The Influence of Individual Resistance Training Variables on Muscle Strength: A Systematic Review and Meta-analysis

BACKGROUND: Resistance training (RT) is used to develop muscle strength for a variety of health and performance benefits. Because of the complexity of variable integration in a RT programme, it is unclear how manipulating RT variables influences the overall dosage (sets × repetitions × exercises × intensity × frequency × duration) expressed as a relative dosage (arbitrary units [au]) or absolute dosage (kilograms) and its effect on muscle strength development. OBJECTIVES: We aimed to investigate how RT volume, intensity and dosage influence muscle strength, and if any individual prescription variable is more important than others for developing muscle strength. METHODS: Four databases were systematically searched. Only randomised controlled trials that recorded dynamic muscle strength and provided sufficient training variable data were included. Meta-regressions were performed on pooled muscle strength data, individually for the quadriceps and chest muscle groups, and RT dosage calculations. Quadratic non-linear regressions were performed to investigate if a change in volume, intensity, duration and dosage as continuous variables, as well as frequency, sex and age as categorical variables predicted the change in muscle strength. RESULTS: There were 157 articles that contained appropriate data for analysis. A significant dose response for muscle strength for all outcomes was identified (p < 0.01). A plateau in muscle strength was identified at 887,000 au for chest and 773,000 au for quadriceps strength, where further increasing the dosage did not maintain or increase the rate at which muscle strength developed. Non-linear models identified volume and intensity as significant predictors of the relationship between dosage and muscle strength development for relative chest strength. Duration was a significant predictor for relative quadriceps strength. CONCLUSIONS: There is a non-linear dose-response effect for RT dosage and muscle strength, indicating there is no further benefit obtained from increasing dosage beyond 773,000-887,000 au. The variables that influence muscle strength are different between muscle groups, suggesting that the interaction between dosage and individual variables may differ between muscle groups and therefore, to optimise muscle strength development, specific training variables should be prioritised when developing RT programmes. These findings reflect relative changes in strength among primarily untrained individuals and a clear relationship with absolute strength in trained populations could not be determined.