Trends of High Altitude Training in Sports: Issues for Consideration

Dr. Satish Kanaujia, IIT(BHU) , Email: skanaujia.hss@itbhu.ac.in

Prof. Archana Singh, MMV, BHU, Email:asanoushka@gmail.com

Athletes in every sport are now realizing the performance benefits that altitude training can have on strength, power and endurance. The main purpose of high altitude training is to stimulate the production of red blood cells in order to increase the efficiency of transporting oxygen to the muscles. An increase in red blood cells prevents muscle fatigue in endurance sports and helps to improve athlete’s performance in competitions. There are three main altitude exposure techniques that are utilized in sports. These are Live High- Train Low, Live High – Train High and Live Low- Train High. Typically high altitude is considered to be around 6500 to 8000 feet, while low altitude can be anywhere from sea level to up about 4000 feet. The difference between high and low altitude are significant enough to have a noticeable effect on training. Simulated altitude training is done through things like oxygen masks and high altitude tents sleeping in (live high, train low). Now these are great pieces of equipment that definitely have their benefits – but the problem is, they can be very uncomfortable and restrictive. It benefits healthier respiratory system, improved endurance and higher red blood cells concentration and allow the athletes to experience increased endurance and speed, less fatigue and improved recovery.

Altitude Training:-Altitude training as a way to improve the performance of athletes is a common knowledge. Pro athletes such as runners as well as non-professional athletes undergo this type of training to increase their capability to exercise. Altitude training is an extreme method of endurance training that athletes use to have leverage over other athletes in terms of performance in competitions. People who normally use this training are cyclists, football players and runners, among others. This training is practiced by athletes by staying and training in high altitudes, at around 8,000 feet above sea level, for several weeks. This is because in high places, there is less oxygen and the air is thinner and as a result, the body learns to adapt to the reduced amount of oxygen.

Recently, endurance athletes have used several novel approaches and modalities for altitude training including: (i) normobaric hypoxia via nitrogen dilution (hypoxic apartment); (ii) supplemental oxygen; (iii) hypoxic sleeping devices; and (iv) intermittent hypoxic exposure (IHE).A normobaric hypoxic apartment simulates an altitude environment equivalent to approximately 2000 to 3000m (6560 to 9840ft). Athletes who use a hypoxic apartment typically ‘live and sleep high’ in the hypoxic apartment for 8 to 18 hours a day, but complete their training at sea level, or approximate sea level conditions. Several studies suggest that using a hypoxic apartment in this manner produces beneficial changes in serum erythropoietin (EPO) levels, reticulocyte count and red blood cell (RBC) mass, which in turn may lead to improvements in postaltitude endurance performance. Supplemental oxygen is used to simulate either normoxic (sea level) or hyperoxic conditions during high-intensity workouts at altitude. This method is a modification of the ‘high-low’ strategy, since athletes live in a natural terrestrial altitude environment but train at ‘sea level’ with the aid of supplemental oxygen (Wilber, 2001).

Live-High, Train-Low:- One suggestion for optimizing adaptations and maintaining performance is the live-high, train-low principle. This training idea involves living at higher altitudes in order to experience the physiological adaptations that occur, such as increased erythropoietin (EPO) levels, increased red blood cell levels, and higher VO2 max, while maintaining the same exercise intensity during training at sea level. Due to the environmental differences at high altitude, it may be necessary to decrease the intensity of workouts. Studies examining the live-high, train-low theory have produced varied results, which may be dependent on a variety of factors such as individual variability, time spent at high altitude, and the type of training program. For example, it has been shown that athletes performing primarily anaerobic activity do not necessarily benefit from altitude training as they do not rely on oxygen to fuel their performances.

Altitude training can produce increases in speed, strength, endurance, and recovery by maintaining altitude exposure for a significant period of time. Opponents of altitude training argue that an athlete's red blood cell concentration returns to normal levels within days of returning to sea level and that it is impossible to train at the same intensity that one could at sea level, reducing the training effect and wasting training time due to altitude sickness. Altitude training can produce slow recovery due to the stress of hypoxia. Exposure to extreme hypoxia at altitudes above 16,000 feet (5,000 m) can lead to considerable deterioration of skeletal muscle tissue. Five weeks at this altitude leads to a loss of muscle volume of the order of 10–15%(Wilber, 2001).

Exercise scientists were challenged to create the positive effects of higher altitude within an environment of normal oxygen concentration (normoxic). For endurance runners, this could be as simple as supplying oxygen while training at altitude. For dynamic athletes, it might require a more elaborate approach, such as sleeping in hypoxic tents while living and training at sea level. Either way, the concept of "Live High, Train Low" was born. This model exposes the athlete to a hypoxic environment for a number of hours per day, typically accomplished by sleeping in an altitude tent. The athlete trains normally and does not have to relocate or travel to gain either the high altitude exposure or low normoxic training. This simulated "living high" (2,500–3,000 meters above sea level) for eight to 10 hours a day and "training low" (below 1,200 meters above sea level) for 18 days has shown significant improvements, which may last as long as 15 days after exposure. A recent study shows an increase in RBC mass in as few as 10 days, but the length of time and extent of adaptation is highly individualized. Furthermore, repeated sprints in hypoxia have elicited greater training adaptations than repeated sprints at normoxia, but the gains seem to be more anaerobic in nature and may have been further improved with the Live High, Train Low scenario.

