Effect of Exercise Intensity and Thermal Strain on Wildland Firefighters' Central Nervous System Fatigue

The arduous conditions (i.e., harsh environmental conditions, high physical and mental demands) in which wildland firefighters (WFFs) have to perform their work during wildfire suppression can lead to states of both physical and mental fatigue. Although several studies have delved into the first type of fatigue, there is a paucity of research on the decrease in WFFs’ cognitive performance. A decreased cognitive performance has been observed throughout multi-day suppression tasks, which could lead to poor decision-making and have unintended consequences on deployments. To our knowledge, the acute effect of tasks performed by WFFs on cognitive fatigue has not been studied. Therefore, the aim of this study was to analyze the effect of performing a specific circuit, which simulated the tasks performed by WFFs in their deployments, on central nervous system fatigue. Ten WFFs (34.4 ± 5.6 yr, 182.9 ± 6.1 cm and 92.8 ± 14.9 kg) participated in the study, who performed a field test composed of 4 specific tasks commonly used in their deployments. Each task was executed for 5 min interspersed by 90 s of recovery. This interval bout was repeated twice with 10 min of recovery in between. During the test heart rate (HR) and core temperature (CT) response were monitored. Both variables were used to calculate the physiological strain index (PSI). In addition, before and at the end of the field test, subjects’ critical flicker fusion (CFF) threshold was measured. The results obtained showed that the WFFs performed a high-exercise demand (mean HR, 85.3 ± 2.5% of maximal HR; CT, 38.3 ± 0.4 ºC and PSI, 6.0 ± 0.7). Despite this, CFF threshold measurements showed an increase (6.0 ± 6.0%, p < 0.05) in the sensory sensitivity threshold, suggesting an exercise induced increase in cortical arousal. Significant (p < 0.05) relationships between sensory sensitivity and time spent at high percentage of maximal HR (>90%), TC and PSI were found (r = -0.71, -0.74 and -0.69, respectively). In conclusion, the specific field test led an enhancement of sensory sensitivity and cortical arousal. However, the correlations found seem to indicate the potential negative effect of high-intensity exercise and thermal strain on central nervous system fatigue.