Intermittent Hypoxic Training and Relevance in Sports Activities
On 28 August 2004 the Moroccan Hicham El Guerrouj won the men�s 5,000m Olympic
final in a time of 13:14.39. The race had been close: only 10 seconds separated
the first seven competitors, and less than one second had divided the
medallists. In elite endurance events such as the 5,000m, where seconds can make
the difference between success and failure, it is no surprise to learn that
athletes and coaches are constantly striving for legitimate advantage over their
rivals.
In the early 1990s, Benjamin Levine, a researcher at the University of Texas,
seemed to have made such a discovery. By exposing college athletes to low
concentrations of oxygen (hypoxia) during rest, and normal conditions (normoxia)
during exercise, Levine and his colleagues were able to show that 5,000m times
could be improved by an average of 13.4 seconds in elite college athletes.
Although this �live high, train low� approach to intermittent hypoxic training
(IHT) is now widely used by endurance athletes across the world, a number of
problems and controversies still exist with the technique. Here, we look at the
issues and provide some practical guidance for those who wish to incorporate
this technique into their training programme.
Air pressure: the basics
It is worth spending a few moments considering the movement of oxygen inside the
body. Unlike solids and liquids, gases expand in all directions and occupy the
space in which they�re contained. This makes units of weight and volume
redundant; instead gases are measured in units of pressure. In an enclosed space
filled with gas, molecules are continually colliding with each other and the
walls that contain them. As more gas molecules are added, the collisions become
more frequent and the pressure exerted on the
walls of their container increases. This pressure can be expressed in a host of
different units. Here, I use two common units: the kilopascal (KPa) and the
millimetre of mercury (mmHg).
At sea level the pressure exerted by all the gases in the atmosphere (nitrogen,
oxygen, carbon dioxide and a host of rare gases) adds up to 101 KPa (760 mmHg).
As 21% of the atmosphere is made up of oxygen, the pressure exerted by oxygen
alone (often referred to as the �partial pressure of oxygen�) is approx 21 KPa
(160 mmHg).
As you climb to altitude, the number of molecules in the atmosphere falls,
leading to fewer collisions and a fall in pressure. On the summit of Mt
Kilimanjaro (5,895m) the atmospheric pressure is 50 KPa (380 mmHg); on Mt
Everest (8,850m) the atmospheric pressure stands even lower at 34 KPa (253
mmHg).
Despite these tremendous changes, the proportion of each gas in the atmosphere
remains the same. Therefore, to calculate the partial pressure of oxygen the
atmospheric pressure is multiplied by the proportion of oxygen (0.21).
Clearly, the simplest way to expose athletes to hypoxia is to encourage them to
live at altitude. Unfortunately, this often proves too costly and time
consuming, so two alternatives are available:
*Hypobaric chamber: These devices are constructed from reinforced steel and are
usually operated by medical specialists. They work by removing an equal
proportion of gases from the chamber, thereby reducing the atmospheric pressure
inside. This is exactly what happens during a climb to altitude.
*Hypoxic device: Instead of lowering pressure, these devices remove only oxygen
and replace the missing space with nitrogen gas. This maintains a normal
atmospheric pressure whilst reducing the partial pressure of oxygen, creating a
hypoxic environment at sea level. This arrangement is much simpler to organise
than a hypobaric chamber and is the easiest way for sea level athletes to
experience hypoxic conditions. Such devices come in all shapes and sizes, from
large living quarters to small portable face mask systems.
However, it is not yet clear whether these systems produce the same responses as
those seen in Levine�s volunteers. Why is all this important? In order for gases to move into the lungs there needs
to be a pressure difference. The greater the pressure difference, the greater
the movement of gas. Think of the rapid movement of air when a large party
balloon is burst. As the air inside is at a much higher pressure than its
surroundings, when the balloon bursts the molecules at high pressure pass easily
into the atmosphere. In the same way, at sea level the partial pressure of
oxygen is much higher in the atmosphere than inside the body
(normally 12 KPa or 99 mmHg), so oxygen therefore moves eagerly down the body�s
airways, through the blood stream and into the tissues. However, at high
altitude, the partial pressure of oxygen in the atmosphere is much lower and the
movement of gas through the body and into the cells is much, much slower. In
order to cope with this challenge, the body adapts in two ways. Firstly, it
improves oxygen delivery to the cells, and secondly it encourages the various
cells themselves to cope with smaller amounts of oxygen. It is these adaptations
that are harnessed by a �live high, train low� regime to improve aerobic
performance at sea level.
According to the eminent physiologist John West, �The underlying rationale is
that sleeping at high altitude increases the red cell concentration of the blood
and thus endows some advantage, whereas the actual training should be at sea
level where the aerobic machinery can be driven to its maximum.� Let�s take the
two points raised by West in turn.
i. Changes in red cell concentration
After just two hours of breathing 10% oxygen, the first physiological changes
can be seen in the circulation. The production of erythropoietin, a hormone
synthesised by the kidneys, rapidly increases and immediately sets to work
coaxing the bone marrow into releasing large quantities of red blood cells. For
the athlete, this is great news as an increase in red cell concentration means a
rise in the oxygen-carrying capacity of the blood and a fall in cardiac output
(the amount of blood ejected by the heart in a minute),
which results in the tissues having a longer period of time to extract oxygen.
