Purpose Infrared thermography can be used to investigate the kinetics of thermoregulatory processes during exercise. The kinetics of the thermal contrast index (TCI), associated with activated perforasomes, and correlation with selected cardiovascular, metabolic, and subjective parameters have not been investigated during exercise. Methods After a 5 min warm-up, n = 21 male cyclists performed an incremental test on a cycling ergometer until volitional exhaustion, followed by 15 min passive recovery. Lactate, heart rate, energy expenditure, oxygen consumption, and rating of perceived exertion were monitored continuously. Skin temperature (Tsk) and TCI from anterior and posterior thigh were taken every 5 min following the TISEM checklist. TCI was calculated based on the temperature difference of the 10% hottest and 10% coldest pixels within the ROI. Results TCI increased significantly in 27 of 36 total pairwise comparisons of consecutive exercise increments (p < 0.0001 to <0.05, Delta 2.95 °C), while Tsk decreased in 20 increments (p < 0.0001 to <0.05, Delta 1.85 °C) from rest to exhaustion. During the recovery period, TCI declined significantly at 5-min post-exercise (400 W: 4.33 °C vs. post 5: 2.93 °C, p < 0.0001), whereas Tsk increased significantly after 10 min (400 W: 30.10 °C vs. post 10: 31.33 °C, p < 0.01). TCI showed stronger positive correlations with the internal load parameter (r = 0.48 to 0.72, all p < 0.0001) compared to the inverse correlations of Tsk (r = -0.27 to -0.41, p < 0.0001). Conclusions This study shows TCI reflects load changes more sensitively and correlates more strongly with physiological parameters than Tsk, highlighting TCI`s potential as a real-time load surrogate.
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