competency [15].
The largest effect size in this meta-analysis was for the squat jump. This meta-analysis included only isolated resistance training interventions, suggesting that these effects occur with resistance training in the absence of any form of power training (such as plyometrics) and therefore this may explain the larger effect on a single squat jump which does not involve a plyometric element (counter movement) in comparison to the vertical jump and standing long jump, which do. In support of this, van Hooren et al. identifies that “In the CMJ, the athlete starts from a standing position and initiates a downward movement, which is immediately followed by an upward movement leading to takeoff. In contrast, during the SJ, the athlete descends into a semi-squat position and holds this position for approximately 3 seconds before takeoff.” [30] Therefore, this clarifies the difference between the two assessment outcomes of squat jump and vertical jump.
Plyometrics as a mode of training is included in previously published reviews and scrutiny of the effect sizes across the studies suggests that inclusion of plyometrics leads to greater enhancement of FMS. Behringer et al. [16] reported a medium effect size for jumping; however, this was both vertical jump and standing long jump combined, and it was not specified whether the vertical jump had a counter movement, or whether the analysis included squat jumps, which do not have a plyometric element. Harries et al. [17] reported a positive effect of resistance training on vertical jump performance (mean difference (MD) = 2.09, 95% CI − 0.01 to 4.20, Z = 1.95, P = 0.05) and a larger effect for studies that combined plyometric with resistance training (MD = 3.03, 95% CI = 0.83 to 5.24, Z = 2.69, P = 0.007) or included plyometric training alone (MD 5.47 [1.95, 9.00], Z = 3.04 [P = 0.002]). Lesinski et al. [19], who also included plyometrics as a training mode, similarly reported a large, significant effect size for vertical jump (SMDwm = 0.80; I2 = 67%; χ2 = 137.47; df = 46; P < 0.001) (although again it is not clear whether this included a counter movement, or whether squat jumps were also included in the analysis). Also, whilst Lesinski et al. [19] did exclude uncontrolled trials, direct comparisons with the present review are not wholly appropriate as several of the studies included in their review involved plyometric training. It is not surprising that plyometric exercise including jumping would result in an improvement in jump performance. However, the results from the current meta-analysis would suggest that the development of strength has a key role to play. This strength development could be associated with the quality and coordination of movement rather than power development alone, which could be more relevant for non-athletic populations by producing better movement and therefore more positive physical activity experiences.
For the current review, it is important to be cautious when drawing conclusions from the jumps data both because of the number of studies (there were 25 vertical jump data sets compared to 10 squat jump and 14 standing long jump data sets) and because of the high heterogeneity across the studies that included the squat jump. The moderator analysis indicates that this may be explained by sex and sport status and is discussed further below.
For the outcome of the sprint, there was a small, significant effect which suggests that adaptations occur that might impact on speed. Supporting this, studies have shown significant correlations between maximal squat strength and sprint performance in youth [18, 31]. Behringer et al. [16] reported an effect size of 0.54 (95% CI 0.34–0.74), which included both shuttle runs and straight sprints and similar results were published by Lesinski et al. [19] (SMDwm = 0.58; I2 = 41%; χ2 = 55.74; df = 33; P < 0.01) Taken together, these findings imply that resistance training has a positive impact on sprint performance.
There was a small but significant effect size for throw outcomes; however, Behringer et al. [16] reported a large effect size for throwing 0.99 (95% CI: 0.19–1.79). In the present review, it is important to note that out of the 6 data sets, two included a handball throw [32, 33]. This task is sport specific and therefore the specific technique required to play the sport may have influenced the results.
It should be noted that both the reviews from Behringer et al. [16] and Harries et al. [17] combined controlled trials and uncontrolled trials for the analyses, which has implications for comparing results to the present review. For uncontrolled trials, it is difficult to ascertain if any intervention effects are due to the normal process of growth and maturation. Equally, for the studies that include participants taking part in performance sport, the effect of normal training cannot be controlled for; to investigate intervention effects in youth populations it is critical to include a control group to ensure appropriate interpretation of results.
Given the lack of studies that investigated the role of isolated resistance training in improving FMS using process-oriented assessment batteries, the current review instead examined individual product oriented FMS outcomes. Nevertheless, we have demonstrated that resistance training has a significant effect on all assessed outcomes, which suggests a positive effect on overall movement. This has positive implications for creating strategies to develop FMS and ultimately encourage a healthier and more active lifestyle.
