Passive Heat Stimuli as a Systemic Training in Elite Endurance Athletes: A New Strategy to Promote Greater Metabolic...

Objectives: The ability to efficiently regulate body temperature is crucial during endurance activities such as trail running, especially during competitive events in hot conditions. Over the past decade, passive hyperthermia exposure has grown significantly in popularity as a means of improving acclimatization and performance in hot environments. The present study aims to compare the physiological changes that occur in a group of professional athletes due to passive sauna exposure (80-90 °C) and their own response to maximal aerobic performance. Methods: Twelve professional trail runners (eight men and four women) were tested in three conditions: (i) baseline; (ii) before; and (iii) after (a) passive dry sauna exposure and (b) a maximal endurance test. In both cases, physiological parameters such as heart rate, tympanic temperature, arterial and muscle oxygen saturation, and blood concentrations of glucose, total cholesterol, high-density lipoprotein (HDL) and hemoglobin were measured. Results: Sauna exposure produced similar trends in cardiovascular and metabolic responses to those occurring during exercise, but at a much lower physiological level. Glucose and HDL levels were both significantly elevated (or tended to be so) after sauna and exercise (p < 0.03 and p < 0.01, respectively). Athletes who mobilized the sum of substrates (glucose and HDL) performed the exercise test faster (r = -0.76; p < 0.004). The response of arterial oxygen saturation (decreased) was similar during sauna and exercise, but opposite at the muscular level (increased during sauna and decreased during exercise). Additionally, inter-individual variability in responses was noted for most of the other parameters, suggesting the existence of 'responders' and 'non-responders' to thermal stimuli. Conclusions: The physiological responses of trained endurance athletes are moderately impacted by passive sauna use. However, individual changes could be correlated with endurance performance and optimizing individualization. Heat stimuli promote different physiological responses in terms of cardiac function, oxygen kinetics and substrate mobilization, albeit to a lesser extent than exercise. Greater substrate mobilization during maximal endurance exercise was found to be correlated with better performance. Further studies are needed to explore the concepts of metabolic flexibility, as described here, and how heat exposure may improve systemic health and performance.