The Resistance Training Dose-Response: Meta-Regressions Exploring the Effects of Weekly Volume and Frequency on Muscle Hypertrophy and Strength Gain

Background: Weekly set volume and frequency are used to manipulate resistance training (RT) dosage.Previous research has identified higher weekly set volume as enhancing muscle hypertrophy and strength gains, but the nature of the dose-response relationship still needs to be investigated.Mixed evidence exists regarding the effects of higher weekly frequency.Objective: Before meta-analyzing the volume and frequency research, all contributing RT sets were classified as direct or indirect, depending on their specificity to the hypertrophy/strength measurement.Then, weekly set volume/frequency 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 for muscle hypertrophy and strength, utilizing 67 total studies of 2,058 participants.All models were adjusted for the duration of the intervention and training status.Results: The relative evidence for the 'fractional' quantification method was strongest; therefore, this quantification method was used for the primary meta-regression models.The posterior probability of the marginal slope exceeding zero for the effect of volume on both hypertrophy and strength was 100%, indicating that gains in muscle size and strength increase as volume increases.However, both best fit models suggest diminishing returns, with the diminishing returns for strength being considerably more pronounced.The posterior probability of the marginal slope exceeding zero for frequency's effect on hypertrophy was less than 100%, indicating compatibility with negligible effects.In contrast, the posterior probability for strength was 100%, suggesting strength gains increase with increasing frequency, albeit with diminishing returns.Conclusions: Distinguishing between direct and indirect sets appears essential for predicting adaptations to a given RT protocol, such as using the 'fractional' quantification method.This method's dose-response models revealed that volume and frequency have unique dose-response relationships with each hypertrophy and strength gain.The dose-response relationship between volume and hypertrophy appears to differ from that with strength, with the latter exhibiting more pronounced diminishing returns.The dose-response relationship between frequency and hypertrophy appears to differ from that with strength, as only the latter exhibits consistently identifiable effects.dose-response relationships between weekly set volume and muscle hypertrophy, weekly set volume and muscle strength gains, weekly frequency and muscle hypertrophy, and weekly frequency and muscle strength gains using various quantification methods. METHODSThis meta-analysis was conducted according to Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines (20).Pre-registration on the Open Science Framework (https://osf.io/r958n)using the International Prospective Register of Systematic Reviews template was used, though some of the methods have changed since the original pre-registration. Inclusion CriteriaInclusion criteria included studies, pre-prints, theses, or abstracts (author correspondence permitting) 1) published or pre-printed before June 2024; 2) available in English; 3) employed a dynamic RT intervention with eccentric and concentric training utilizing a randomized experimental design (either within-or between-group) lasting a minimum of 4 weeks in healthy participants; 4) did not involve participants > 70 years old; 5) compared at least two groups featuring differences in set volume and/or frequency while controlling for the load (±5% of 1RM or ±2RM) and proximity to failure (failure, non-failure, or mixed); 6) included pre-intervention and post-intervention measurements of muscle hypertrophy with a direct, site-specific measurement (ultrasound, computed tomography, magnetic resonance imaging, muscle biopsies) or included pre-intervention and post-intervention measurements of dynamic (up to a 10RM), isometric, or isokinetic maximal strength; 7) not retracted or called into question by Vigotsky et al. (21). Search StrategyPubMed/Medline and Google Scholar databases were searched for studies until April 2023.The following search terms were used for PubMed/Medline: ("resistance training" OR "resistance exercise" OR "strength training") AND ("musc*" OR "hypertrophy" OR "muscle mass" OR "muscle thickness" OR "growth" OR "cross sectional area" OR "fat free mass" OR "lean body mass" OR "limb circumference" OR "muscle strength" OR "strength") AND ("volume" OR "dose" OR "dose response" OR "frequency" OR "multiple sets" OR "single sets" OR "sessions per week" OR "sessions").The following search terms were used for Google Scholar: allintitle: ("resistance training" OR "resistance exercise" OR "strength training") AND ( "musc *" OR hypertrophy OR "muscle mass" OR "muscle thickness" OR growth OR "cross sectional area" OR "fat free mass" OR "lean body mass" OR "limb circumference").Screening was performed using abstrackr (http://abstrackr.cebm.brown.edu).Titles, abstracts, and full texts were examined for inclusion; data was extracted from full texts that met the inclusion criteria.JP and JR performed this process.Following screening and exclusion of studies deemed not to meet the inclusion criteria, the reference lists of included studies and publications that cited the included studies were also screened for inclusion.Finally, studies published between April