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Nóbrega, SR, Ugrinowitsch, C, Pintanel, L, Barcelos, C, and Libardi, CA. Effect of resistance training to muscle failure vs. volitional interruption at high- and low-intensities on muscle mass and strength. J Strength Cond Res 32(1): 162–169, 2018—The purpose of this study was to investigate the effects of resistance training (RT) at high- and low-intensities performed to muscle failure or volitional interruption on muscle strength, cross-sectional area (CSA), pennation angle (PA), and muscle activation. Thirty-two untrained men participated in the study. Each leg was allocated in 1 of 4 unilateral RT protocols: RT to failure at high and low intensities, and RT to volitional interruption (repetitions performed to the point in which participants voluntarily interrupted the exercise) at high (HIRT-V) and low (LIRT-V) intensities. Muscle strength (1 repetition maximum [1RM]), CSA, PA, and muscle activation by amplitude of the electromyography (EMG) signal were assessed before (Pre), after 6 (6W), and 12 (12W) weeks. 1RM increased similarly after 6W (range: 15.8–18.9%, effective size [ES]: 0.41–0.58) and 12W (range: 25.6–33.6%, ES: 0.64–0.98) for all protocols. All protocols were similarly effective in increasing CSA after 6W (range: 3.0–4.6%, ES: 0.10–0.24) and 12W (range: 6.1–7.5%, ES: 0.22–0.26). PA increased after 6W (∼3.5) and 12W (∼9%; main time effect, p < 0.0001), with no differences between protocols. EMG values were significantly higher for the high-intensity protocols at all times (main intensity effect, p < 0.0001). In conclusion, both HIRT-V and LIRT-V are equally effective in increasing muscle mass, strength, and PA when compared with RT performed to muscle failure.