Machines and free weight exercises: a systematic review and meta-analysis comparing changes in muscle size, strength, and power

INTRODUCTION: The aim of this study was to compare changes in muscle size, strength, and power between free-weight and machine-based exercises. EVIDENCE ACQUISITION: The online databases of Pubmed, Scopus, and Web of Science were each searched using the following terms: "free weights" OR barbells OR dumbbells AND machines" up until September 15, 2020. A three-level random effects meta-analytic model was used to compute effect sizes. EVIDENCE SYNTHESIS: When strength was tested using a free-weight exercise, individuals training with free-weights gained more strength than those training with machines (ES: 0.655; [95% CI: 0.269, 1.041]). When strength was tested a machine-based exercise incorporated as part of the machine-based training program, individuals training with machines gained more strength than those training with free-weights (ES: -0.784 [95% CI: -1.223, -0.344]). When strength was tested using a neutral device, machines and free-weight exercises resulted in similar strength gains (ES: 0.128 [95% CI: -0303, 0.559]). There were no differences in the change in power (ES: -0.049 [95% CI: -0.557, 0.460]) or muscle hypertrophy (ES: -0.01 [95% CI: -0.525, 0.545]) between exercise modes. CONCLUSIONS: Individuals looking to increase strength and power should consider the specificity of exercise, and how their strength and power will be tested and applied. Individuals looking to increase general strength and muscle mass to maintain health may choose whichever activity they prefer and are more likely to adhere to.