Moderator analysis
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 the sex of participants was a moderator for squat jump, sex of training group was a moderator for squat jump, sport status was a moderator for squat jump and standing long jump, the type of control group was a moderator for the standing long jump, and additionally quality score was a moderator for sprint (see Table 2).
Sex of participants and sex of training group
The outcome of squat jump displayed high heterogeneity. The sex of participants (males or females) and sex of training group (i.e. the training group was designed for either males, females or mixed sex) may explain this variance, with more of an effect on males and the male training groups.
In adolescents, it has been reported that during puberty, sex differences in muscular strength occur with boys demonstrating accelerated gains [34]. However, it has been suggested that there is no clear evidence of any difference in strength between pre-pubescent girls and boys [35]. As this meta-analysis included both children and adolescents, it is difficult to make conclusions based on this data. Additionally, for squat jump there was only one study that included females and nine that included males.
Sport status
For squat jump, and standing long jump distance, there was more of an effect of resistance training on those involved in sport compared to those who were not identified as being involved in a specific sport (e.g. identified as ‘school children’). Recent research has found an association between FMS and participation in organised sports [36]. Those study participants who take part in sport may therefore already have well-developed FMS at baseline, greater competency with the resistance training, and therefore would be more susceptible to further gains. Those who do not participate in sport might not display as much competency in their movement at baseline and therefore it could take longer to make observable improvements. However, it is important to note that the ‘not sport’ group may have included children who take part in sport; it was just not reported in the study as a ‘sport’ group (e.g. a football team).
Age and pubertal stage
There was no moderator effect of age or pubertal stage on any of the outcomes and although Behringer et al. [16] proposed that younger children may experience a greater effect of resistance training due to the degree of neuromuscular adaptation that occurs, Lesinski et al. [19] reported no difference in the effect between pubertal stages or for chronological age. These previous reviews have examined effects in athlete groups, so taken together, it appears that gains in FMS are likely, irrespective of age and maturity status. Morgan et al. [37] identified that some children (particularly older) may experience a ‘ceiling effect’ with some FMS measures. However, ceiling effects are less likely to occur with product assessments because there is always the possibility of performing better when the scoring is related to speed, distance or accuracy [37].
Type of control and quality score
There was a large imbalance of studies for type of control group, with 13 studies being ‘no strength control’ versus only 1 study being ‘sport only’. For the quality score, there were nine studies that were ‘weak’ versus only one study being ‘moderate’ and only one study being ‘strong’ and therefore it is not possible to make conclusions based on this data. In particular for the quality score, with only one study being strong, this has implications for interpreting the results as well as suggesting that more quality studies should be undertaken to investigate this topic further.
Strengths and limitations
There were a number of strengths of this review. There should be strong confidence in the main findings given the rigorous review process. Strict inclusion/exclusion criteria resulted in an analysis of 33 data sets that examined the effects of resistance training on FMS in 542 youths from 11 countries. Additionally, it is the first review to have included resistance training only interventions, rather than include interventions that include plyometric training, which may be more relevant for a sporting population who may be aiming to improve performance.
This review builds on previous reviews, but with the inclusion of non-sporting populations. The context of this review was that resistance training might be a worthwhile intervention to help improve FMS in inactive youth; thus the inclusion of non-sporting participants was important. Although the meta-analysis conducted by Behringer et al. [16] also included non-athletes, 7 years later, an update to build on the data is beneficial.
There was high compliance reported in the included studies. For the studies who reported it, compliance was 89%. As well as a strength of the current review, high compliance adds substance to the potential for resistance training as a viable mode of intervention to improve FMS.
There are also limitations apparent 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 78 participants), frequency, duration and programme content. The frequency ranged from 1 to 3 times a week and duration ranged from 6 to 104 weeks. Programmes also involved a mixture of sets and reps with a range of intensities. The forest plot (Fig. 3) also signifies large variation in the individual studies’ results. There was also an indication of the presence of publication bias which should be considered when interpreting the results.
A limitation of the moderator analysis was that not all of the studies reported data to enable a thorough investigation, and there were not equal numbers of studies in all comparisons, so limited conclusions can be made based on this additional level of analysis. Evaluating the quality of the papers included, there was found to be a mixture of quality of studies, with only 33.3% of the studies classified as strong.
Finally, all of the studies included used product-oriented, rather than process-oriented, outcomes. Therefore this meta-analysis does not inform us about how the movements are performed. This supports previous research that has concluded that the use of process and product assessments should be used to comprehensively capture levels of movement competency in human movement [9, 10].
