Freedom of Choice? Effects of Self-Selected Rest Intervals on Muscular Adaptations

This study aimed to compare the effects of self-selected versus fixed inter-set rest intervals on muscular adaptations and training time efficiency during lower-body resistance training (RT) in resistance-trained adults. Forty resistance-trained participants were randomized to a self-selected rest interval group (SELF; n = 21) or a fixed ~2-minute rest interval group (FIXED; n = 19). Participants completed 8 weeks of supervised lower-body RT, performing four sets of 8–12 repetitions for the squat, leg extension, standing calf raise, and seated calf raise. Outcomes included ultrasound-derived muscle thickness (MT), dynamic and isometric strength, muscular endurance, training time, and session RPE. Both groups demonstrated similar improvements in MT, strength, and muscular endurance, with no meaningful between-group differences. Domain-level analyses supported these findings, showing trivial treatment effects for hypertrophy (−0.1 SD; 95% CrI: −0.2 to 0.0) and performance (0.0 SD; 95% CrI: −0.3 to 0.3). FIXED completed training sessions ~29% faster than SELF (56.0 ± 4.5 vs. 78.5 ± 15.4 min), while session RPE was similar (8.2 ± 1.0 vs. 8.1 ± 0.7 AU). Self-selected rest intervals produced comparable muscular adaptations to fixed rest intervals but required substantially more time. Prescribing ~2-minute rest intervals appears to optimize training efficiency without compromising adaptations.