## 4 Discussion  
The present study aimed to evaluate the effects of including induced mental fatigue during a HIIT training session on physical load, fatigue, and mental fatigue. The main findings of the present study suggested that physical efforts were not significantly different between the mentally fatiguing and the control HIIT conditions. Therefore, mental fatigue did not impair physical performance during the HIIT. Despite this, the presence of induced mental fatigue caused higher increases in subjective feelings of physical and mental fatigue in comparison with conventional HIIT. The absence of significant differences in physical efforts between the mentally fatiguing and control conditions warrants a thorough discussion to provide potential explanations for these findings. One possible reason for the non-significant results could be the design of the HIIT protocol used in the study. It was shown that variations in the intensity, duration, and recovery intervals of HIIT can influence its impact on physical performance [36]. Additionally, the duration of the mental fatigue induction may have played a role in the lack of significant differences, although the markers of mental fatigue indicate that this duration significantly increased the feelings of mental fatigue between the HIIT with incongruent Stroop and the conventional Stroop. In this line of reasoning, the duration of the cognitive task can affect subsequent physical performance [37; 38]. Furthermore, individual variations in response to the cognitive task could have contributed to the non-significant findings. Other authors highlighted the existence of interindividual differences in susceptibility to mental fatigue and its impact on physical performance [39; 40]. By considering these factors, it becomes evident that the interaction between mental fatigue and physical efforts is complex and multifaceted. Future studies should further investigate the optimal design of HIIT protocols, the duration of mental fatigue inductions, and individual differences to better understand the underlying mechanisms and provide practical recommendations for coaches and practitioners. We hypothesized that mental fatigue would negatively influence the physical performance (behavioral outcomes) of tennis practitioners during HIIT. On the contrary, the results of the present study showed that the physical efforts quantified with mean and maximum HR and mean and maximum speed were not significantly different between the mentally fatiguing and the control HIIT conditions. Therefore, this hypothesis should be rejected. Despite previous authors reporting impairments in physical efforts during specific anaerobic-aerobic tests [19; 20; 21], it is also certain that most of these authors suggested that these impairments were caused by an increase in the values of RPE [41]. Higher values of RPE would cause a significant decrease in physical performance as a reduction in the time to exhaustion [42] or in the performance in aerobic performance competitions [12]. Although the physiological mechanisms involved in this phenomenon are unclear, the accumulation of adenosine in the brain appears to be a candidate [43; 44; 45]. However, it has also been suggested that there are brain mechanisms mediated by motivation that could counterattack this negative effect of increased RPE on physical performance [1]. In the present study, a significant increase in general fatigue (analogous to RPE) was also observed in the presence of mental fatigue without significative changes in objective (i.e., heart rate and speed variables) indicators of physical performance. Therefore, the presence of induced mental fatigue increased subjective feelings of fatigue during HIIT without changing physical performance. Future studies should test the effect of motivation as a moderator in the relationship between mental fatigue, RPE, and physical performance. In this regard, studies should be performed analyzing the electroencephalogram (EEG)signal before, during, and after mental fatiguing tasks and exercises.