RESUMEN
Background: Whole-body hyperthermia (WBH) has an adverse effect on the nervous system and neurophysiological performance. In the present study, we examined whether short-duration whole-body immersion in 45°C water (HWI-45°C), which produces a strong neural and temperature flux without inducing WBH, can increase or impair neurophysiological performance in humans. Methods: Fifteen men (aged 25 ± 6 years) were enrolled in this study and participated in three experiments: 1) a brief (5-min) immersion of the whole body in 37°C water (WI-37°C); 2) a brief (5-min) HWI-45°C; and 3) a control trial in a thermoneutral condition at an ambient temperature of 24°C and 60% relative humidity. Before and after the immersions, neuromuscular function (electromyographic activity, reflexes, electrically and voluntary induced torque production, voluntary muscle activation level) were tested. To provoke central inhibition, the participants performed a sustained 2-min maximal voluntary contraction (MVC). Results: Thermophysiological strain was greater after HWI-45°C than after WI-37°C. Electrophysiological modulations of motor drive transmission and peripheral modulations of muscle contractility properties in response to HWI-45°C seemed to have little effect on central activation of the exercising muscles and no effect on MVC production. Conclusion: Although exposure to acute noxious heat was effective in evoking neuromuscular excitability, the increases in core temperature (â¼0.2°C) and muscle temperature (â¼0.6°C) did not induce moderate or severe WBH. These changes did not seem to affect central structures; that is, there were no additional increases in central and/or peripheral fatigue during a sustained 2-min MVC.
RESUMEN
PURPOSE: This study investigated the effects of thermal stimulation on corticomotor excitability with transcranial magnetic stimulation (TMS). MATERIAL AND METHODS: Participants consisted of healthy young adults (n = 20) and seniors (n = 15). Each experimental session consisted of a baseline (BL) assessment, followed by a warming and a cooling protocol. At BL, recordings of motor evoked potentials (MEPs) and skin temperature were performed with the index finger covered with a 'neutral' gel pack (24 °C). For warming, the same measurements were performed, but with the index covered with a warmed gel pack (45 °C). The gel pack was kept for 5 min, and the measurements were performed at 1 min during warming and 5 and 10 min post. After a break, participants were tested with the cooling protocol (gel pack 10 °C) by repeating the same sequence as in the warming. RESULTS: The two thermal protocols induced the desired range of skin temperatures (warming = 35-45°; cooling = 13-24°). For MEP modulation, the primary analysis revealed no main effects or interactions, owing to the variability of responses to either warming or cooling stimulation. Further analysis of individual responses revealed that modulation, when present, was short-lasting and was characterized by a depression in about half of the participants. Facilitation was also observed, but only in smaller clusters, especially with cooling (13/35). Modulation in MEP amplitude did not correlate with changes in skin temperature. CONCLUSION: These results are consistent with previous reports regarding variability in response to sensory stimulation protocols. In the case of thermal stimulation, such variability likely reflects individual differences in the influences exerted by thermal afferents centrally.