As runners when we think of altitude (referenced from sea level) we are primarily considering the effect on the oxygen carrying system of the body. This begins with oxygen from the ambient air being drawn into the lungs where it diffuses across the ultra thin membrane of the alveoli into the adjacent bloodstream. This oxygenated blood then flows on to the heart where the heart then pumps it to the working muscles. In the working muscles structures called mitochondria then utilise the oxygen that is absorbed into the cells of muscles to burn triglycerides (fatty acids) to create energy. At low to moderate training intensities (for most runners) the fat burning metabolic pathway is the dominant mechanism through which energy is created to fuel ongoing muscle contraction.
In order for this system to operate efficiency no matter the circumstances, the body is armed with hypoxic sensors or receptors that constantly detect the level of oxygen saturation in the blood. More precisely this is the saturation of oxygen held in the haemoglobin of red blood cells transported in the blood stream. This hints at 2 critical intrinsic controlling variables that will become very important in the efficiency of this system: the red blood cell count in a given volume of blood and the quantity of haemoglobin that can capture and hold the oxygen.
the red blood cell count in a given volume of blood and the quantity of haemoglobin
When we move from sea level to higher altitude the molecules that make up the air are free to be further apart and consequently the barometric air pressure drops. This means that at altitudes above sea level for a given volume of air that you inhale into your lungs there will be less molecules of oxygen (at all altitudes the ambient air contains the same percentage of oxygen, approximately 21%). In this context we often refer more specifically to the partial pressure of oxygen being reduced. At this point there are 2 key factors to keep in mind. Firstly that the number of oxygen molecules available to be diffused into the bloodstream are lower and secondly that the pressure of the oxygen and also potentially the air in general is lower. In the case where there is less oxygen available to enter into the blood and the pressure difference driving it to enter the bloodstream is lower the result is a relative state of hypoxia.
we often refer more specifically to the partial pressure of oxygen being reduced
Initially as the amount of oxygen in the bloodstream reduces the body will not respond dramatically to it. As the brain receives signals that indicate minor hypoxia it will then instruct the heart to beat with greater force and at a higher rate. If the change in oxygen level is small the brain may decide that even this is not necessary. As runners, we may be aware that at certain altitudes above sea level we feel a little short of breath with higher than normal heart rate whilst running. This problem solving mechanism continues until a critical threshold is reached where the brain deems it necessary to summon back-up.
The primary back-up mechanism of note is the endocrine system and the production of EPO (erythropoeitin) from the kidneys. EPO acts as a signalling chemical that triggers the production of new red blood cells from the bone marrow. Whilst the perpetual background processes of the body include constant replenishment of new red blood cells, the additional stimulus provided by the presence of extra EPO causes even more red blood cells to be produced than would have otherwise. Controlled scientific studies have identified this process happening in athletes after just one day of exposure to altitude. However simply adding more red blood cells to the bloodstream is not enough to counteract the state of relative hypoxia. Haemoglobin is also required to sit within these red blood cells to capture and hold the oxygen. In order to build more haemoglobin a surplus of iron above that required for normal daily function is also necessary, as iron is the primary building block of haemoglobin.
iron is the primary building block of haemoglobin
In effect then, increasing the number of solid structures (also termed the hematocrit) that can carry oxygen in the blood is the body’s primary way of adapting to altitude and ensuring that enough oxygen is getting to working muscles to fuel the muscles during exercise. As I’ll describe later this is not the only adaptation option available to be the body but it does appear to be the primary solution the body uses for altitudes in a range just above what we can live at without adaptation. What are commonly known in running as ‘moderate’ altitudes or training altitudes.
For many runners interested in knowing how to avoid or target significant altitudes a key question is at what altitude does the body start to initiate a true adaptation to the relative hypoxia? Real world data from around the world taken from a range of different altitudes suggests that for the average runner this will happen somewhere in the region of 1700m to 1750m above sea level. And hence why many of the world’s most popular altitude training centres are just above this level, generally between 1700m (example: Boulder, USA) and 3000m (example: Bekoji, Ethiopia).
As a resident of Cochrane (AB), Canada I can personally attest to the validity of this rule of thumb. Cochrane is situated at 1150m above sea level with peaks up to 1300m. At this altitude it is possible for more observant runners to notice a very slight increase in difficulty in breathing and sustaining challenging running paces. However because this altitude is not high enough to bring about a true adaptation in the runners blood profile there is no significant altitude training benefit when returning to lower altitudes for running. Although people may agree or disagree as to whether they felt a sea level run was easier or not, physiologically there is in fact no difference in their blood.
The first point to make on deliberate altitude training for endurance sports is that not everyone responds positively. Some people are responders (a good example is Paula Radcliffe from Great Britain) and some are not (a good example is Alistair Cragg from South Africa). In fact some notable studies have shown that some athletes exposed to altitude actually experienced a negative overall effect on their training. Some of this reported negative effect could simply be due to an incorrect training protocol being followed whilst at altitude. This is turn highlights a second critical point, that in those athletes that respond a unique and personal training plan is required as every athlete will respond at different times and in different ways. There are some genes that have been identified and postulated as being influencers on this altitude response. However the picture is still largely unclear as genes generally interact in large numbers rather than in isolation and currently this area of genetics research is still in its infancy.
some athletes exposed to altitude actually experienced a negative overall effect on their training
Hypobaric and Hypoxic (the real world)
Real world altitude training requires both careful selection of training venue and a large dose of both time and patience on the part of the athlete. The optimal training altitudes range from the aforementioned starting level of 1700m to 3000m. Above 3000m most athletes will endure such a high level of hypoxia that much of the training performed will be rendered ineffective. This is because whilst the body will be forced to adapt, the lack of oxygen and resulting impact on muscle performance will mean that workouts will be run at much slower pace and ultimately the runner could have gotten more training benefit from high quality workouts performed at lower level.