Live-High, Train-High

In the live-high, train-high regime, an athlete lives and trains at a desired altitude. The stimulus on the body is constant because the athlete is continuously in a hypoxic environment. Initially VO2 max drops considerably: by around 7% for every 1000 m above sea level) at high altitudes. Athletes will no longer be able to metabolize as much oxygen as they would at sea level. Any given velocity must be performed at a higher relative intensity at altitude.

Alternative Models to Live High and Train High to Prepare for Altitude Competitions:- There is no well-controlled study comparing the effects between the classical live high and train high concept and other combinations, i.e., live high and train low or live low and train high, to prepare for altitude endurance competitions. However, based on the presented effects of acclimatization to altitude on performance and the fact that optimal performance is achieved after acclimatization to the altitude where the competition takes place, live high and train high is very likely the optimal concept (Chapman et al., 2016). Moreover, this assumption is supported by logistic aspects and reasons of familiarization with the conditions at the competition site. Hence, living at altitude with only short interruptions for some training sessions at lower altitudes may represent, when logistically possible, an effective alternative to the classical concept. Moreover, intermittent exposures to hypobaric hypoxia may also represent an option to prepare for altitude training/competitions. Beidleman and colleagues demonstrated physiologic adaptations and improved time-trial exercise performance at altitude (4,300 m) after 7 days (4 h/day) of intermittent high-altitude (4,300 m) exposures, with or without exercise training during exposures (Beidleman et al., 2008). These researchers also showed improved muscular performance after 3 weeks of intermittent hypobaric hypoxia (4 h/day, 5 days/week) which was closely related to increased resting SaO2 post exposure (Beidleman et al., 2003). In contrast, no beneficial effects on endurance performance at altitude were found after 1 week of normobaric intermittent hypoxia exposures (2 h at rest plus two 25 min of exercise) (Beidleman et al., 2009).When time for optimal altitude acclimatization is not available due to logistic reasons, short-term arrival strategies (2 – 14 h before the competition) might be a (sub-optimal) option (Chapman et al., 2013; Foss et al., 2017).

Psychological Changes in Performance:-Research on psychological factors impacting exercise performance can be classified into two categories: the influence of situational factors on exercise performance (i.e., psychological factors that vary over time and depend on environmental influences/changes of the human organism) and the connection between exercise performance and personality traits (i.e., psychological factors, which are believed to be somewhat stable over time). Although there is some research on the connection between responses to altitude (acute mountain sickness) and personality traits (Missoum et al., 1992; Niedermeier et al., 2017a), altitude is considered an environmental factor which affects mainly situational psychological aspects. Therefore, the primary focus of this section is on situational factors (Burtscher et al. (2018),

Situational Psychological Aspects

Without exposure to moderate altitude, the following situational psychological aspects have been studied with respect to endurance performance: cognitive functions, role of mood state in training and competition, overtraining syndrome/high levels of stress, use of psychological strategies.Cognitive functions were shown to be associated with performance-related variables. Impaired cognitive functions were reported with difficulties to regulate exercise intensity and an appropriate pacing (Van Biesen et al., 2016). Several aspects of cognitive functions can be negatively influenced by altitude, e.g., detection of visual stimuli, short term memory, spatial memory, motor speed/precision, complex reaction time, decision making, cerebral function (Virues-Ortega et al., 2004; Ainslie et al., 2013). Although the effects become more relevant at higher altitude (i.e., 3,500 m and above), moderate altitude has more subtle effects, out of which some are detectable, e.g., increased complex reaction time at 1,500 m (Denison et al., 1966). Furthermore, the exercise to exhaustion during a competition might amplify the adverse effects on cognitive function (Moore et al., 2012). Similarly, adverse effects on performance due to acute mountain sickness as reported in Shukitt-Hale et al. (1991) might be increased, even though acute mountain sickness plays a minor role in moderate altitudes with a prevalence of 9% at 2,850 m (Maggiorini et al., 1990). Decision making was shown to be impaired with a riskier behavior in (simulated) altitude (Pighin et al., 2012; Davranche et al., 2016). Decision making is of special importance in the pacing process during an endurance competition (Hettinga et al., 2017). It should be considered that with riskier decision making at altitude, there is the danger of overpacing during the competition. However, the peak in risky decision making occurs during acute exposure (i.e., after 3 h) to hypoxic conditions and time in hypoxic conditions showed a risk-reducing effect on decision making (Niedermeier et al., 2017b).

