Meta-analysis — 2024 — RPE & autoregulation · Hypertrophy · Strength
Exploring the Dose–Response Relationship Between Estimated Resistance Training Proximity to Failure, Strength Gain, and Muscle Hypertrophy: A Series of Meta-Regressions
What did the researchers investigate?
The dose-response between proximity to failure, estimated as repetitions in reserve (RIR), and gains in strength and muscle size, using exploratory meta-regressions.
What did they find?
Strength gains were similar across a wide range of RIR. Muscle growth increased as sets were ended closer to failure. Because RIR was estimated from study descriptions, the exact relationship remains unclear.
What does this mean for training?
For muscle growth, finishing sets with few reps left matters; for strength, there is more room to stop earlier. Effort is worth tracking.
How does FitX AI use this evidence?
FitX AI asks how many reps you had left after every set and raises the effort across the block, ending close to failure in week 4.
Where FitX AI's own choices go beyond this paper
RPE mesocycle. The 7, 8, 9, 10 ladder and the 5-week length are FitX AI design choices. The papers support training near failure and RPE/RIR as effort scales, not this exact schedule.
Reps-left feedback. Reps-left is self-reported; accuracy varies between people, so one rating moves targets only a little.
8-15 working range. 8-15 is a practical default range, not a research-derived limit. 6-10 for barbell squats and hinges, and RPE 9 as their ceiling, are FitX AI choices to limit the fatigue of the most demanding lifts: no paper sets these numbers. A range the user has set is never changed.
Limitations
Exploratory, with RIR estimated rather than measured in most studies, and only modest model fit. The authors advise caution.