The residual effect of sauna induced heat stress on cognition, neuromuscular function, and stress hormones level ; Pirties sukelto šiluminio streso liekamasis poveikis kognityvinei ir neuroraumeninei funkcijoms bei streso hormonų kaitai

Finnish sauna bathing, an activity that promotes relaxation and wellbeing, has become increasingly popular worldwide. Regular sauna bathing is associated with reduced risks of fatal cardiovascular diseases and allcause mortality (Laukkanen, Khan, Zaccardi, & Laukkanen, 2015), and acts as a protective lifestyle factor against common memory diseases (e.g., dementia and Alzheimer’s disease) (Laukkanen, Kunutsor, Kauhanen, & Laukkanen, 2017). Sauna bathing has also been used as a thermal therapy to treat pain, aches, and other symptoms of rheumatic disease (Isomäki, 1988; Nurmikko & Hietaharju, 1992). The usual sauna visit consists of short stays (5–20 min) in dry air (relative humidity (rh) of 10–20%) at a relatively high temperature (air temperature (Ta) 80–100 °C), which induces short-term mild to severe whole-body hyperthermia (Leppäluoto, Tuominen, Väänänen, Karpakka, & Vuor, 1986). Hyperthermia is a physiological stress (Moran, Shitzer, & Pandolf, 1998), under such thermal conditions, the heat-stressed brain operates at lower levels of blood and energy supply, resulting in impairments in brain neural networks (Nybo, 2010; Rasmussen, Stie, Nybo, & Nielsen, 2004) and metabolism (Nybo, Møller, Volianitis, Nielsen, & Secher, 2002) which lead to suppression of cognitive function (Racinais, Gaoua, & Grantham, 2008) and motor inhibition (Brazaitis et al., 2015; Nybo & Nielsen, 2001). However, little is known about the functional residual consequences of recovering body temperatures from moderate whole-body hyperthermia in a thermoneutral environment on the effectiveness of cognition, motor performance and stress hormones level. This knowledge gap is surprising given that in the heat-stressed brain, the water, hormonal and neurotransmitter balances are disturbed and brain morphology is altered (Kiyatkin, 2005, 2010), and cognitive modulation (dysfunction or arousal) may persist into the recovery period after removal from the sauna heat or heat per se. Also disturbance in water and hormonal balance, due to induced central fatigue (Roelands & Meeusen, 2010; Zhao et al., 2015), can modulate motor performance. The aim of the research There are no studies showing post-sauna residual consequences when body temperature naturally decreases to initial level. The aims of this study were to induce whole-body moderate hyperthermia (rectal temperature (Tre) ~38.5 °C; (Lucas et al., 2015)) and simultaneously stress the brain (~0.3 °C warmer than arterial blood entering the brain; Nybo et al., 2002) in subjects using a traditional sauna, and to investigate the post-sauna residual consequences on brain neural network arousal, information processing, cognitive and neuromuscular performance, and stress hormones level. Research objectives 1. To investigate post-sauna residual consequences on brain neural network arousal and cognitive processing (information processing and decision making) in cortical level and cognitive performance (spatial rotation ability and working memory) on behavior level when body temperature naturally decreases to preheating level. 2. To examine post-sauna residual consequences on stress hormones (free cortisol, dopamine, noradrenaline and serotonin) level and investigate how these changes influence neuromuscular performance when body temperature naturally decreases to initial level. Research hypotheses 1. A higher electroencephalography (EEG) alpha power, appearing mostly in a state of relaxation, is associated with reduced cortical activity (Klimesch, 1999) and is inversely related to cognitive processing (Lang, Lang, Kornhuber, Diekmann, & Kornhuber, 1988). We expected that postsauna 90 min recovery would produce enhanced relaxation in the brain neural network, which would be characterized by an increase in resting EEG alpha activity, and lead to impairments in cognitive processing (stimulus discrimination, evaluation, and categorization) and cognitive (reaction time and accuracy). In view of the greater complexity (anatomical and physiological) of the auditory system compared with the visual system (Duncan, Kosmidis, & Mirsky, 2005), greater post-sauna heat-stress modulation of cognitive processing would be more pronounced in tasks with auditory than with visual stimuli. Since the tasks performed with higher complexity are more vulnerable to stressors than simple tasks (Hocking, Silberstein, Lau, Stough, & Roberts, 2001), we expect that with altered brain homeostasis during heat stress (Shibasaki, Namba, Oshiro, Crandall, & Nakata, 2016; Shibasaki, Namba, Oshiro, Kakigi, & Nakata, 2017), reduced cortical activity after heat exposure (Nybo et al., 2002) will induce fatigue, impair attention and concentration, and as a result, disturbed spatial rotation ability and working memory. 2. We hypothesized that during recovery 2 h after passive hyperthermia induced by a sauna without cooling or fluid replacement, free cortisol concentration would remain elevated and that this elevated cortisol level would improve muscle contractile properties (Pääkkönen & Leppäluoto, 2002) during maximal voluntary contraction (MVC) and electrically induced muscle contractions. Decreased noradrenaline level (Berridge & Foote, 1991) and increased serotonin level (Yu et al., 2011) may induce greater central fatigability (Roelands & Meeusen, 2010; Zhao et al., 2015). These factors can provoke greater force fluctuation (Keller-Ross, Schlinder-Delap, Doyel, Larson, & Hunter, 2014; Mesin, Dardanello, Rainoldi, & Boccia, 2016) during the contraction of ankle plantar flexor at 50% of an MVC for 120 s, and lower muscle steadiness should lead to greater overall (central and peripheral) neuromuscular fatigability. Originality, novelty and practical significance of the research Little is known about residual sauna effect. Studies investigating postheat recovery usually use different cooling methods (fans, cold shower, ice packs, cooling costumes and etc.; Hannuksela & Ellahham, 2001; Kukkonen-Harjula et al., 1989; Wanda Pilch et al., 2013; Shibasaki et al., 2017; Thomas, Cheung, Elder, & Sleivert, 2006). Cold stimuli in hyperthermic subjects can blunt the natural residual consequences of postheat recovery on perception, behavior, and performance (Cabanac, Massonnet, & Belaiche, 1972; Craig, 2002; Gagge, Stolwijk, & Hardy, 1967). To the best of our knowledge, the multipurpose post-sauna-recovery effect (with no any cooling methods) on cognition, motor performance, and hormonal level have not been investigated yet. The findings of this study highlight the physiological response to natural post-sauna recovery. We believe that the results of this study will provide facts for better understanding of heat residual effect on human physiological state and will be useful for practical implications in occupational and recreational health as well as well-being. CONCLUSIONS 1. Recovery to normothermia after a sauna leads to a greater resting neural network relaxation followed by increases in cognitive processing economy for a given oddball task. By contrast with our expectation, the auditory processing was not more affected by post-sauna recovery than was visual processing. Post-sauna recovery modifications in ERP components (stimulus processing) were insufficient to affect cognitive performance (reaction time and accuracy) in both visual and auditory task modalities. Residual heat stress increases the level of sleepiness and induces higher feeling of fatigue, but that does not impair attention and concentration. Moreover, residual heat stress improves spatial rotation function. 2. We show that post-sauna recovery was accompanied by slowed salivary free cortisol diurnal kinetics, whereas the heat stress-induced increased NA, DA, and 5-HT levels (McMorris et al., 2006) did not persist into the 2 h recovery after the sauna. Although recovery to normothermia after the sauna caused greater acceleration of muscle contractile properties and decreased muscle steadiness, sustained (120 s) isometric submaximal contraction (50% of MVC) did not provoke greater neuromuscular fatigability.