Mood state (including positive/negative affect, arousal, state anxiety, and in a wider sense perceived effort) affects several domains of training and competition (Raglin, 2001): Firstly, exercise performance is associated to the level of arousal/state anxiety. In general, very high and very low levels of state anxiety seem to be unfavorable for good performance Secondly, negative affect is associated with lower endurance performance (Renfree et al., 2012). Altitude levels showed an adverse effect on mood states, i.e., persons reported to feel less vigorous and more fatigued at an altitude of approximately 3,000 m compared to 2,200 m (Shukitt-Hale et al., 1990). In endurance competitions, where persevering pain discomfort is a central aspect, perceiving a higher level of fatigue during competition is a disadvantage for the athlete. There is evidence that carbohydrate (CHO) supplementation can lead to a decreased perceived effort of exercise performance at altitude (Oliver et al., 2012a). Therefore, CHO supplementation prior to the competition – beside the known physiological benefits (compare Section “Carbohydrate Requirements”) – can also result in positive psychological effects. Similar results have been found for the supplementation of caffeine at moderate hypoxia, i.e., approximately 2,500 m altitude (Smirmaul et al., 2017). Thirdly, mood state is used as an indicator for overtraining syndrome.

Use of Psychological Strategies:-Regarding the importance of psychological factors, it is not surprising that several psychological strategies exist to enhance endurance performance and for an optimal mental preparation of the athlete. The strategies include: association and dissociation, imagery training, self-talk, goal setting (Tuffey, 2000; Hatzigeorgiadis et al., 2011; McCormick et al., 2015), out of which association might be mostly influenced by altitude.

Association is a cognitive strategy, where athletes focus on the signals of the body including fatigue, pain, muscle soreness and use this information for pace regulation. In contrast, dissociation is a technique, where athletes use various means to distract themselves of the unpleasant sensations of the body. Means of distractions are ranging from listening to music, mentally constructing a house, to doing mathematical operations. Both approaches increased endurance performance with medium- to large-sized effect sizes (McCormick et al., 2015). Association is reported to be more effective in elite endurance athletes, whereas dissociation is effective in non-elite endurance athletes (Raglin and Wilson, 2000). Imagery training (i.e., mentally simulating situations prior to the competition), self-talk interventions (i.e., “my legs are strong and powerful” prior to and during the competition), goal setting (prior to the competition) were also shown to be effective in enhancing endurance performance (McCormick et al., 2015). Imagery training might include the sensation of the environment at moderate altitude, e.g., mild hypoxia and/or lower temperature, to be as realistic as possible. Moreover, these techniques might be less influenced by altitude compared to association.

Nutritional Aspects When Preparing for Endurance Competition at Altitude:-Nutrition can have a major impact on the physiological adaptations associated with altitude training and competition. First, because satiety hormones are influenced by hypoxia, and in further consequence, appetite and energy intake are suppressed (Karl et al., 2018). The most important prealtitude consideration may be iron status (Constantini et al., 2017). It has been shown that the risk of illness and respiratory infection is increased during altitude exposure (Walsh and Oliver, 2016). This may be due to direct effects of hypobaric hypoxic conditions accompanied by oxidative stress but may be also the result of inadequate nutrition and recovery (Ross and Martin, 2015). Key nutrition-related concerns include the need for iron, higher energy and fluid requirements, adequate protein intake for preventing body mass loss, and a higher demand for antioxidant-rich foods to maintain robust immunity.

Appropriate Iron Needs:-Iron is necessary for optimal erythropoietic adaptation to altitude exposure. Suboptimal iron status (i.e., ferritin < 30 ng/mL) may result from limited energy and iron intake, poor bioavailability, or increased iron demands due to high training loads, environmental factors (hypoxia-induced erythropoiesis, haemolysis, sweating), menstrual blood losses, and genetics (Pedlar et al., 2018). Therefore, iron status should be at a high level (ferritin > 50 ng/mL) before attempting altitude training (Clenin et al., 2015).

Energy and Hydration Needs:-Energy and fluid requirements are higher at altitude. Weight loss is a common phenomenon at altitude because of hypoxia-induced appetite suppression combined with an increase in basal metabolic rate (Butterfield et al., 1992). Within the first days of acclimatization, athletes are at high risk of dehydration due to increased respiratory water loss by enhanced ventilation, increased urinary water loss, and an increase in basal metabolic rate. Thus, athletes should be encouraged to drink sufficient fluids while at altitude. Overall, authors recommend regular monitoring of body mass and urine osmolality during altitude training to ensure proper hydration and to prevent overdrinking since both hypohydration and hyperhydration impair performance and present a risk to health (Maughan and Meyer, 2013).

