HYPOXIA AND HYPEROXIA

Hypoxia is a state in which the oxygenation of certain tissues, organs or the whole body is insufficient in relation to their oxygen demand. The physiological cause of hypoxia in the human body is exposure to natural (hypobaric hypoxia) or simulated (normobaric hypoxia) high-altitude conditions. Hypobaric hypoxia occurs in high mountains and results from an O₂ deficit in the air caused by its “thinning” as atmospheric pressure drops. Up to an altitude of about 13,000 m above sea level the percentage composition of atmospheric gases does not change — oxygen remains at 20.9%. As altitude increases, barometric pressure decreases, reducing the amount of O₂ in one cubic metre of air, while its percentage share remains constant. The drop in atmospheric pressure with altitude causes a proportional decrease in the partial pressure of oxygen in the blood of people staying at altitude. Similar conditions can be simulated at sea level in hypobaric (pressure) chambers. Another way to simulate high-altitude conditions is to create normobaric hypoxia. In the simplest terms, this consists in a controlled reduction of the O₂ percentage in the breathing mixture, replacing it with an increased concentration of nitrogen, which is neutral for the human body. The air composition changes without any change in atmospheric pressure. This also lowers the partial pressure of oxygen in the blood, as during a stay at high altitude, but at constant barometric pressure. Importantly, the body then activates almost identical adaptive mechanisms aimed at restoring homeostasis.

Hypoxia reduces exercise capacity, especially during prolonged efforts lasting more than two minutes. Interestingly, no deterioration is observed for short-term and static efforts.

The direct cause of reduced exercise capacity in hypoxia is the lower partial pressure of oxygen in arterial blood (PaO₂), which limits O₂ transport to the tissues and consequently reduces maximal oxygen uptake (VO₂max) and exercise capacity.

The body’s basic adaptive mechanism to hypoxia — and the one that directly improves exercise capacity — is improved O₂ transport by the blood. Limited O₂ availability increases the secretion of erythropoietin (EPO), the hormone that plays the main regulatory role in red blood cell production. This was initially the main reason why endurance athletes took up altitude training. Research in recent years also shows that, in addition to improved blood oxygen capacity, athletes can gain other, non-haematological benefits from hypoxia, such as enhanced angiogenesis, higher glycolytic enzyme activity, and increased buffering capacity of blood and tissues.

As a result, systematic exposure and physical training in a hypoxic environment improve exercise capacity both in normoxia and hypoxia, as adaptive changes in the body deepen. In the light of scientific reports, altitude hypoxia should be considered an ergogenic aid that improves human exercise capacity.

The literature to date shows that hypoxia is an extremely effective training and therapeutic tool, which is why it is widely used in medicine (treatment of obesity, asthma, type II diabetes), rehabilitation (cardiac rehabilitation, rehabilitation of athletes) and sport (improvement of physical fitness and exercise capacity). It should be noted that the key to correctly stimulating the body’s adaptive mechanisms is the proper selection of exposure time and hypoxia intensity, as well as appropriate training and therapeutic stimuli.

The therapeutic properties of hypoxia are revealed through the activation of hypoxia-inducible factor (HIF-1), which directly affects the expression of more than 200 genes and enzymes. This translates into a range of beneficial changes in the body that can be used therapeutically. For example, training in hypoxia increases leptin secretion, helping to reduce appetite. An increase in the number of endothelial glucose transporters (GLUT) has also been found, which directly affects blood glucose levels and the activity of glycolytic pathway enzymes. Systematic exercise in hypoxia has also been observed to reduce body weight and cholesterol levels. Moreover, hypoxic training is an excellent solution for people with obesity, because even low-intensity exercise brings significant health improvement without excessive load on the musculoskeletal system. When considering the health benefits of hypoxic training, one cannot overlook the increased activity of nitric oxide synthase (NOS), a strong vasodilator. Increased NOS activity lowers arterial blood pressure — a highly desirable response in people with hypertension.

The origins of altitude training date back to the 1968 Summer Olympics in Mexico City, held for the first time in history at an altitude above 2,000 m (~2,240 m above sea level). Both during preparations and after the games, many new questions arose about training methodology in high-altitude conditions. During the competition it was observed that sprinters and jumpers set many new world records, while the opposite trend was seen in endurance events, where results were significantly weaker than in previous years.

