Exercise‐heat stress with and without water replacement alters brain structures and impairs visuomotor performance

Effects of exercise‐heat stress with and without water replacement on brain structure and visuomotor performance were examined. Thirteen healthy adults (23.6 ± 4.2 years) completed counterbalanced 150 min trials of exercise‐heat stress (45°C, 15% RH) with water replacement (EHS) or without (~3% body mass loss; EHS‐DEH) compared to seated rest (CON). Anatomical scans and fMRI Blood‐Oxygen‐Level‐Dependent responses during a visuomotor pacing task were evaluated. Accuracy decreased (P < 0.05) despite water replacement during EHS (−8.2 ± 6.8% vs. CON) but further degraded with EHS‐DEH (−8.3 ± 6.4% vs. EHS and −16.5 ± 10.2% vs. CON). Relative to CON, EHS elicited opposing volumetric changes (P < 0.05) in brain ventricles (−5.3 ± 1.7%) and periventricular structures (cerebellum: 1.5 ± 0.8%) compared to EHS‐DEH (ventricles: 6.8 ± 3.4, cerebellum: −0.7 ± 0.7; thalamus: −2.7 ± 1.3%). Changes in plasma osmolality (EHS: −3.0 ± 2.1; EHS‐DEH: 9.3 ± 2.1 mOsm/kg) were related (P < 0.05) to thalamus (r = −0.45) and cerebellum volume (r = −0.61) which, in turn, were related (P < 0.05) to lateral (r = −0.41) and fourth ventricle volume (r = −0.67) changes, respectively; but, there were no associations (P > 0.50) between structural changes and visuomotor accuracy. EHS‐DEH increased neural activation (P < 0.05) within motor and visual areas versus EHS and CON. Brain structural changes are related to bidirectional plasma osmolality perturbations resulting from exercise‐heat stress (with and without water replacement), but do not explain visuomotor impairments. Negative impacts of exercise‐heat stress on visuomotor tasks are further exacerbated by dehydration.