For the first time altitude training was applied in the training process almost fifty years ago. To make it more effective, its methodology has been constantly modified. As a result three concepts of high-altitude training have been accepted: live high-train high (LH-TH), live high-train low (LH-TL) and intermittent hypoxic training (IHT). Each of them combines the effects of acclimatization and different training procedures, what influences adaptation and performance (Levine and Stray-Gundersen, 1997; Wilber et al., 2007; Czuba et al., 2011). For example, the main factor limiting the effectiveness of the LH-TH concept is that many athletes cannot maintain high training intensity while staying at an altitude for a longer period of time and consequently decrease their level of endurance (Wilber et al., 2007). To overcome this problem Levine and Stray-Gundersen (1997) introduced the LH-TL method. This approach assumes that athletes should live at medium altitudes (2000-3000 m) in order to improve blood oxygen capacity, whereas training itself should be performed at an altitude not greater than 1200 m. This method allowed athletes to improve their blood oxygen capacity, while maintaining high training intensity (Wilber et al., 2007). Recently, significant attention in sport sciences has been given to IHT, which theoretically, may cause more pronounced adaptive changes in muscle tissues in comparison to traditional endurance training under normoxic conditions (Czuba et al., 2013). Improvement in sea-level performance and increased maximal oxygen uptake (([VO.sub.2max])) after IHT cannot be explained by changes in blood variables alone, but are also associated with non-hematological adaptive mechanisms (Czuba et al., 2011).
Recently, Millet et al. (2010) introduced the possibility of combining different hypoxic training methods including intermittent hypoxic exposure at rest (IHE) during interval -training (IHIT) or LH-TL associated with IHT (LHTLH). Whereas reports on the impact of IHT on enchased glycolysis and buffering capacity (Gore at al., 2007), scientists have also discussed the potential benefits of this method for anaerobic performance. Therefore more recently, Millet at al. (2013) based predominantly on different mechanisms subdivided the IHT methods to: continuous low intensity (30 minut) exercise in hypoxia--CHT, interval training in hypoxia--IHT and repeated sprint training in hypoxia--RSH.
The basic mechanism improving ([VO.sub.2max]) and thereby exercise capacity during both LH-TH and LH-TL training concepts is the stimulation of erythropoiesis, which results in increased blood oxygen capacity (Levine and Stray-Gundersen, 1997; Melissa et al., 1997). Another major factor improving exercise capacity after LH-TL training has been attributed to LL-TH-induced greater buffering capacity of muscle tissue (Gore et al., 2001) and/or to a decrease in exercise energy cost (Saunders et al., 2004a). The two mechanisms were also used to explain why exercise capacity increases after either LL-TH or IHT training, when its application did not lead to improved blood oxygen capacity (Czuba et al., 2011; Katayama et al., 2004; Rodriguez et al., 2004; Roels et al., 2005; 2007). However, there is a consensus in the literature that IHT training can improve exercise capacity by reducing exercise energy cost (Bonnetti et al., 2009; Czuba et al., 2011), increasing muscle tissue buffering capacity (Gore et al., 2001) and stimulating the activity of glycolytic enzymes (Katayama et al., 2004).
Available data with regard to effects of altitude training on the energy cost of working muscles are unequivocal (Green et al., 2000; Gore et al., 2001; Levine and Stray-Gundersen, 1997; Svedenhag et al., 1991; Saltin et al. 1995). The LH-TL training has been found not to reduce the energy cost in normoxia (Levine and StrayGundersen, 1997; Telford et al., 1996), but in several studies (Gore et al., 2001; Katayama et al., 2003; Marconi et al., 2005; Neya et al., 2007; Saunders et al., 2004b; Schmidt et al. 2006) significant reductions in exercise energy cost after various procedures of altitude training were registered.
Recently, Lundby et al. (2007) observed that sub-maximal V[O.sub.2] is unchanged during and 2-3 days after chronic exposure from moderate altitude (2500-3000 m) up to altitudes of 4300 m. However, in the study of Schmitt et al. (2006) energy cost of exercise did not change immediately after LH-TL protocol but 15 days later was reduced in the non-specific activity. In specific activity this improvement was observed in both groups (LH-TL and control).