Carbohydrate Requirements:-At altitude, the stress response to exercise is enhanced, and thus CHO requirements are higher than at sea level (Katayama et al., 2010). CHO consumption before exercise in hypoxia can mitigate some of the negative symptoms of high altitude, like less oxygen saturation and ventilation (Golja et al., 2008). However, the train-low strategy should be considered only before traveling to altitude and should be avoided in the days leading up to altitude. Although reduced oxygen can inhibit mTOR (Liu et al., 2006), possibly contributing to muscle deterioration during hypoxia (Debevec et al., 2018), leucine supplementation did not prevent loss of fat-free mass during a 13-day trek to Everest Base Camp in a double-blind randomized study (Wing-Gaia et al., 2014). It needs to be established whether leucine directly interacts with mTOR during hypoxic conditions, which could attenuate loss of fat-free mass during longer duration high altitude exposure.

Protein for Lean Body-Mass Retention:-Weight loss and body composition changes are an unfortunate consequence of sustained hypobaric hypoxia, with lean body mass (LBM) comprises approximately 60–70% of body weight loss during high-altitude (>5,000 m) exposure (Wing-Gaia, 2014). Possible mechanisms of altitude-induced muscle wasting include downregulation of muscle protein synthesis and suboptimal energy and protein intake. Thus, it appears reasonable that a higher protein diet at altitude may be useful to improve retention of LBM.

Antioxidant Supplements:-It has been shown that antioxidant status is impaired under hypoxic conditions and even remained impaired for 2 weeks following an altitude training camp (Pialoux et al., 2010). Although low oxygen pressure seems to be favorable to low ROS production, high altitude exposure can lead to enhanced ROS generation due to up-regulation of the mitochondrial electron transport chain, xanthine oxidase, and nitric oxide synthase (Dosek et al., 2007). High-altitude training appears to weaken both the enzymatic and non-enzymatic antioxidant systems (Quindry et al., 2016).

Probiotics and Vitamin D to Prevent Infections:-Prolonged intense exercise is associated with a transient depression of immune function and a heavy schedule of altitude training and competition can lead to immune impairment in athletes. This is associated with an increased susceptibility to upper respiratory tract infection (URTI) (Walsh and Oliver, 2016). Despite the increased UVB radiation from sunlight, vitamin D supplementation should be considered in athletes who stay at high altitude as vitamin D may influence iron metabolism and thus erythropoiesis (Smith and Tangpricha, 2015).

Suggestions for Preparation for Endurance Competitions at Altitude :-The optimum for achieving better results in endurance competitions at altitudes is to be born or at least live permanently and train at such altitudes (Fulco et al., 2007), or to move to altitude at any time over the course of the sporting carrier. With regard to sea-level performance, live high (at natural altitude) and train low seem to be the most promising protocol in athletes (Bonetti and Hopkins, 2009). However, athletes mostly compete at low altitudes with only rare competition events at higher altitudes. In such matters, the preparation period for the altitude competition depends on the time interval between competitions, the importance of the altitude competition, and prior individual experiences and may vary between some hours and about 2 weeks. In more recent time, coaches and athletes are also faced with the opportunity to use artificial altitude (normobaric hypoxia chambers) for preparation purposes offering multiple combinations of artificial and real altitude exposures with and without training. In order to accelerate the regeneration processes at altitude, inhalation of oxygen of lowland concentration can be used (via a mask or tent) Chapman et al., 2010). It is also important for successful handling of altitude for athletes to be mentally prepared for the fact that training at higher altitude will be much more demanding compared to low altitudes. Hypoxic training before going to altitude was suggested as a method potentially improving the ability to tolerate discomfort at altitude and thus improve exercise performance (Álvarez-Herms et al., 2016). Importantly, training intensity must be modified with regard for the given environment. Load can gradually be increased, but the rising intensity must be carefully monitored. Over the course of sojourns at higher altitudes there are generally three basic critical periods (Suchý et al., 2009), which should be respected when planning the training.

Benefits and Risks of Altitude Training:-Altitude training refers to exposing the body to hypoxic environments (those which limit the amount of oxygen reaching the tissues) long enough to elicit physiological adaptations. These adaptations enhance the body's ability to use oxygen and increase the athlete's aerobic capacity. The result is an athlete who can move for extended periods of time at a slightly higher pace and recover more quickly between bouts of exercise. The effects are caused mainly by the production of more red blood cells (RBC), which carry oxygen through the body. There also seems to be a dose-dependent response, or maybe even a threshold of sorts for these adaptations. Studies commonly cite 7,000 feet above sea level as the necessary "dose" of altitude to get the desired effects. Also, the process takes about two to three weeks to deliver maximum benefits. The effects are not permanent, as RBC mass returns to normal about 15 days after the athlete drops below the aforementioned altitude.

Athletes Who May Benefit:-At first glance, it may seem like only distance runners and endurance athletes would benefit from altitude training. Although these athletes benefit significantly, almost all athletes stand to reap some gains from this training. Even if the physiological adaptations don't directly carry over to their performance at sea level, athletes who compete at high altitudes from time to time should strongly consider acclimating before competition.

