Core-Perception Coupling: Relationships among Core Temperature, the Rating of Perceived Exertion, and Thirst during...

Hydration status modulates both exercise performance capacity and exercise tolerance. This study aimed to examine within-trial relationships among core temperature (Tc), ratings of perceived exertion (RPE), and thirst during moderate continuous cycling, and to test whether baseline hydration status would moderate these associations. Thirty trained men (34.7 ± 6.1 years; VO₂max 48.8 ± 5.8 ml·kg⁻¹·min⁻¹) completed up to 120 min of cycling at 50% Wmax under thermoneutral conditions. Participants were randomized to an experimental group receiving structured hydration counseling (EXP, n = 16) or a control group following habitual fluid intake (CON, n = 14). Hydration status was verified via urine specific gravity; Tc (CORE heat-flux sensor), the heart rate, RPE (0-10), and thirst (0¬-10) were recorded every 15 min. Analyses included repeated-measures tests, Spearman correlations, and ANCOVA adjusting for baselines and VO₂max. Tc rose from rest and tended to plateau by ~60-75 min, whereas RPE and thirst increased throughout. Hydration status predicted higher RPE (partial η² = 0.41, p < 0.001) with Tc not independently significant (p = 0.81); VO₂max was not a significant predictor, although the effect estimate suggested an inverse association with RPE (p = 0.058). For thirst, hydration status remained significant (partial η² = 0.27-0.50, p ≤ 0.005). Comparing the two groups, the CON group showed greater increases in RPE (ΔRPE mean +0.23 ± 0.05, p < 0.001; ΔRPE peak +0.53 ± 0.10, p < 0.001) and thirst (ΔThirst mean +0.23±0.04, p < 0.001). EXP participants more often completed the 120-min trial (75.0% vs. 28.6%; χ² = 10.57, p = 0.014). Perceptual coupling was strong (ΔRPE-ΔThirst r_s≈0.84), with ΔRPE inversely related to VO₂max and ΔTc modestly. Starting exercise euhydrated reduces perceived effort and thirst during prolonged, thermoneutral cycling, while Tc per se adds little explanatory power for RPE after adjustment. Simple field markers (thirst, heart rate) can flag emerging strain, particularly in less fit individuals.