Randomised trial — 2016 — Hypertrophy · Strength · Recovery
Longer Interset Rest Periods Enhance Muscle Strength and Hypertrophy in Resistance-Trained Men
What did the researchers investigate?
Whether 1-minute or 3-minute rests between sets changed strength, muscle endurance and muscle thickness (by ultrasound) in 21 young, resistance-trained men over 8 weeks. Everything else was held constant: 3 full-body workouts a week of 3 sets of 8-12RM on 7 exercises.
What did they find?
Maximal strength was significantly greater with 3-minute rests for both the squat and the bench press. Muscle thickness was significantly greater in the front of the thigh, with a trend toward greater growth in the triceps (p = 0.06). Both groups improved upper-body muscle endurance, with no significant difference between them.
What does this mean for training?
In experienced young men, longer rests produced more strength and thigh growth over eight weeks than minute-long rests.
How does FitX AI use this evidence?
One of the studies behind the rest timer's longer recommendations. It compared only 1 and 3 minutes, so it cannot set the exact 2, 2.5 and 3 minute defaults; those remain 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.
Limitations
One small study (21 men, 8 weeks) in young, experienced lifters, so results may not apply to women, beginners or older adults. A pooled analysis of muscle growth (Singer 2024) found little added benefit beyond about 90 seconds, so findings across studies are not fully consistent.