The effectiveness of resistance training in children. A meta-analysis.
Many recent studies have reported that resistance training can be effective in producing strength gains among prepubescents. These studies appear to refute the early claims of ineffectiveness of resistance training in children. A meta-analysis procedure combines the results of individual empirical studies and estimates a standardised effect, termed effect size. This effect size is based on the scores of the control and experimental groups before and after training. It defines the difference between the gain of the experimental and control groups, divided by the standard deviation of the pooled variances of both groups. A literature search revealed 28 studies which described a resistance training programme for girls and boys under the age of 12 and 13 years, respectively. Presumably, these children were pre- or early-pubescents. However, only 9 of these studies provided the necessary data to calculate the effect size and could be included in the analysis. The majority of the studies showed a gain in strength between 13 and 30%. The overall mean effect size was found to be 0.57. This signifies that following training, the average child in the resistance training group was above 71.6% of the children in the control group. The effectiveness of resistance training can be influenced by factors such as age and maturation, gender, as well as the frequency, duration and intensity of the training programme. The studies included in the analysis examined participants of varying ages and did not demonstrate a clear influence of age. Most studies examined only boys or a mixed group of boys and girls. Therefore, the influence of gender on the effectiveness of resistance training in prepubescents cannot yet be determined. Nevertheless, in the few studies where boys and girls were examined separately, no difference was found in the effect of resistance training between genders. It appears that a training frequency of twice per week is sufficient to induce strength gains in children. However, the minimal, or for that matter optimal, duration and intensity are not clear. Some of the weaknesses observed in the reviewed studies include: (i) the lack of control for a possible learning effect; (ii) non-randomisation into the training and control groups; (iii) no report of adherence rate; (iv) a reliance on boys as study participants; and (v) too little information on the type, volume and intensity of training. Future studies should take these weaknesses into consideration.