## 4 Discussion
The EEG signal should be complemented with performance and subjective perception to explore all spheres. The results would elucidate the physiological reason behind the findings. Our results also showed a significant reduction in the mood state after the HIIT without cognitive tasks. In contrast, the values of mood state stayed stable after the HIIT with the Stroop task. We hypothesized that this reduction in the mood state could be related to motivation and boredom. The repetitive nature and the general fatigue induced by a HIIT session could make that participant's perceived mood state worse. On the contrary, the appearance of a novel task (such as the incongruuous Stroop) could have reduced the effects of boredom on mood states. Nevertheless, differences between sessions were not found. Thus, future studies should explore the role of Stroop or other cognitive tasks during physical exercise practice as a way to maintain motivation and decrease boredom during practice. The HRV provides information about the balance between the parasympathetic and sympathetic nervous systems [34]. Therefore, the HRV could reflect the interaction between brain and heart [46]. Thus, previous studies have used the HRV to prevent athletes from overtraining [47; 48]. Our results showed that the RMSSD had been reduced after the HIIT sessions. Similar results have been previously reported after exercise [49; 50; 51]. This could be due to increased sympathetic modulation due to the extra cardiac output required while exercising [52; 53]. In addition, previous studies have found that RMSSD could be sensitive enough to decrease during cognitive processing [54] or emotional stimuli [55]. However, our results did not show statistically significant differences between the two types of HIIT (with or without cognitive tasks). This could mean that although mental fatigue was perceived in the HIIT with cognitive load session, it was not enough to increase the fatigability of the autonomic nervous system. Thus, future studies should examine different types of cognitive tasks to explore if the autonomic modulation could change. Regarding the efforts reported during both protocols, the higher mental load and mental fatigue reported during the HIIT with cognitive load can be explained by the cognitive effort performed during the Stroop. Despite the fact that a single HIIT implies cognitive load, caused by self-pacing, attention, or the efforts to remember the execution of the HIIT [56; 57], the introduction of cognitively demanding tasks may increase the cognitive effort caused during a HIIT. This could confirm previous studies investigating Stroop's incongruent efficacy to induce cognitive load and fatigue in sports [57; 58; 2; 59]. Sustained attention might cause the cognitive effort, response inhibition, cognitive flexibility, and working memory demand that Stroop implies [1]. Therefore, conducting the incongruent Stroop task during the recovery periods of HIIT could induce mental fatigue in tennis players. Although it is difficult to generalize all this information based on two isolated training sessions, the results suggest important pre-indices and practical applications. In this regard, single HIITs produce significant increases in mental fatigue; therefore, coaches should not perform HIITs during competitions because of the perceived physical and mental fatigue after this training [59]. Indeed, adding an external (i.e., Stroop) cognitive demanding task could significantly enhance the increase caused by HIIT trainings on mental fatigue. Furthermore, the results of this investigation are relevant since they could confirm that subjective perceptions of mental fatigue can be induced using shorter cognitive stimuli during exercise practice. This information could be important for coaches in managing mental fatigue due to the use of a cognitive task in HIIT during season periods where resistance to mental fatigue has to be worked [60; 61].