The significantly better results in sprints, jumps and throws were due to improved aerodynamics in the thinner air caused by lower atmospheric pressure. On the other hand, that same reduced pressure at altitude directly impairs the ability to perform endurance-type efforts.

Over more than four decades of altitude training, several methodological approaches using hypoxia in sports training have appeared in the literature and in practice.

The first altitude training concept, developed in the late 1960s, was the “live high — train high” (LH-TH) procedure. It involves both living and training at 2,000–3,000 m above sea level for several weeks. Numerous studies have shown that lower altitudes do not stimulate erythropoiesis sufficiently, while higher altitudes require much longer adaptation, which disturbs the training process. Extended adaptation time also means longer periods of reduced training loads, which can lower an athlete’s competitive form.

The traditional LH-TH method is aimed primarily at improving exercise capacity by stimulating erythropoiesis and increasing the oxygen capacity of the blood. Its weakness, however, is that the large reduction in training load required for altitude acclimatisation can lead to a loss of form after returning to sea level.

In the late 1990s an alternative concept appeared: “live high — train low” (LH-TL), a modification of the original LH-TH procedure. The new approach was designed to take full advantage of adaptation to hypoxia while minimising the reduction in exercise capacity that disrupted training programmes in LH-TH. LH-TL assumes that the athlete stays at 2,000–3,000 m above sea level for 3–4 weeks, while training is carried out no higher than 1,000 m. The authors of this unconventional solution, Levine and Stray-Gundersen (1997), confirmed its effectiveness in their research: a significant 5% increase in maximal oxygen uptake (VO₂max) and a 9% increase in blood haemoglobin concentration in subjects living at 2,500 m and training at 1,250 m for 28 days, along with significant improvement in 5,000 m run times. No such changes were observed in the control group.

The LH-TL method has also been modified — an interesting variant was proposed by Stray-Gundersen et al. (2001): living and low-intensity training at 2,500 m for four weeks, with high-intensity interval training performed at 1,250 m (live high — base train high — interval train low; HiHiLo). The study found a significant 3% increase in VO₂max and a significant improvement in 3 km run time, by ~6 s on average, associated with significant increases in haemoglobin concentration and haematocrit. Similar changes were observed in other studies (Czuba et al.) based on the HiHiLo protocol. The training effectiveness of LH-TL has been confirmed in further research as well.

Initially the LH-TL method used only the natural environment (hypobaric hypoxia), regularly transporting athletes from 2,000–3,000 m to below 1,000 m for training. Such a solution requires complex logistics and considerable time and money. However, the dynamic development and popularisation of equipment generating normobaric hypoxia has made the LH-TL concept available to most athletes, even amateurs.

The latest research on LH-TL using normobaric hypoxia has shown significant increases in both haematocrit and blood haemoglobin concentration.

In recent years, thanks to the development of normobaric hypoxia technology, much attention has also been paid to another altitude training method — “live low — train high” (LL-TH). In the LL-TH model, athletes spend the day in normoxia, while training takes place in hypoxia. This form of training most often uses normobaric hypoxia, usually obtained by oxygen filtration or by diluting the oxygen concentration with nitrogen. Two approaches are distinguished within LL-TH. The first involves only short exposure (90–180 minutes) to normobaric hypoxia at rest (Intermittent Hypoxic Exposure — IHE); it is used in medicine, but athletes show no improvement in exercise capacity because the stimulus is too short.

Sport uses the second approach — training in hypoxia (Intermittent Hypoxic Training — IHT). Its theoretical basis is the assumption that the stress of hypoxia combined with training stress produces greater adaptive changes than training in normoxia.

This form of altitude training most often simulates hypoxia corresponding to 2,000–3,500 m above sea level. A major advantage is that time spent outside the chamber can be used for other training activity. Recovery after IHT takes place in normoxia, which prevents the negative symptoms of prolonged hypoxia and shortens post-training regeneration. This approach does not force a reduction of training loads during sessions in normoxia.

IHT does not change haematological parameters — the hypoxia exposure is too short — but a number of other beneficial adaptations improve exercise capacity. The procedure has been shown to enhance anaerobic capacity and reduce the energy cost of work by improving muscle tissue buffering and increasing glycolytic enzyme activity. Relatively recent studies have shown significant improvement in anaerobic performance and short-duration efforts.

In summary, the effectiveness of this form of training depends above all on properly planned methodology — the type of effort, its volume and intensity, and the selection of the hypoxic stimulus.