## 1 Introduction
Among the mechanisms by which heat stress induces an increase in muscle mass, apart from calcineurin, the scientific literature describes an increase in the activity of serine-threonine phosphatase [20]. In turn, [PERSON] et al. observed no clear benefits of muscle heating in terms of hypertrophy during resistance training. At the same time,the authors also did not observe statistically significant differences in the dynamic force measured by 3-RM knee extension. Thermal stimuli also did not negatively affect the measured variables [21]. Heat stress can act as an effective post-exercise treatment and appears to be clinically beneficial for people who have difficulty participating in sufficient physical training, such as elderly or injured athletes [22; 23]. Interactions between relevant heat shock proteins after heat therapy and factors related to muscle hypertrophy are proven, but the mechanisms driving them remain unclear [24]. Among the factors regulating muscle mass, attention is paid to the effect of the thermal stimulus on the inhibition of protein degradation and mitochondrial clearance [22]. Heat has been shown to have a beneficial effect on muscle atrophy [24; 25]. Numerous beneficial angiogenic effects of heat and improvement of microcirculatory function are observed [14; 26; 27; 28]. Heat application compared to continuous moderate-intensity training in sedentary young men induced a similar increase in skeletal muscle capillarization [29]. The literature reports that heat stimulation increases shear stress, which may act as a secondary signal to stimulate angiogenesis further and contribute to its increase by releasing factors into the circulation that activate endothelial cell proliferation and migration [30]. The anti-inflammatory effect of heat is also well described in research [31]. In post-exercise muscle regeneration methods, it is assumed that warm stimuli have a relaxing effect on muscle tone and improve muscle elasticity and stiffness; still, more research is needed in this area [32; 33; 34; 35]. The evidence describing the mechanisms and the minimum duration of the therapy needed to achieve changes in the biomechanical properties of muscles is insufficient [36]. The analgesic effect of a warm stimulus is quite common; however, research results vary [32; 37]. This is primarily the result of a different methodology (time of application), different forms of its exposure (conduction, convection, radiation), and variable types of pain measures (neuropathic, receptor) [38]. Most authors evaluate the analgesic effect of heat as positive, an effect also seen in DOMS (delayed onset muscle soreness) syndromes [39; 40; 41; 42]. Researchers still did not observe significant differences in pressure pain threshold (PPT) compared to baseline measurements [43].In contrast, studies found that high-intensity thermal stimulation of human muscles causes muscle pain, possibly mediated by polymodal muscle nociceptors [44].The authors suggest that muscle thermo-sensitivity is weaker than nociception if a small volume of muscle tissue is stimulated. Our study aimed to investigate the effect of GR-HCT duration (5 vs. 10 min) on important muscle biomechanical parameters such as muscle tension, stiffness, elasticity, pressure pain threshold, isometric strength, and tissue perfusion in MMA athletes. Our central hypothesis was that a longer GR-HCT stimulation time would show higher statistically significant changes in the measured parameters. Determining the effective duration of this therapy without reducing its efficacy may help to optimize recovery processes in sports in the future. The results of this pilot study focused on generating practical suggestions regarding the timing of GR-HCT.