Drawbacks:-Altitude exposure is not without its share of issues. During altitude training, athletes may experience decreased REM sleep. The hypoxic environment can hinder breathing for some, and significantly affect sleep, which could potentially decrease the athlete's rate of recovery. The reward seems to outweigh this particular drawback, as multiple studies confirm the positive adaptations of altitude exposure. Exposure to high altitudes reduces the athlete's ability to recover between sets of anaerobic and fatiguing exercise. This may be due to a number of peripheral or central factors, but no matter the cause, the result is a gradual decline in the athlete's ability to perform an otherwise normal practice. This accelerated fatigue could lead to decreased psychological awareness during training, fewer repetitions (slowing the rate of skill acquisition or refinement) and potential overall de-conditioning, all of which support the idea of seeking normoxic training environments.

Advantages of Altitude Training:- Altitude training is not for everyone since our bodies react differently to the impacts of training in elevated grounds. During altitude training, athletes may experience decreased sleep. The hypoxic environment can hinder breathing for some, and significantly affect sleep, which could potentially decrease the athlete's rate of recovery. It is crucial to understand that not all athletes respond to altitude exposure the same way. Monitoring an athlete's heart rate, sleep, mood and training performance can prevent altitude training from causing more harm than good. It's also important to hydrate properly and increase one's carbohydrate intake, since the lack of oxygen saturation often increases reliance on anaerobic energy. Antioxidant supplementation is also an option, since exposure to high altitudes increases oxidative stress, especially when combined with exercise.

1. Higher Red Blood Cells Concentration:-The kidneys are responsible in the release of the hormone known as erythropoietin. This hormone stimulates the bone marrow to produce red blood cells that are important in the supply of oxygen in the body. When a person trains at high altitudes, there is less supply of oxygen. This causes the kidneys to increase the release of erythropoietin and consequently, more oxygen is carried by the blood to the organs and tissues.

2. Improved Endurance and Overall Performance:-The measure of the amount of oxygen in the body that can be converted into energy from the food a person eats is known as VO2. This works through the conversion of the eaten food into adenosine triphosphate. Conversely, in terms of measuring how much an athlete has improved when it comes of performance, the term is known as VO2 max. This condition is achieved by training at high altitudes and ensures improved performance and endurance.

3. Strengthened Respiratory System:- In higher altitudes, there is a reduction in the percentage of oxygen molecules as the altitude rises due to lesser barometric pressure. When an athlete trains for several weeks at high altitudes, the body learns to adapt to the reduced oxygen supply and the respiratory muscles are strengthened.

4.Longer Effects:-Proponents of altitude training posit that by spending weeks training in high altitudes, muscle metabolism takes place and so does improved performance and endurance. This is because there is an increase in the supply of red blood cells that are instrumental to carrying oxygen to different parts of the body. What makes this effective is that even if the athlete competes in lower altitudes, the concentration in red blood cells remains high.

5.Comfortable:-Supporters of altitude training say that the cool air present above sea level offers a level of comfort for athletes. This is because exercising is mostly done in lower grounds while the athlete can spend 12 hours sleeping and resting in higher grounds.

Disadvantages of Altitude Training

1. It Affects Immunity:-One of the drawbacks of training in high altitudes is in a person’s immunity to diseases. With a weakened immune system, an athlete can be susceptible to pathogens that can lead to ailments. It is important for the body to have proper nutrition while training since it also increases the amount of nutrients needed.

2. Exposure to Stress:-Another negative effect of this extreme training method is the increase in cortisol levels in the body under stress. Training in high altitudes results to lesser oxygen supply. Because of this, the lungs and the heart have to work harder. With lower oxygen supply as elevation rises, the body needs to compensate this by increasing the production of cortisol. This can lead to muscle breakdown.

3.It Weakens Endurance:-Critics of altitude training claim that even if the body increases the production of red blood cells as oxygen is reduced, there is no assurance that the muscles will still have the same amount of oxygen it needs for performance. This is also because different people react differently to stressors and that not all can adapt to higher altitudes. It may affect the endurance of an athlete while it will not have an effect on another.

4. Negative Increase in Red Blood Cell Production:-In some cases opponents of high altitude trainings argue that lingering in places where air is thinner might increase the production of red blood cells to supply the different organs and tissues of the body. While it is helpful to familiarize the body with lower oxygen supply, the red blood cell increase can also affect the viscosity of the blood or its concentration. If this happens, flow of blood is restricted and it becomes slower, which is not also healthy. This is on top of the concern that this type of training entails expenses and can costly which might not be practical for some athletes considering that not all have access to high altitudes.

