Results of this study indicated that passive heat acclimation (MHA) induced a certain impact on selected physiological/haematological parameters and skin temperature.
Prediction of EET test was that it strongly affected on skin temperature over muscles of lower and upper limbs. However, interesting and bigger changes occurred in torso, especially on back.
The drop of skin temperature over the selected regions of the upper part of body seems to be associated with cooling effect of sweat evaporation. Sweating is the most important thermoregulatory reaction by which heat balance is maintained following a rise in body temperature during endogenous exercise-induced heat load. During exercise, the highest evaporative heat loss following take place from back and chest , what was also observed in our study of cross-skiers.
It is known that heat dissipation, under conditions of exercise-thermal stress, is mediated primarily by evaporation of sweat. Heat acclimation has been shown to enhance sweat production by eliciting changes in the sensitivity of eccrine glands, total sweat output and distribution of gland activity. The physical training induces peripheral adaptations, while heat acclimation produces both peripheral and central modifications. It is suggested that repeated cutaneous heat stimulation may be essential to the development of central sudomotor changes [28, 29].
Untrained non-heat acclimated individuals are able to physiologically wet a maximum of approximately 72% of their skin surface. Increasing the number of sweat glands activated per surface area permit a more complete saturation of the skin surface and a great evaporation. Physiologically, the ability to achieve the better dynamics of sweating an improved acclimation status facilitated by a higher local sweating rate on both the arm and back mediated by a greater number of simultaneously activated sweat glands, which potentially facilitated an enhanced distribution of sweat on the skin surface and the attainment of greater rates of maximal evaporative heat loss .
Tyler et al.  have shown that resting temperature of skin is unaffected by HA of any duration, but HA has a large effect on reducing mean T skin during exercise. In our study, despite the lack of MHA effect on resting temperature (T1) there was a large effect of MHA on reducing mean superficial temperature over muscle zones after exercise in upper part of body (Table 1). According to our previous article, the marked skin temperature drop after similar exercise was visible in upper body in cross-country skiers and in swimmers in lower part of body , indicating that as changed to surface temperature distribution depends on the type of sport practiced.
It was reported that normal skin temperature pattern is affected for many hours after exercise more in some areas than others, depending on the activity of the structures below the ROI and, in a different way, according to the type of performed exercise . Our studies show that repeated sauna baths a slight influence on the pattern of skin temperature changes with respect to effort and restitution in elite athletes. Character of plots of changes in skin body temperatures is the same, but its amplitude is slightly different ΔT2–1 and ΔT3-1 in the anterior part of lower extremities (knees, vastus lateralis) and for torso back (trapezius muscle neck and back), respectively (Fig. 2).
We also showed that regular sauna baths had a moderate, positive effect on increasing resting PV (+ 2.13%). Data from meta-analysis  indicated that the size of PV increase after HA was influenced by the number of days of heat exposure the hydration state of the person. In trained men, daily post-exercise sauna bathing for 10–15 thirty minute sessions over 2–3 weeks (80–100 °C, 10–20% RH) caused an increase plasma volume 7–18% .
The MHA did not alter VO2; however, some reduction in rest HR after MHA was observed by us (Table 2). Willmott et al.  indicated no statistically significant changes HR in resting or after short-term HA. However, a significant heart rate decreases of 4% (after twice daily heat stress exposure for 2 days) and 6% (after once daily heat exposure for 4 days) was demonstrated during exercise. These authors reported that HR adjustment was typically concurrent with hypervolemia.
Application of sauna bathing changes only a little thermoregulatory process in elite athletes. Probably, the endurance athletes are exposed to endogenous thermal load, potentially constraining adaptation to heat during training process [18,19,20,21]. There are relative small information about MHA effect on vasodilatation, vasoconstriction and sweat evaporation to exercise in professional athletes. Therefore, further studies are needed in order to understand the mechanisms of thermoregulation, especially taking into account thermal stimulations using in biological regeneration of sportsmen as well as in medical treatment.
The main limitation of our study was the small number of participants recruited from the elite athletes. The second limitation could be relatively large variability in body sensitivity on sauna baths among the subjects.