The main reason of these discrepancies may be ascribed to different methodological approaches adopted by researchers, different sports level and genetic predispositions of subjects, as well as the level of hypoxia (altitude) and time of exposure (Chapman et al., 2014; Gore et al., 2013)
Another significant factor to consider is the type of hypoxia used, because physiological adaptation to hypobaric vs. normobaric hypoxia is different, particularly with regard to lung fluid regulation (Millet et al., 2012). Bonetti and Hopkins (2009) reported in their meta-analysis that the LHTL protocol in natural conditions (hypobaric hypoxia) currently provides the best stimulus for enhancing endurance performance in elite and sub elite athletes, while some artificial protocols (normobaric hypoxia) are effective only in sub elite athletes. This may also explain why attempts at making the above methods more effective are being constantly made.
One of the more interesting modifications in the LH-TL protocol was proposed by Chapman et al. (1998). According to the new solution, athletes should stay and perform low-intensity training at an altitude of 2500 m for four weeks, while high-intensity interval training should be performed at 1250 m. This method is known as HiHiLo (live high-base train high-interval train low). Stray-Gundersen et al. (2001) found a significant 3% increase in ([VO.sub.2max]) accompanied by improvement of several hematological variables (hemoglobin mass and hematocrit) that resulted in significant improvement of running time (5.8 s, 3 km race) after the HiHiLo protocol. Therefore, one can assume that the HiHiLo protocol may be an effective training method, particularly applicable to cross-country skiers and biathletes during the preparatory period.
Many winter sports athletes, in order to perform specific training on ice or snow during the preparatory period travel to the mountains and stay at high altitude. These procedures have been adopted by biathletes and cross-country skiers. The described above version of HiHiLo training is still not used frequently and reports on its effectiveness are limited. In particular, there are no data regarding the effects of HiHiLo training and a reduced energy cost of exercise in normoxia. Thus, we investigated if the HiHiLo procedure performed in hypobaric hypoxia conditions affects biathletes' aerobic capacity and exercise energy cost in normoxia. Additionally, erythropoietin and selected hematological variables were evaluated, as potential factors influencing aerobic capacity and endurance performance.
Fifteen highly-trained biathletes participated in the study. The basic anthropometric data of the athletes (body height- BH, body mass-BM, % of fat--FAT, fat-free mass-FFM) and their training experience are presented in Table 1. All subjects held valid medical examination certificates and had no contraindications for participating in the experiment. The study design was approved by the Bioethics Committee for Scientific Research at the Academy of Physical Education in Katowice.
The selected biathletes were randomly divided into experimental (H) and control (C) groups. The experiment was carried out in the middle of the preparatory period of the annual training cycle (September-October). The H group trained in hypobaric hypoxia for three weeks (3 microcycles) in natural settings (Glaitner Hochjoch, Italy) according to the HiHiLo procedure as described by Chapman et al. (1998), whereas their control counterparts trained in normoxic conditions (~600 m, Duszniki Zdroj, Poland). The athletes from the H group beside the LH-TL procedure performed specific aerobic endurance training in hypoxia. Both groups of athletes performed the same training protocol with respect to relative training workloads.
The participants from the H group stayed at an altitude of 2015 m and performed aerobic endurance training on skis (70-85 % of threshold heart rate--H[R.sub.LT]; ~130-150 bpm) four times in each microcycle at 3000 m (12 altogether). The total training volume in hypoxia in the group H was 30 hours. Each microcycle included three sessions of high-intensity interval training (95-105% H[R.sub.LT]; ~170-180 bpm) intended to increase the athletes' strength endurance, during which they skirolled at an altitude of 1000 m (19 h) and then returned to 2015 m. The total training volume in normoxia in the H group equaled 18 hours. The subjects in the group C followed the same training protocol with skirollers in normoxia. The total training volume in normoxia in the group C was 49 h. Exercise protocol
Before the altitude training began all subjects underwent the first series of examinations (S1). The experimental training protocol consisted of three microcycles involving incremental exercise and a one-week regeneration period, followed by the second series...