The Resistance Training Dose-response: Effects Of Weekly Volume On Muscle Hypertrophy

PURPOSE: This meta-analysis explored the nature of the continuous dose-response relationship between weekly resistance training (RT) set volume and muscle hypertrophy. METHODS: Each RT intervention included healthy participants <70 years old, utilized a dynamic (RT) intervention ≥4 weeks differing in set volume and/or frequency while controlling for other RT variables, and measured hypertrophy using a direct, site-specific measurement. All contributing RT sets were classified as direct or indirect: direct sets were those in which the measured muscle(s) was likely to be the primary force generator in the exercise while indirect sets were those in which the measured muscle(s) was likely to be meaningfully trained but not the primary force generator. Then, weekly set volume for indirect sets was quantified as 1 for ‘total,’ 0.5 for ‘fractional,’ and 0 for ‘direct.’ A series of multi-level meta-regressions were performed, with all models adjusted for frequency, intervention duration, and training status. RESULTS: The multi-level meta-regression models for the effect of weekly set volume on hypertrophy included 35 studies, 220 effects, and 1,032 participants. The relative evidence was strongest for the ‘fractional’ quantification method (2 x Log(BF) = 9.48 to 10.29). Model comparisons revealed the square root model was the best fit, with a marginal slope of 0.24% [95% CrI: 0.15%, 0.33%]. CONCLUSIONS: The best fit meta-regression model indicated a positive dose-response relationship whereby higher weekly ‘fractional’ set volumes resulted in greater muscle hypertrophy, though with diminishing returns. These findings align with previous meta-analytic work, and expands on them by including new data and exclusively site-specific, direct hypertrophy measures. No clear plateau in the dose-response relationship was identified; however, there is additional uncertainty at higher volumes, and the data remain compatible with multiple functional forms.