# Acute Effect of Passive Hyperthermia on Lactate Concentrations
## 4 Discussion  
The present study evaluates the acute effect of exposure to high temperatures before and after an anaerobic lactic test in normothermia on lactate concentrations before and after 5 min of recovery. In addition, the effect of this pre-heat application on sports performance in an anaerobic test is evaluated. In our study, it was possible to observe multiple significances in the measurements taken during the tests. Firstly, referring to Figure 3, a significance of \(p<0.05\) was observed in the haematocrit values obtained between the pre-test and post-test measurements. The haematocrit values increase at the end of the test, which was observed in the three different tests performed. This could be due to haemoconcentration as a result of fluid loss and the reduction in plasma volume that occurs during physical activity, which also increases in warm environments [19]. Therefore, there is an increase in the percentage of red blood cells in whole blood in the early stages of exercise [20]. This haemoconcentration must be taken into account when measuring lactate. For this reason, the lactate values were corrected by the Van Beaumont Equation through haematocrit [21]. Elsewhere, between the lactate measurement pre-test and post-test, and between pre-test and Rec (Figure 4), of the three tests, a significant difference of \(p<0.05\) can be shown. These significant differences could be due to the fact that, when treating the test as a lactic anaerobic test, a logarithmic increase in lactate is produced by anaerobic glycolysis [4]; therefore, it is logical that these significant differences occur between pre- and post-exercise conditions. The application of heat just before the test produces an increase in lactate concentrations at the end of the test, but there were no significant changes. It has been reported that when exercising under anaerobic conditions, the use of glycogen via the glycolytic pathway predominates [2]. In addition, this mechanism is exacerbated in situations of heat stress, causing an increase in anaerobic glycolysis. The enzyme lactate dehydrogenase can convert pyruvate to lactate [5]. Therefore, an increase in blood lactate concentrations is expected to occur during intense exercise and even more under heat stress conditions before the test. Furthermore, the high temperatures have a potential to affect substrate supply and metabolite release upon modifying vasodilation [22; 23]. However, the application of the acute passive thermal effect of 10 min duration just before the test shows a greater elimination of lactate concentrations in blood when performing a passive 5 min recovery in normothermia. Figure 5: This figure shows the comparison between three conditions. In the hyperthermic pre-test condition, the pre-test lactate values are after the sauna bath. All values were corrected for haemoconcentration. It should be noted that there was no significant difference in lactate concentration between the post-test and after 5 min of recovery in the hyperthermia pre condition (Figure 4B), while in the first test (normothermia; Figure 4A), there was a significant increase after 5 min of recovery. In this respect, it is known that the sweating rate increases after exposure to heat [24]. Hence, it is possible that sweating was greater in the second test and that there was higher lactate elimination through sweating [25]. A recent study reported that heat acclimatization reduces sweat lactate [26]. Another possible reason is the elimination of lactate through urine [27; 28]; however, heat inhibits urine production through the action of ADH and the renin angiotensin system [1], so this pathway is implausible. In contrast, when applying the acute passive thermal stimulus for 5 min immediately after the end of the test (post hyperthermia) and looking at Figure 4C, we can observe that there is a significant decrease in Rec lactate concentrations compared to the post-test measurement.