Meta-analysis — 2024 — Hypertrophy · Recovery
Give it a rest: a systematic review with Bayesian meta-analysis on the effect of inter-set rest interval duration on muscle hypertrophy
What did the researchers investigate?
How the rest between sets affects muscle growth, using Bayesian meta-analysis of 9 randomised studies (19 measurements).
What did they find?
Results suggest a small benefit to resting more than 60 seconds, perhaps through better performance on later sets. No appreciable difference was found when resting more than 90 seconds. Training to failure or not did not change this.
What does this mean for training?
Rest long enough to perform well on the next set: more than a minute, with little to gain past about 90 seconds for growth.
How does FitX AI use this evidence?
When FitX AI fits your plan to your session length, it allows about 2 minutes per set for big lifts and 90 seconds for small ones, including rest. The rest timer recommends 2 to 3 minutes, which is longer than the roughly 90 seconds past which this analysis saw no difference in muscle growth; the longer defaults rest on the strength evidence below and are product choices.
Where FitX AI's own choices go beyond this paper
Session length. The per-set time estimates (2.75 and 2.25 minutes, plus warm-up) are planning figures, not research findings.
Rest timer. The 180, 150 and 120 second defaults and the three exercise categories are FitX AI product choices; the timer is a recommendation and never forces a rest. The evidence is mixed. For muscle growth alone, the pooled analysis found little difference beyond about 90 seconds, while trained lifters gained more strength (and in one trial more thigh growth) with rests of about 2-3 minutes. No paper sets different rests for heavy compound, other compound and isolation exercises.
Session check. Flagging a heavy squat or hinge with a top of range above 12 and 3+ sets, more than 6 hard sets of those lifts in a session, and three same-movement exercises for one muscle are FitX AI heuristics. Exercise order (early exercises get more reps and progress more for strength) is from reviews logged as candidates, not yet in the library. Fatigue varies by person, so these are pointers, never limits.
Limitations
9 studies with substantial heterogeneity in results.