5. Possible Dehydration:-Person undergoing this extreme training is prone to dehydration. This is because as the elevation becomes higher, breathing and heart rate increase. Increased respiratory rate can result to more moisture loss. Moreover, since the athlete is exposed to cool and dry air, dehydration is more likely to occur unless fluid intake is increased. Another possibility can be the hastened evaporation of moisture from the skin since there is lowered air pressure.

Conclusion:-Altitude training is not for everyone since our bodies react differently to the impacts of training in elevated grounds. It also has benefits and drawbacks that need to be considered. It is crucial to understand that not all athletes respond to altitude exposure the same way and individual differences is there. Monitoring an athlete's heart rate, sleep, mood and training performance can prevent altitude training from causing more harm than good. To ensure an athlete or any individual interested in altitude training, it is important to undergo tests and consult a medical practitioner before subjecting the body to this type of training. In summary, physiological adaptations that occur with training at altitude can be positively influenced by appropriate nutrition. It's also important to hydrate properly and increase one's carbohydrate intake, since the lack of oxygen saturation often increases reliance on anaerobic energy. Antioxidant supplementation is also an option, since exposure to high altitudes increases oxidative stress, especially when combined with exercise. In the case of an endurance competition at moderate or high altitude, the diet in the weeks prior to altitude exposure is of utmost importance, especially from the perspective of improving iron status and overall health.

References:

• Ainslie, P. N., Wilson, M. H., and Imray, C. H. (2013). “Cerebral circulation and brain,” in High Altitude: Human Adaptation to Hypoxia, eds E. R. Swenson and P. Bärtsch (New York, NY: Springer), 141–170.

