Mental fatigue impairs the number of repetitions to muscular failure in the half back‐squat exercise for low‐ and mid‐ but not high‐intensity resistance exercise.

The study aimed to analyze the acute effect of mental fatigue on the maximum number of repetitions in a resistance exercise session with different intensities in resistance‐trained adults. Eighteen young men aged between 18 and 25 years old (age, 22.1 ± 2.0 years; body weight, 82.5 ± 6.6 kg; height, 177.4 ± 5.2 cm; half‐back squat 1‐RM, 106.7 ± 21.9 kg) were recruited for the study. Each participant performed two trials (i.e., control and mental fatigue) in a random and balanced order for 30‐min. The participants performed three sets of half back‐squat exercise to failure, with intensities of 50%, 70%, and 90% of 1RM with a passive recovery interval of 5‐min between sets. The intensity was randomized and counterbalanced, and the order was maintained in both conditions for the same subject. We assessed resistance training using the number of repetitions to failure and perceived effort and checked the mental fatigue subjectively and objectively. The participants in the mental fatigue condition presented a significantly increased perception of mental fatigue (p < 0.05) and reduced pupil diameter (p < 0.05). The number of repetitions were significantly lower for the 50 (p < 0.05) and 75% 1RM (p < 0.05) in the mental fatigue condition, but the 90% 1RM remained similar (p > 0.05). Also, the perceived effort showed significantly higher results for the 50 (p < 0.05) and 75% 1RM (p < 0.05) in the mental fatigue condition, but the 90% 1RM remained similar (p > 0.05). Then, this study showed that mental fatigue reduced resistance exercise performance for low‐ and mid‐intensity but not for high‐intensity. Highlights: Mental fatigue reduces the number of repetitions to failure during low‐ and moderate‐intensity resistance exercise.Mentally fatigued individuals experience a higher‐than‐normal perception of effort during low‐ and moderate‐intensity resistance exercise.Measuring mental fatigue immediately before a resistance exercise session might be a rapid and valuable tool to autoregulate the exercise intensity and/or volume of resistance training sessions. [ABSTRACT FROM AUTHOR]