Evaluating the Effects of Complex Training on Athletic Performance: A Systematic Review

Background. Complex training (CT), which combines heavy resistance exercises with plyometric movements, is widely used to enhance strength, power, speed, agility, and jumping performance in athletes. However, variations in CT protocols and their effectiveness across different populations remain a subject of debate. Objectives. This systematic review and meta-analysis aimed to evaluate the effects of CT on athletic performance, particularly in football and soccer players, and to identify key factors influencing its efficacy. Materials and methods. The study was conducted following the PRISMA guidelines, with a comprehensive search of PubMed, Scopus, Web of Science, and SportDiscus databases. Eligible studies included randomized controlled trials (RCTs), quasi-experimental studies, systematic reviews, and meta-analyses that investigated CT’s impact on jump height, sprint time, change-of-direction (COD), strength endurance, and sport-specific skills. Data were extracted and analyzed, with effect sizes (Cohen’s d) and heterogeneity statistics (I², Q-test p-value) reported. Results. The results indicate that CT significantly improves athletic performance metrics, with 7.4% gains in vertical jump height (alternating CT), 2.4% reductions in sprint time, and marked agility enhancements (p < 0.001). Subgroup analysis revealed greater benefits for youth athletes (SMD = 2.01) and professionals (SMD = 1.58 for sprinting and 1.53 for COD performance). Substantial improvements were also observed in strength endurance, lower-body power, and kicking accuracy. Conclusions. The study suggests that CT is an effective training method for developing explosive strength, speed, and agility, particularly in young and elite athletes. Given low-to-moderate heterogeneity (I² = 10%–25%), CT consistently enhances performance across different populations. Future research should focus on optimizing CT protocols for specific athlete groups to maximize performance gains.