Abstract
Objective
The purpose of this study was to identify potential phenotypic and biological factors that relate to directional changes in subjective feelings of mental and physical energy and fatigue during exercise-heat stress.
Methods
Fifteen subjects (female = 6; 29 ± 9 years; 71 ± 14 kg) completed 90 min of cycling in the heat with (euhydration) or without fluid (dehydration). Subjective feelings of mental and physical energy and fatigue were measured using visual analog scales, and subjects were bifurcated based on the directionality of mood. Changes in mood were correlated with baseline phenotypic characteristics and biofluid composition.
Results
Subjective feelings of mental (median change: 60.5 (hypohydrated) vs. 18.0 (euhydrated), p = 0.03) and physical fatigue (77.0 (hypohydrated) vs. 57.0 (euhydrated), p < 0.01) were greater post-exercise with dehydration, while feelings of mental (−27.0 (hypohydrated) vs. −18.5 (euhydrated), p = 0.17) and physical energy (−47.0 (hypohydrated) vs. −44.0 (euhydrated), p = 0.39) were not different compared to euhydration. Higher body mass index (p = 0.07), fat-free mass (0.03), and a more coordinated cardiorespiratory system (p < 0.01) were related to a decrease in mental energy and increased mental fatigue. Changes in blood carbon dioxide (p = 0.02) were greater in subjects who reported an increase in mental energy, while subjects who reported an increase in mental fatigue had lower decreases in blood chloride (p = 0.04). Subjects who reported an increased physical energy had a lower decrease in hemoglobin (p = 0.04).
Conclusions
Our findings support previous work suggesting baseline characteristics and blood biomarkers may be associated with mood states.
Fatigue: Biomedicine, Health & Behavior (2026) 14(3), 256–271