• Álvarez-Herms, J., Julià-Sánchez, S., Gatterer, H., Blank, C., Corbi, F., Pagès, T., et al. (2016). Anaerobic training in hypoxia: a new approach to stimulate the rating of effort perception. Physiol. Behav. 163, 37–42. doi: 10.1016/j.physbeh.2016.04.035
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Beidleman, B. A., Muza, S. R., Fulco, C. S., Cymerman, A., Sawka, M. N., Lewis, S. F., et al. (2008). Seven intermittent exposures to altitude improves exercise performance at 4300 m. Med. Sci. Sports Exerc. 40, 141–148. doi: 10.1249/mss.0b013e31815a519b
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Beidleman, B. A., Muza, S. R., Fulco, C. S., Jones, J. E., Lammi, E., Staab, J. E., et al. (2009). Intermittent hypoxic exposure does not improve endurance performance at altitude. Med. Sci. Sports Exerc. 41, 1317–1325. doi: 10.1249/MSS.0b013e3181954601
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Bonetti, D. L., and Hopkins, W. G. (2009). Sea-level exercise performance following adaptation to hypoxia: a meta-analysis. Sports Med. 39, 107–127. doi: 10.2165/00007256-200939020-00002
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Burtscher et al. (2018), “Preparation for Endurance Competitions at Altitude: Physiological, Psychological,Dietary and Coaching Aspects. A Narrative Review.” Front. Physiol.,2 | https://doi.org/10.3389/fphys.2018.01504
• Butterfield, G. E., Gates, J., Fleming, S., Brooks, G. A., Sutton, J. R., and Reeves, J. T. (1992). Increased energy intake minimizes weight loss in men at high altitude. J. Appl. Physiol. 72, 1741–1748. doi: 10.1152/jappl.1992.72.5.1741
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Chapman, R. F., Karlsen, T., Ge, R. L., Stray-Gundersen, J., and Levine, B. D. (2016). Living altitude influences endurance exercise performance change over time at altitude. J. Appl. Physiol. 120, 1151–1158. doi: 10.1152/japplphysiol.00909.2015
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Chapman, R. F., Laymon, A. S., and Levine, B. D. (2013). Timing of arrival and pre-acclimatization strategies for the endurance athlete competing at moderate to high altitudes. High Alt. Med. Biol. 14, 319–324. doi: 10.1089/ham.2013.1022
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Chapman, R. F., Stickford, J. L., and Levine, B. D. (2010). Altitude training considerations for the winter sport athlete. Exp. Physiol. 95, 411–421. doi: 10.1113/expphysiol.2009.050377
• CrossRef Full Text | Google Scholar
• Clenin, G., Cordes, M., Huber, A., Schumacher, Y. O., Noack, P., Scales, J., et al. (2015). Iron deficiency in sports - definition, influence on performance and therapy. Swiss Med. Wkly. 145:w14196. doi: 10.4414/smw.2015.14196
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Constantini, K., Wilhite, D. P., and Chapman, R. F. (2017). A clinician guide to altitude training for optimal endurance exercise performance at sea level. High Alt. Med. Biol. 18, 93–101. doi: 10.1089/ham.2017.0020
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Davranche, K., Casini, L., Arnal, P. J., Rupp, T., Perrey, S., and Verges, S. (2016). Cognitive functions and cerebral oxygenation changes during acute and prolonged hypoxic exposure. Physiol. Behav. 164(Part A), 189–197. doi: 10.1016/j.physbeh.2016.06.001
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Debevec, T., Ganse, B., Mittag, U., Eiken, O., Mekjavic, I. B., and Rittweger, J. (2018). Hypoxia aggravates inactivity-related muscle wasting. Front. Physiol. 9:494. doi: 10.3389/fphys.2018.00494
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Denison, D. M., Ledwith, F., and Poulton, E. C. (1966). Complex reaction times at simulated cabin altitudes of 5,000 feet and 8,000 feet. Aerosp. Med. 37, 1010–1013.
• PubMed Abstract | Google Scholar
• Dosek, A., Ohno, H., Acs, Z., Taylor, A. W., and Radak, Z. (2007). High altitude and oxidative stress. Respir. Physiol. Neurobiol. 158, 128–131. doi: 10.1016/j.resp.2007.03.013
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Foss, J. L., Constantini, K., Mickleborough, T. D., and Chapman, R. F. (2017). Short-term arrival strategies for endurance exercise performance at moderate altitude. J. Appl. Physiol. 123, 1258–1265. doi: 10.1152/japplphysiol.00314.2017
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Fulco, C. S., Zupan, M., Muza, S. R., Rock, P. B., Kambis, K., Payn, T., et al. (2007). Carbohydrate supplementation and endurance performance of moderate altitude residents at 4300 m. Int. J. Sports Med. 28, 437–443. doi: 10.1055/s-2006-924515
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Golja, P., Flander, P., Klemenc, M., Maver, J., and Princi, T. (2008). Carbohydrate ingestion improves oxygen delivery in acute hypoxia. High Alt. Med. Biol. 9, 53–62. doi: 10.1089/ham.2008.1021
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Hatzigeorgiadis, A., Zourbanos, N., Galanis, E., and Theodorakis, Y. (2011). Self-talk and sports performance: a meta-analysis. Perspect. Psychol. Sci. 6, 348–356. doi: 10.1177/1745691611413136
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Hettinga, F. J., Konings, M. J., and Pepping, G. J. (2017). The science of racing against opponents: affordance competition and the regulation of exercise intensity in head-to-head competition. Front. Physiol. 8:118. doi: 10.3389/fphys.2017.00118
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Karl, J. P., Cole, R. E., Berryman, C. E., Finlayson, G., Radcliffe, P. N., Kominsky, M. T., et al. (2018). Appetite suppression and altered food preferences coincide with changes in appetite-mediating hormones during energy deficit at high altitude, but are not affected by protein intake. High Alt. Med. Biol. 19, 156–169. doi: 10.1089/ham.2017.0155
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Katayama, K., Goto, K., Ishida, K., and Ogita, F. (2010). Substrate utilization during exercise and recovery at moderate altitude. Metabolism 59, 959–966. doi: 10.1016/j.metabol.2009.10.017
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Liu, L., Cash, T. P., Jones, R. G., Keith, B., Thompson, C. B., and Simon, M. C. (2006). Hypoxia-induced energy stress regulates mRNA translation and cell growth. Mol. Cell. 21, 521–531. doi: 10.1016/j.molcel.2006.01.010
