## 1 Introduction  
Physiological determinants of performance in endurance sports are maximal oxygen uptake (\(\text{VO}_{2}\text{max}\)), oxygen consumption at lactate thresholds (ventilatory thresholds), velocity (or power output) at metabolic thresholds, submaximal economy and percentage of slow-twitth muscle fibers [1, 2]. As exercise intensity increases up to maximal (\(\text{VO}_{2}\text{max}\)), exercise metabolism changes. When the intensity exceeds the first ventilatory threshold (VT1), metabolic acidosis develops and this threshold appears to be a sensitive indicator of the development of metabolic acidosis [3], and at VT1, there is the greatest intensity of aerobic metabolism. The second ventilatory threshold (VT2) (respiratory compensation point) is useful in the evaluation of aerobic performance [4, 5] because exceeding intensity at VT2 induces lactate accumulation in the blood and hyperventilation, leading to fatigue caused by decompensated metabolic acidosis [6]. In sports, special attention is given to \(\text{VO}_{2}\text{max}\) and VT2 regarding training methods, supplementation or environmental conditions that increase \(\text{VO}_{2}\text{max}\) and oxygen uptake and power (or speed) at VT2. Recently, one of the most popular methods of supporting physical training is the inclusion of hypoxia in training [7, 8]. Physiological adaptations in response to hypoxia are well documented and include increases in maximal aerobic capacity, capillary density, hemoglobin mass and erythrocyte count due to increased erythropoietinproduction under hypoxia. A number of hypoxic training protocols have been developed with varying effectiveness by using different combinations of training at varying altitudes and utilizing both natural and artificial altitudes [9]. Training under hypobaric hypoxia is often difficult to implement due to travel time, cost and athlete engagement [10]. For this reason, the live low-train high (LL-TH) training method is gaining popularity and becoming an important method of hypoxic training in modern sports. Intermittent hypoxic training (IHT) is a method in which athletes live at low altitudes, but train under hypoxic conditions (LL-TH), and it is believed that IHT can enhance performance improvements compared to similar training at sea level [11]. In this method, the athlete stays in hypoxia only for training but spends most of the time during the day in normoxia. IHT causes only minor disruptions to athletes' daily routines, allowing them to maintain their normal lifestyles [12]. Importantly, the quality of sleep and recovery is well preserved, as athletes do not sleep in hypoxic conditions [10]. However, published data reported inconclusive effectiveness of IHT in improving VO2 max and hematological parameters [13; 14]. After IHT, improvements in aerobic and anaerobic capacity are reported [15; 16], no effects [17] or even negative effects of this training are indicated (reduced speeds and power output, reduced oxygen flux in athletes) [18]. [PERSON] et al. [19] recently emphasized that, despite the extensive literature, clear evidence and practical guidance on how hypoxic training allows to increase endurance at sea level is still lacking. The two main methods of endurance training are moderate-intensity continuous training and interval training. Interval exercise typically involves repeated bouts of relatively intense exercise interspersed by short periods of recovery. These two methods have different efficiencies in improving endurance performance and still not shown what is the optimal model of interval training in terms of intensity, duration and frequency to induce improvements in endurance [20]. The choice of training workload in interval training is usually made in relation to VO2 max, maximal heart rate (HRmax) or maximal workload [20], which does not take into account the exercise metabolism (metabolic thresholds).