The Effects of Unstable Strength Training on Lower Limb Stability in Adolescent Volleyball Players in China

Background: This study aimed to compare the effects of unstable strength training (UST) and traditional strength training (TST) on lower-body stability in adolescent volleyball players in China. Methods: This stratified randomized controlled trial recruited 62 eligible athletes from Shandong Province. Participants were assigned to either the UST group or the TST group, and both groups completed a 10-week training program. Results: Multivariate analysis of variance (MANOVA) for within-group effects revealed statistically significant improvements in all dependent variables for both the UST and TST groups (p < 0.05), FMS [F = 35.112, p < 0.001, η2 = 0.377]; and balance ability differences, left-side score (LS) [F = 8.268, p < 0.001, η2 = 0.125], right-side score (RS) [F = 8.094, p < 0.001, η2 = 0.122]. Furthermore, after controlling for covariates, MANCOVA analysis for between-group effects still showed statistically significant differences between the UST and TST groups on all post-test dependent variables. These differences included: functional differences, FMS [F = 34.412, p < 0.001, η2 = 0.389]; LS [F = 8.079, p < 0.01, η2 = 0.130]; and RS [F = 8.532, p < 0.001, η2 = 0.136]. Conclusion: UST is more effective than TST in improving athletes’ lower-body stability performance. Future studies should explore the application of UST in other sports and examine its effects on parameters beyond functional movement and balance.