The end result is something not far from finding the Holy Grail: an increase in
the maximum oxygen consumption (VO2 max) and, with it, a rise in the athlete�s
maximum work rate.
In addition to this profound change, a number of studies have also pointed
towards other adaptations that occur within the muscles themselves. This is
hardly surprising as hypoxia triggers the activation of HIF-1? (hypoxic
inducible factor -1?), which is responsible for stimulating the production of
proteins from a range of different genes.
ii. Training at sea level
In a series of experiments, Levine and his colleagues found that resting and
training at altitude (�live high, train high�) fails to produce improvements in
performance. Although training in a hypoxic environment feels considerably
harder than at sea level, athletes are unable to reach their maximum work rates
or levels of oxygen consumption. This is like driving a car at speed in third
gear � although it feels much harder, progress is slow.
When healthy, well acclimatised volunteers ascend to altitude, VO2 max falls
prodigiously. This inability to use oxygen at higher altitudes results in a fall
in maximum aerobic power of approx 1% for every 100m gained above 1,500m of
altitude. Therefore, the end result for any athlete who trains at high altitude
for any length of time is a fall in the levels of exercise intensity and a
general reduction in their overall level of physical fitness.
The evidence for �live high, train low�
With more than 15 years of evidence now available, we still have uncertainty and
controversy, as well as areas of agreement in the research. Three preliminary
observations are worth making:
i. Quality: Quality research in this area is expensive and time consuming. The
best studies have been funded by substantial grants from either the
International Olympic Committee or national sports agencies. Much of the work is
otherwise poorly financed and this is reflected in the small sample sizes,
absence of control groups and insufficient follow-up times that characterise
many of these studies. Any conclusions drawn from such work need to be treated
with a great deal of caution and are mostly avoided here.
ii. Participation: Most participants in �live high, train low� trials are young,
white and male elite athletes. This makes it difficult to apply these results to
a wider spectrum of the �normal� population.
iii. Specificity: The majority of work has focused upon the performance of
middle distance runners. Although some work has been undertaken on other elite
endurance athletes (skiers, swimmers and cyclists), it would be a leap of faith
to apply the findings to other endurance events.
How much hypoxia is good?
From the limited evidence available, an altitude of between 1,600m and 3,000m
seems necessary to generate a consistent increase in red cell concentration. At
altitudes below 1,600m there is little change, while above 3,000m athletes run
the risk of incurring problems that can cancel out any potential improvements
they have gained.
In order to obtain improvements in red cell concentration, VO2max and athletic
performance, Levine�s athletes lived at an altitude of 2,500m (equivalent to 16%
oxygen) for up to 20 hours a day while training at sea level, for four weeks.
Few studies have come close to emulating such a lengthy period of exposure, but
the results from those that have are worth mentioning here. Despite some
agreement with Levine�s findings, a number of studies have shown either no
improvement or have identified enhancements in
performance without a change in red cell concentrations.
Recent research has shown that the HIF-1?protein not only stimulates the
formation of red cells but has also been linked with improvements in muscle
efficiency. This is characterised by improvements in blood flow, the supply of
glucose and the clearance of lactic acid from working muscles. These changes may
be of considerable benefit to the Olympic 5,000m finalist and therefore offer a
clear advantage over a simple blood transfusion or �blood doping�, which often
occurs before major sporting events.
On 28 August 2004 the Moroccan Hicham El Guerrouj won the men�s 5,000m Olympic
final in a time of 13:14.39. The race had been close: only 10 seconds separated
the first seven competitors, and less than one second had divided the
medallists. In elite endurance events such as the 5,000m, where seconds can make
the difference between success and failure, it is no surprise to learn that
athletes and coaches are constantly striving for legitimate advantage over their
rivals.
Why is all this important? In order for gases to move into the lungs there needs
to be a pressure difference. The greater the pressure difference, the greater
the movement of gas. Think of the rapid movement of air when a large party
balloon is burst. As the air inside is at a much higher pressure than its
surroundings, when the balloon bursts the molecules at high pressure pass easily
into the atmosphere. In the same way, at sea level the partial pressure of
oxygen is much higher in the atmosphere than inside the body
(normally 12 KPa or 99 mmHg), so oxygen therefore moves eagerly down the body�s
airways, through the blood stream and into the tissues. However, at high
altitude, the partial pressure of oxygen in the atmosphere is much lower and the
movement of gas through the body and into the cells is much, much slower. In
order to cope with this challenge, the body adapts in two ways. Firstly, it
improves oxygen delivery to the cells, and secondly it encourages the various
cells themselves to cope with smaller amounts of oxygen. It is these adaptations
that are harnessed by a �live high, train low� regime to improve aerobic
performance at sea level.
From the limited evidence available, an altitude of between 1,600m and 3,000m
seems necessary to generate a consistent increase in red cell concentration. At
altitudes below 1,600m there is little change, while above 3,000m athletes run
the risk of incurring problems that can cancel out any potential improvements
they have gained.