Bioelectrical Responses to Resistance Training Performed to Momentary Failure or with Repetitions in Reserve: A Within-Subject Analysis of Phase Angle and Impedance Components

Resistance training (RT) may alter tissue electrical properties, but whether proximity to momentary muscular failure influences longitudinal bioelectrical responses remains unclear. This study compared eight weeks of unilateral RT performed to failure (FAIL) or with repetitions in reserve (RIR) on segmental phase angle (PhA), resistance (R), and reactance (Xc). Nineteen resistance-trained adults (11 men: age 20.4 ± 1.8 years, height 172.7 ± 5.6 cm, body mass 74.9 ± 7.6 kg; 8 women: 24.8 ± 4.7 years, 164.1 ± 8.0 cm, 63.0 ± 10.1 kg) completed twice-weekly unilateral preacher curl and leg extension training, with contralateral limbs randomized to FAIL or RIR. Outcomes were assessed at baseline, week 4, and week 8 using Bayesian Gaussian mixed-effects models, with sex-specific interaction analyses treated as exploratory. Week-8 FAIL-RIR PhA contrasts were −0.079 (95% CrI: −0.440 to 0.286) and −0.065 (−0.390 to 0.256) in women’s and men’s arms, and −0.014 (−0.329 to 0.298) and 0.011 (−0.284 to 0.302) in their legs. With female as the reference category, week-8 protocol × time estimates for R were 0.922 (95% CrI: −23.693 to 25.946) in the arm and −4.106 (−17.415 to 9.360) in the leg; corresponding Xc estimates were 0.191 (−2.301 to 2.643) and −0.762 (−2.506 to 0.958), respectively. These findings are limited to segmental limb-level bioelectrical estimates and should not be interpreted as localized responses of the specifically trained muscles.