Resistance Training Load Effects on Muscle Hypertrophy and Strength Gain : Systematic Review and Network Meta-analysis

Purpose: To analyse the effect of resistance training (RT) performed until volitional failure with low-, moderate- and high-loads on muscle hypertrophy and muscle strength in healthy adults; and assess the possible participant-, design-, and training-related covariates which may affect the adaptations. Methods: Using PRISMA guidelines, MEDLINE, CINAHL, EMBASE, SPORTDiscus, and Web of Science databases were searched. Including only studies that performed sets to volitional failure, the effects of low- (>15 RM), moderate- (9-15 RM), and high-load (≤8 RM) RT were examined in healthy adults. Network meta-analysis was undertaken to calculate the standardised mean difference (SMD) between RT loads in overall and subgroup analysis involving studies deemed high-quality. Associations between participant-, design-, and training-related covariates with SMD's were assessed by univariate and multivariate network meta-regression analysis. Results: Twenty-eight studies involving 747 healthy adults were included. Although no differences in muscle hypertrophy between RT loads were found in overall (P= .113 - .469) or subgroup analysis (P= .871 - .995), greater effects were observed in untrained participants (P= .033), and participants with some training background who undertook more RT sessions (P= .031 - .045). Muscle strength improvement was superior for both high-load and moderate-load compared to low-load RT in overall and subgroup analysis (SMD= 0.60 - 0.63 and 0.34 - 0.35, respectively; P< .001 - .003), with a non-significant but superior effect for high- compared to moderate-load (SMD= 0.26 - 0.28, P= .068). Conclusion: While muscle hypertrophy improvements appear to be load independent, increases in muscle strength are superior in high-load RT programs. Untrained participants exhibit greater muscle hypertrophy while undertaking more RT sessions provides superior gains in those with previous training experience.