In-Season Resistance Training Strategies for Soccer: A Systematic Review and Meta-Analysis of Isoinertial Devices Versus Conventional Methods on Change of Direction, Power, and Sprint Performance

Background: Flywheel training [FWT] is increasingly implemented to enhance lower‑limb power in soccer players; however, its comparative effectiveness versus conventional resistance training [CRT] for soccer‑specific performance outcomes remains unclear. Objective: To systematically review and meta‑analyze randomized controlled trials [RCTs] comparing FWT with CRT on countermovement jump [CMJ], sprint performance [10 m, 20 m, 30 m], and change‑of‑direction [COD] ability in soccer players. Methods: PRISMA‑compliant searches were conducted in PubMed, Web of Science, and Scopus up to August 14, 2025. Eligible studies were RCTs involving healthy soccer players completing ≥4 weeks of FWT or CRT and reporting at least one target outcome. Risk of bias was assessed using Cochrane RoB 2, and certainty of evidence using GRADE. Random‑effects meta‑analyses were performed, with heterogeneity quantified by I². Eight RCTs [n = 216; 196 males, 20 females; elite youth to professional levels] were included. Results: FWT produced significantly greater improvements in CMJ performance compared with CRT [SMD = 1.73; 95% CI: 0.59–2.87; p = 0.003; I² = 0%]. In contrast, CRT was superior for short‑distance sprint performance, with significant between‑group differences favoring CRT in 10 m [MD = 0.027 s; p = 0.041; I² = 0%] and 20 m sprints [MD = 0.044 s; p = 0.023; I² = 0%]. No significant differences were observed for 30 m sprint performance (p = 0.096; I² = 0%] or COD ability (p = 0.275], although COD outcomes exhibited substantial heterogeneity [I² = 79%]. Conclusions: Both FWT and CRT effectively enhance explosive performance in soccer players. FWT preferentially improves vertical jump performance, whereas CRT yields greater benefits in short‑distance sprint acceleration. Integrating both modalities within a periodized training program may provide the most comprehensive performance adaptations.