Combining HIIT Plus Cognitive Task Increased Mental Fatigue but Not Physical Workload in Tennis Players

Mental fatigue can significantly impair physical performance during sports practice. However, previous studies have not analyzed the impact of combining High-Intensity Interval Training (HIIT) with a cognitive task on physical performance. In this line, we developed the present research to investigate the specific aspects of mental and physical fatigue as well as the potential impact of combining HIIT with a cognitive task. Therefore, the objective of this study was to compare mental and physical fatigue indicators between single HIIT and an incongruent Stroop task added to the HIIT protocol. A cross-sectional study was conducted with 32 tennis players (25 men and 7 women, mean age = 21.40). Two HIIT sessions were performed: (1) one with the incongruent Stroop task presented during rests, and (2) another without the Stroop task during rests. Mental fatigue, mental load, and wellness were measured. Additionally, mean and maximum heart rate, mean and peak speed, and heart rate variability through RMSSD were recorded using a heart rate monitor. Mann-Whitney U tests and Wilcoxon tests were performed to explore differences between during-session loads and pre- and post-HIITs. The results showed that mixed HIIT with the incongruent Stroop task caused higher increases in mental fatigue (p = 0.04) and the ratio of perceived exertion (p = 0.001) from pre- to post-HIIT. However, no significant differences in external (speed) and internal (heart rate) factors were observed between the protocols. This suggests that subjective feelings of physical exertion increased significantly due to mental fatigue. However, it did not decrease the physical workload during HIIT for tennis players. Therefore, incorporating short periods of the incongruent Stroop task during HIIT could be used to develop tolerance to mental fatigue without a decrease in the workload.