Effects of 12 Weeks of Interval Block Resistance Training Versus Circuit Resistance Training on Body Composition, Performance, and Autonomic Recovery in Adults: Randomized Controlled Trial.

Interval block resistance training (IBRT) and circuit resistance training (CRT) are periodization models aimed at enhancing neuromuscular and metabolic adaptations. This study aims to evaluate the effects of a 12-week IBRT program compared to CRT on body composition, muscle strength, speed, functional capacity, and autonomic recovery in young Chilean adults. A randomized, parallel, double-blind study was conducted with 30 participants assigned to IBRT (n = 15) or CRT (n = 15). Assessments included body mass index (BMI), waist circumference, right-hand grip strength, the running anaerobic sprint test (RAST), the 6 min walk test (6 MWT), and heart rate variability (HRV) indices: low-frequency to high-frequency ratio (LF/HF) and root mean square of successive differences (RMSSD, a time-domain HRV metric reflecting parasympathetic activity). Statistical analyses included -tests and ANCOVA. Groups were similar in age (IBRT: 25.2 ± 3.19; CRT: 23.27 ± 3.69, = 0.14) and BMI (IBRT: 21.56 ± 2.22; CRT: 22.36 ± 1.70 kg/m, = 0.40). Both groups improved significantly in waist circumference (IBRT: -1.85%; CRT: -2.37%), grip strength (IBRT: +5.47%; CRT: +4.02%), RAST (IBRT: -2.67%; CRT: -1.04%), 6 MWT (IBRT: +4.53%; CRT: +2.17%), LF/HF (IBRT: -11.43%; CRT: -5.11%), and RMSSD (IBRT: +5.36%; CRT: +3.81%) (all ≤ 0.01). IBRT produced significantly greater gains in 6 MWT (B = 19.51, 95% CI: 0.79 to 38.23, = 0.04). Both IBRT and CRT effectively improved body composition, muscle strength, speed, functional capacity, and autonomic recovery. However, IBRT demonstrated a superior effect on aerobic capacity.