Systematic review — 2017 — Hypertrophy · Recovery
The effects of short versus long inter-set rest intervals in resistance training on measures of muscle hypertrophy: A systematic review
What did the researchers investigate?
Whether short (60 seconds or less) or long (over 60 seconds) rests between sets lead to different muscle growth, in a systematic review of six studies of at least four weeks that measured changes in muscle mass.
What did they find?
Both short and long inter-set rest intervals may be useful for gains in muscle hypertrophy. Newer findings in trained participants, using measures sensitive enough to detect changes in muscle size, suggest a possible advantage for long rest intervals. Few studies had similar designs, so the authors say more research is needed to separate the two approaches.
What does this mean for training?
Rest need not be tightly controlled for muscle growth, though experienced lifters may gain a little more with longer rests.
How does FitX AI use this evidence?
Part of the reason the rest timer is a recommendation you can end early rather than a rule. It does not set the timer's 2 to 3 minute defaults, which are product choices.
Where FitX AI's own choices go beyond this paper
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 length. The per-set time estimates (2.75 and 2.25 minutes, plus warm-up) are planning figures, not research findings.
Limitations
Only six studies met the criteria, and the authors note the evidence was too thin to clearly separate the two approaches.