Whole body heat stress increases motor cortical excitability and skill acquisition in humans

Abstract

Objective: Vigorous systemic exercise stimulates a cascade of molecular and cellular processes that enhance central nervous system (CNS) plasticity and performance. The influence of heat stress on CNS performance and learning is novel. We designed two experiments to determine whether passive heat stress (1) facilitated motor cortex excitability and (2) improved motor task acquisition compared to no heat stress.

Methods: Motor evoked potentials (MEPs) from the first dorsal interosseus (FDI) were collected before and after 30 min of heat stress at 73 °C. A second cohort of subjects performed a motor learning task using the FDI either following heat or the no heat condition.

Results: Heat stress increased heart rate to 65% of age-predicted maximum. After heat, mean resting MEP amplitude increased 48% (p<0.05). MEP stimulus-response amplitudes did not differ according to stimulus intensity. In the second experiment, heat stress caused a significant decrease in absolute and variable error (p<0.05) during a novel movement task using the FDI.

Conclusions: Passive environmental heat stress (1) increases motor cortical excitability, and (2) enhances performance in a motor skill acquisition task.

Significance: Controlled heat stress may prime the CNS to enhance motor skill acquisition during rehabilitation.

Keywords: Cortical excitability; Heat stress; Motor learning.

Figure 1

Schematic timeline of the group receiving heat stress for experiment 1 (upper figure) and experiment 2 (lower figure). In both experiments there were control conditions consisting of ambient room temperature exposure.

Figure 2

Representative examples of single motor evoked potentials from transcranial magnetic stimulation (TMS) collected from a single male subject during the control condition and the heat condition. MEPs were recorded at the motor hotspot at a fixed intensity of 120% RMT.

Figure 3

The MEP responses to heat stress. (A) The Ratio (post/pre) of the MEP amplitude measured after 30 minutes of no heat stress (Control) or after 30 minutes of heat stress (Heat) (p < 0.05). (B) The Ratio of the change in MEP amplitude ((Heat (post/pre)/Control (post/pre)) for each subject in the study. Six of the eleven subjects (5 male; subjects 1, 3, 4, 8, 10) showed 49% or greater increase in MEP amplitude ratio after the heat condition compared to the control condition. (C) The MEP amplitude ratio for each intensity assessed during the TMS protocol. There was an overall significant main effect for heat stress in the Heat condition, (P = 0.05).

Figure 4

(A) Absolute error, normalized as the percent of block 1 error, depicted for the Control (black bars) and Heat (gray bars) groups showing an overall reduction in error during the Heat condition (p < 0.05). (B) Variable error, normalized as the percent of block 1 error, depicted by for the Control (black bars) and Heat (gray bars) groups showing an overall reduction in error during the Heat condition (p < 0.05). Error bars are standard errors. **indicates the first Block error that was significantly reduced from baseline (P <0.01) with all subsequent Block error groups less than baseline (p <0.01).