Is There a “Window of Opportunity” for Flexibility Development in Youth? A Systematic Review with Meta-analysis

Abstract

Abstract Background Flexibility is an important component of physical fitness for competitive and recreational athletes. It is generally suggested that flexibility training should start from childhood (6–11 years of age) to optimize joint range of motion (ROM) increases; however, evidence is limited and inconsistent. Objective To examine whether there is a difference in the effect of stretching training on flexibility during childhood (6–11 years of age) and adolescence (12–18 years of age). Design Systematic review and meta-analysis. Methods We searched PubMed Central, Web of Science, Scopus, Embase, and SPORTDiscus, to conduct this systematic review. Randomized controlled trials and non-randomized controlled trials were eligible. No language and date of publication restrictions were applied. Risk of bias was assessed using Cochrane RoB2 and ROBINS-I tools. Meta-analyses were conducted via an inverse variance random-effects model. GRADE analysis was used to assess the methodological quality of the studies. Results From the 2713 records retrieved 28 studies were included in the meta-analysis (n = 1936 participants). Risk of bias was low in 56.9% of all criteria. Confidence in cumulative evidence was moderate. We found that stretching was effective in increasing ROM in both children (SMD = 1.09; 95% CI = 0.77–1.41; Z = 6.65; p < 0.001; I2 = 79%) and adolescents (SMD = 0.90; 95% CI = 0.70–1.10; Z = 8.88; p < 0.001; I2 = 81%), with no differences between children and adolescents in ROM improvements (p = 0.32; I2 = 0%). However, when stretching volume load was considered, children exhibited greater increases in ROM with higher than lower stretching volumes (SMD = 1.21; 95% CI = 0.82–1.60; Z = 6.09; p < 0.001; I2 = 82% and SMD = 0.62; 95% CI = 0.29–0.95; Z = 3.65; p < 0.001; I2 = 0%, respectively; subgroup difference: p = 0.02; I2 = 80.5%), while adolescents responded equally to higher and lower stretching volume loads (SMD = 0.90; 95% CI = 0.47–1.33; Z = 4.08; p < 0.001; I2 = 83%, and SMD = 0.90; 95% CI = 0.69–1.12; Z = 8.18; p < 0.001; I2 = 79%, respectively; subgroup difference: p = 0.98; I2 = 0%). Conclusions Systematic stretching training increases ROM during both childhood and adolescence. However, larger ROM gains may be induced in childhood than in adolescence when higher stretching volume loads are applied, while adolescents respond equally to high and low stretching volume loads. Registration: INPLASY, registration number: INPLASY202190032; https://inplasy.com/inplasy-2021-9-0032/