Functional Changes in Motor Cortical Brain Regions following Passive and Exertional Heat Stress

Abstract

Endurance performance is impaired when exercising in the heat. Hyperthermia-induced fatigue cannot be entirely explained by alterations in peripheral mechanisms. The central nervous system may have key roles in hyperthermia-induced fatigue and thus warrant further investigations. PURPOSE: To evaluate functional changes in the brain following passive or exertional heat stress. METHODS: Five moderately-trained athletes (mean ± SD: age 23.6 ± 1.7 years; body fat 9.3±2.0%; VO2max 58±6 ml kg-1 min-1) underwent a motor task-based blood-oxygen-level dependent (BOLD) fMRI scan while donning a water-perfused thermal suit in a familiarization and four randomized, counterbalanced trials. fMRI scans were conducted after (a) running on a motorized treadmill at 70% VO2max with ingestion of ambient water (EX) or (b) ice slurry (ICE), (c) passive heating via warm water immersion at 41°C (PAH), or (d) rest (CON). Serum osmolality was evaluated from blood samples collected at the start and end of each trial. Rectal temperature (Tre) and heart rate (HR) were assessed using two-way ANOVA with p < 0.05 considered as significant. RESULTS: Participants started all trials euhydrated (mean serum osmolality: 292 ± 1 mOsmol/kg; p= 0.719), with similar baseline HR (66 ± 2 beats/min; p=0.908) and Tre (36.9 ± 0.1°C; p=0.431). Post-intervention percent body mass change were similar across trials (-0.7 ± 0.2%; p= 0.110). Post-intervention Tre for EX (39.4 ± 0.2°C) was similar to PAH trial (39.3 ± 0.2°C; p = 0.59) but higher than ICE (39.0 ± 0.2°C; p = 0.01) and CON trials (36.5 ± 0.2°C; p < 0.01). Mean Tre achieved during post-intervention fMRI scan were 38.5 ± 0.1°C for both EX and PAH trials, 37.7 ± 0.1°C for ICE trial and 36.3 ± 0.1°C for CON trial. Task-based fMRI detected lower BOLD signals from primary motor cortex in PAH (fold changes from baseline: 0.46 ± 0.35) compared to EX trials (1.03 ± 0.21, p = 0.01). BOLD signals were also lower in primary somatosensory cortex for PAH (0.39 ± 0.23) compared to EX (1.04 ± 0.18, p < 0.01) and CON trials (0.98 ± 0.10, p = 0.02). CONCLUSION: Preliminary results from task-based BOLD analysis showed that passive heating led to functional suppression in the sensorimotor areas in the brain. Supported by DIRP Grant, PA No. 9015102335.