Sufficient time is a critical component of a successful training program at altitude. Firstly time will be need to be added at the start of the altitude phase to account for the athlete acclimatising to both the altitude and the likely change in humidity. Depending on the athlete this may take one to two weeks. If the destination is a long way from the athletes’ home then there may also be a time change adaptation required too. After this period more serious workouts can be performed that mimic more closely what the athlete would be able to run if at their regular training location. However it is still common for coaches to formulate their own unique rules of thumb for calculating the difference in running pace for each athlete when they are performing workouts at altitude.
acclimatising to both the altitude and the likely change in humidity
These rule of thumb formulas should take into account the type of workout. What I have commonly noticed in athletes during altitude training stints is that the runner will often run on target pace to begin with but then burn out after a short period of time. Then with a quick rest the runner will feel good again and again set off at target pace before again burning out and needing a rest. Hence the runner will be more capable of sustaining short intervals of high speed running than they will be of attempting to sustain a strong pace on a continuous run for significant length of time (ie. a tempo workout). One interesting side note is that whilst it is true to say that distance runners find it harder to train at altitude, sprinters will generally find it easier. Not only are sprinters not running for boughts long enough to get them into serious oxygen debt (they are mostly burning free ATP and glycogen) but as the air pressure is lower at higher altitude they will also experience less air resistance as their body’s try to move air out of the way at high speed.
sprinters will generally find it easier
A key physiological concept underpinning the planning of altitude training is the understanding of the continual cycle of red blood cell replenishment. The rate at which new red blood cells are created will not only affect the rate at which the athlete can adapt to altitude but it will also influence the length of time that effect continues after the athlete has returned to normal training/racing level. This is why it is common to assume that after 2 weeks much of the training adaptation will be lost after an altitude stint. Quite simply the body has had time to turn over a new army of red blood cells that have now been created in response to normal altitude rather than the training altitude.
What this all means in reality is that most well trained smart distance runners seeking a competitive advantage will aim to reside at an altitude training location for 4-6 weeks continuously (schedule permitting). This will give them enough time to acclimatise and then enough time to get meaningful training in during the period that their bodies are experiencing the largest positive gains in endurance performance. After returning to normal level they will aim to run a key race within 7-10 days – enough time to rest from the travel, to get used to running faster at lower altitude and then apply the advantage before it begins to disappear.
Normobaric and Hypoxic (the artificial method)
For those that can’t or don’t want to travel to higher altitudes for training the alternative, now relatively well known and accepted is using simulated altitude, most commonly an altitude tent. In short an altitude generator takes ambient air and scrubs a given amount of the oxygen content out and then supplies the hypoxic filtered air at a range of adjustable oxygen levels and flow rates. This procedure differs from real altitude in that there is no pressure drop in the air supplied, hence this simulated training is hypoxic but not hypobaric (generally referred to as normobaric). Whereas at altitude you still breathe approximately 21% oxygen in the air, with simulated altitude the percentage of oxygen is artificially lowered. The hypoxic air can then be piped into a sleeping tent, training chamber or face mask.
This system has a number of advantages most clearly that travel and its associated costs and time changes are not necessary. Simulated altitude training also allows control over the level of hypoxia and hence a more bespoke training protocol with a gradual build up. In most cases this kind of training is associated with a protocol of live high – train low. In reality this means the athlete sleeps at simulated altitude during the night and trains at their normal local altitude during the day. A number of well known and well documented studies have demonstrated that live high – train low is on balance the most effective way to gain an altitude training advantage. Put simply the athlete gets the background hypoxic exposure to affect their blood profile but can still train at full intensity at the lower altitude.
live high – train low is on balance the most effective way
More recent detailed work by researchers in the US has indicated that the optimal way to use simulated altitude training is to build up the altitude over a week and then spend the next 3-4 weeks leading up to a race at the target altitude (commonly 2000-3000m). Not only this they have also shown that the athlete needs at least 9 hours of exposure per night and that for people using a tent for less than 12 hours a day, setting the equivalent altitude to 3000m is necessary to provide a significant stimulus. In extensions to this work the researchers have also found that whilst testing college cross country runners, a systematic methodology of 3 nights in the altitude tent followed by a night out of the tent, repeated cyclically for the duration of the stint provides the optimal results.
The results surrounding the research into simulated altitude training conform to the same responder and non-responder split that occurs with real altitude. For those athletes that experience a performance benefit the amount of gain in performance for distance running appears to vary between 3% and 8% with some rare exceptions exceeding 10%. It is reported results like this that have demonstrated that in some cases simulated altitude training appears to be able to work just as well as traveling to a real altitude venue. It could be for this reason that some in the global athletics community have called into question whether simulated altitude should be considered as cheating and should be banned by the IAAF.