Core and skin temperature influences on the surface electromyographic responses to an isometric force and position task.

The large body of work demonstrating hyperthermic impairment of neuromuscular function has utilized maximal isometric contractions, but extrapolating these findings to whole-body exercise and submaximal, dynamic contractions may be problematic. We isolated and compared core and skin temperature influences on an isometric force task versus a position task requiring dynamic maintenance of joint angle. Surface electromyography (sEMG) was measured on the flexor carpi radialis at 60% of baseline maximal voluntary contraction while either pushing against a rigid restraint (force task) or while maintaining a constant wrist angle and supporting an equivalent inertial load (position task). Twenty participants performed each task at 0.5°C rectal temperature (Tre) intervals while being passively heated from 37.1±0.3°C to ≥1.5°C Tre and then cooled to 37.8±0.3°C, permitting separate analyses of core versus skin temperature influences. During a 3-s contraction, trend analysis revealed a quadratic trend that peaked during hyperthermia for root-mean-square (RMS) amplitude during the force task. In contrast, RMS amplitude during the position task remained stable with passive heating, then rapidly increased with the initial decrease in skin temperature at the onset of passive cooling (p = 0.010). Combined hot core and hot skin elicited shifts toward higher frequencies in the sEMG signal during the force task (p = 0.003), whereas inconsistent changes in the frequency spectra occurred for the position task. Based on the patterns of RMS amplitude in response to thermal stress, we conclude that core temperature was the primary thermal afferent influencing neuromuscular response during a submaximal force task, with minimal input from skin temperature. However, skin temperature was the primary thermal afferent during a position task with minimal core temperature influence. Therefore, temperature has a task-dependent impact on neuromuscular responses.

Effects of Motivational Self-Talk on Endurance and Cognitive Performance in the Heat.

PURPOSE: We tested the effectiveness of a 2-wk motivational self-talk (MST) intervention-specific to heat tolerance-on endurance capacity and cognitive function in the heat. METHODS: Eighteen trained male (n = 14) and female (n = 4) cyclists randomly received 2 wk of MST training (n = 9) or a control regimen (CON, n = 9). The experimental protocol was a PRE/POST design consisting of 30 min of cycling at 60% peak power output (PPO) in the heat (35°C, 50% relative humidity, ~3.0 m·s airflow), a 30-min rest period, followed by a time to exhaustion (TTE) test at 80% PPO, and an identical rest period. Executive function, reaction time, and working memory were tested at baseline and each rest period. Key measures included TTE, speed and accuracy on the cognitive tests, rectal temperature, HR, oxygen consumption, and RPE. RESULTS: Group (MST vs CON) × test (PRE vs POST) × time repeated-measures ANOVA revealed that MST significantly increased TTE from PRE (487 ± 173 s) to POST (679 ± 251 s, P = 0.021) concurrent with a higher terminating rectal temperature (PRE, 38.5°C ± 0.2°C; POST, 38.8°C ± 0.4°C; P = 0.023); no TTE (PRE, 531 ± 178 s; POST, 510 ± 216 s; P = 0.28) or rectal temperature (PRE, 38.4°C ± 0.3°C; POST, 38.4°C ± 0.2°C; P = 1.000) changes were found in CON. MST significantly improved both speed and accuracy for executive function from PRE/POST, with no PRE/POST differences for CON on any cognitive measure. There were no interactions (all P > 0.05) for other key measures. CONCLUSION: Motivational self-talk is effective in altering the internal psychophysiological control of exercise and plays a role in improving endurance capacity and executive function in the heat.