Comparison of Muscle Activation between Variable Resistance Training and Free Weight Training: A Systematic Review and Meta-Analysis

Variable resistance training (VRT) can address biomechanical limitations of free weight training (FWT) and enhance performance, but its effect on muscle activation remains unclear. This study aimed to compare muscle activation between VRT and FWT, focusing on the impact of variable resistance load proportions and contraction phases in VRT. A systematic review was conducted using PubMed, Scopus, SPORTDiscus, and Web of Science, including studies of healthy adults with at least six months of training experience performing multi-joint exercises. Muscle activation was measured using surface electromyography (sEMG), and Standardized Mean Differences (SMD) were calculated using a random-effects model. The meta-analysis of 11 studies found no significant overall difference between VRT and FWT (SMD = 0.07; p = 0.493). However, VRT showed greater activation during the concentric phase than both the eccentric phase (SMD = 1.40; p < 0.001) and the FWT’s concentric phase (SMD = 0.32; p = 0.033). Higher variable resistance load proportions in VRT also resulted in greater activation than FWT at equivalent loads (SMD = 0.38; p = 0.036). Thus, under the same load, VRT significantly improves concentric-phase muscle activation over FWT, with higher variable resistance loads providing even greater benefits.