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Maggiorini, M., Buhler, B., Walter, M., and Oelz, O. (1990). Prevalence of acute mountain sickness in the Swiss Alps. Br. Med. J. 301, 853–855. doi: 10.1136/bmj.301.6756.853
• CrossRef Full Text | Google Scholar
• Maughan, R. J., and Meyer, N. L. (2013). Hydration during intense exercise training. Nestle Nutr. Inst. Workshop Ser. 76, 25–37. doi: 10.1159/000350225
• PubMed Abstract | CrossRef Full Text | Google Scholar
• McCormick, A., Meijen, C., and Marcora, S. (2015). Psychological determinants of whole-body endurance performance. Sports Med. 45, 997–1015. doi: 10.1007/s40279-015-0319-6
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Missoum, G., Rosnet, E., and Richalet, J. P. (1992). Control of anxiety and acute mountain sickness in Himalayan mountaineers. Int. J. Sports Med. 13(Suppl. 1), S37–S39. doi: 10.1055/s-2007-1024587
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Moore, R. D., Romine, M. W., O’Connor, J. P., and Tomporowski, P. D. (2012). The influence of exercise-induced fatigue on cognitive function. J. Sports Sci. 30, 841–850. doi: 10.1080/02640414.2012.675083
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Niedermeier, M., Waanders, R., Menz, V., Wille, M., Kopp, M., and Burtscher, M. (2017a). Is acute mountain sickness related to trait anxiety? A normobaric chamber study. Physiol. Behav. 171, 187–191. doi: 10.1016/j.physbeh.2017.01.004
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Niedermeier, M., Weisleitner, A., Lamm, C., Ledochowski, L., Frühauf, A., Wille, M., et al. (2017b). Is decision making in hypoxia affected by pre-acclimatisation? A randomized controlled trial. Physiol. Behav. 173, 236–242. doi: 10.1016/j.physbeh.2017.02.018
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Oliver, S. J., Golja, P., and Macdonald, J. H. (2012a). Carbohydrate supplementation and exercise performance at high altitude: a randomized controlled trial. High Alt. Med. Biol. 13, 22–31. doi: 10.1089/ham.2011.1087
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Pedlar, C. R., Brugnara, C., Bruinvels, G., and Burden, R. (2018). Iron balance and iron supplementation for the female athlete: a practical approach. Eur. J. Sport Sci. 18, 295–305. doi: 10.1080/17461391.2017.1416178
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Pighin, S., Bonini, N., Savadori, L., Hadjichristidis, C., Antonetti, T., and Schena, F. (2012). Decision making under hypoxia: oxygen depletion increases risk seeking for losses but not for gains. Judgm. Decis. Mak. 7, 472–477.
• Google Scholar
• Quindry, J., Dumke, C., Slivka, D., and Ruby, B. (2016). Impact of extreme exercise at high altitude on oxidative stress in humans. J. Physiol. 594, 5093–5104. doi: 10.1113/JP270651
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Raglin, J. S. (2001). Psychological factors in sport performance: the Mental Health Model revisited. Sports Med. 31, 875–890. doi: 10.2165/00007256-200131120-00004
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Raglin, J. S., and Wilson, G. S. (2000). “Psychology in endurance performance,” in Endurance in Sport, eds R. J. Shephard and P. O. Astrand (Oxford: Blackwell Scinece Ltd), 212–222.
• Google Scholar
• Renfree, A., West, J., Corbett, M., Rhoden, C., and St Clair Gibson, A. (2012). Complex interplay between determinants of pacing and performance during 20-km cycle time trials. Int. J. Sports Physiol. Perform. 7, 121–129. doi: 10.1123/ijspp.7.2.121
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Ross, M. L., and Martin, D. T. (2015). “Altitude, cold and heat,” in Clinical Sports Nutrition, 5, eds L. M. Burke and V. Deakin (Sydney: McGraw-Hill), 767–791.
• Shukitt-Hale, B., Banderet, L. E., and Lieberman, H. R. (1991). Relationships between symptoms, moods, performance, and acute mountain sickness at 4,700 meters. Aviat. Space Environ. Med. 62, 865–869.
• PubMed Abstract | Google Scholar
• Shukitt-Hale, B., Rauch, T. M., and Foutch, R. (1990). Altitude symptomatology and mood states during a climb to 3,630 meters. Aviat. Space Environ. Med. 61, 225–228.
• PubMed Abstract | Google Scholar
• Smirmaul, B. P., de Moraes, A. C., Angius, L., and Marcora, S. M. (2017). Effects of caffeine on neuromuscular fatigue and performance during high-intensity cycling exercise in moderate hypoxia. Eur. J. Appl. Physiol. 117, 27–38. doi: 10.1007/s00421-016-3496-6
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Smith, E. M., and Tangpricha, V. (2015). Vitamin D and anemia: insights into an emerging association. Curr. Opin. Endocrinol. Diabetes Obes. 22, 432–438. doi: 10.1097/MED.0000000000000199
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Suchý, J., Dovalil, J., and Peri?, T. (2009). Sou?asné trendy tréninku ve vyšší nadmoøské výšce. ?esk. Kinantropol. 13, 38–53.
• Tuffey, S. (2000). “Psychological preparation of endurance performers,” in Endurance in Sport, eds R. J. Shephard and P. O. Astrand (Oxford: Blackwell Scinece Ltd), 451–457.
• Google Scholar
• Van Biesen, D., Hettinga, F. J., McCulloch, K., and Vanlandewijck, Y. (2016). Pacing profiles in competitive track races: regulation of exercise intensity is related to cognitive ability. Front. Physiol. 7:624. doi: 10.3389/fphys.2016.00624
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Virues-Ortega, J., Buela-Casal, G., Garrido, E., and Alcazar, B. (2004). Neuropsychological functioning associated with high-altitude exposure. Neuropsychol. Rev. 14, 197–224. doi: 10.1007/s11065-004-8159-4
• CrossRef Full Text | Google Scholar
• Walsh, N. P., and Oliver, S. J. (2016). Exercise, immune function and respiratory infection: an update on the influence of training and environmental stress. Immunol. Cell Biol. 94, 132–139. doi: 10.1038/icb.2015.99
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Wing-Gaia, S. L. (2014). Nutritional strategies for the preservation of fat free mass at high altitude. Nutrients 6, 665–681. doi: 10.3390/nu6020665
• PubMed Abstract | CrossRef Full Text | Google Scholar
• Wilber Randall L., (2001) “Current Trends in Altitude Training Sports Medicine volume 31, pages249–265