The Influence of Resistance Training on Joint Flexibility in Healthy Adults: A Systematic Review, Meta-analysis, and Meta-regression

Despite the large body of evidence regarding the effectiveness of muscle stretching exercises, little is known about the effect of resistance training on flexibility. A systematic search was conducted on 9 academic search instruments; inclusion criteria were as follows: healthy adult participants (age >18 years); resistance training intervention (duration >4 weeks); at least one flexibility outcome. Risk of bias was assessed using the RoB-2 and ROBINS-I tools. A 3-level meta-analysis was conducted, with multiple outcomes nested within each study. A moderator analysis was conducted by fitting a meta-regression model. Significance level was set at p<0.05. We included 36 studies (1,469 participants). None of the included papers resulted at a low risk of bias. The pooled effect size for resistance training on flexibility was g=0.6325, with 95% CI: 0.4762 to 0.7888 (p<0.0001). There was a substantial amount of heterogeneity between studies. Exercise intensity was a significant moderator (p<0.0225, high vs low), based on 129 and unique effect sizes, and sex (p=0.0429). Activity level and age were nonsignificant moderators. Resistance training could be implemented as a strategy to improve joint flexibility, with high-intensity protocols resulting in a magnified effect. However, the high overall risk of bias and substantial heterogeneity limit our ability to draw definitive conclusions.