Training in the heat for improved performance
A story about getting ready for cycling in the mountains by training in the heat.

We see a large majority of professional athletes doing an altitude trainings camp every year before major competitions. The main reason for conducting high altitude camps is to acclimatise the athletes to the altitude and to increase the oxygen transport capacity of the blood. In 2019, our head of science Dr Tim Podlogar took part in a cycling event Granfondo Stelvio Santini featuring mountain climbs where he finished third and did training in the heat instead of altitude acclimatisation. And given the success, this helped him to improve his performance. So what was the underlying idea and what have we done?

Partial pressure of oxygen is reduced at the altitude. This means that even-though the oxygen concentrations at the altitude is the same as at the seaside, there is actually less oxygen at the altitude. This means that performance is decreased.

Tim’s critical power at the seaside is around 345W. At 2000 m above sea level it would only be 310W, at 2500m 297W and at 3000m 284W. This is a substantial decrement!

This decrement in physical abilities does not only have implications for pacing during a race but also requires athletes to get ready and try to minimise this loss of performance.

There is no doubt that residing at altitude and thus getting acclimatised to the altitude can make this reduction in performance smaller. However, the other important reason why athletes decide to do altitude training camps is to improve performance at normal altitude due to increased red blood cell count. The effectiveness of such an approach is still questioned in the literature but our view is that altitude can in certain scenarios indeed be beneficial.

Below are two recent articles worth having a read for those interested in hypoxic training:

Acclimation or Acclimatisation

We hear both terms pretty often and it is probably time to explain if they mean different things. As a matter of fact there is a difference.

Acclimatisation takes place in the natural environment. In case of heat acclimatisation this would mean in the summer heat and in case of altitude acclimatisation on a mountain.

Acclimation on the other hand takes place in special facilities, environmental chambers. In those special rooms humidity, temperature and amount of oxygen in the air can be changed so to replicate the conditions athlete would be experiencing at altitude. When it comes to altitude, hypoxia is created by replacing some of the oxygen with nitrogen creating an environment with a lower percentage of oxygen in the air, whereas at the altitude concentration stays the same only pressure is changed. However, the effects are thought to be very similar if not the same.

Environmental chamber at the University of Birmingham, UK

Heat acclimation

There is no doubt that athletes need to get used to the heat in order to compete effectively in the heat. The core temperature is fairly well regulated and is 37ºC at rest. Any deviation from this is not appreciated by the human body and we use various mechanisms to counteract changes. Given the fact that many important competitions (e.g. Olympic Games) take place in very hot climates, preparing athletes for competitions in high heat or humidity has been the subject of numerous studies. The fact is that competition in the heat reduces performance and this reduction is even greater if the athlete is not used to training in the heat.

This is why athletes utilise heat acclimation. This usually involves 1-2 weeks of training in the heat just before the competition. Here are some reasons why heat acclimation is beneficial:

Improved thermal comfort:

  • Lower body core temperature
  • Improved sweating response (quicker onset and higher capacity)
  • Increased perfusion of the skin
  • Better movement efficiency in the heat

Performance improves due to:

  • Lower heart rate
  • Higher stroke volume
  • Better regulation of blood pressure
  • Improved body fluid regulation
  • Improved thirst sensation
  • Bigger water stores within the body
  • Increased plasma volume
  • Improved mental resistance to the heat
  • Reduced loss of electrolytes in the sweat

Tim’s race took place in early June 2019 and the start was in the early morning when the temperatures in alpine valleys are still very low. So, why did he train in the heat?

Tim lived in Birmingham, Great Britain. There are no climbs around. But what he did have was access to the environmental chamber at the University of Birmingham. This chamber is capable of simulating both heat and altitude. When we started planning his training programme, we found that there is almost no evidence that only acute exposure to altitude would be beneficial (i.e. only training but no living at altitude). Given the scientific debate about the effectiveness of altitude training, Tim also decided to take a risk and try something new. In fact, there are many similarities between hypoxic training and training in the heat.


The first thing that happens when you get to the altitude is that your ventilation increases. You have to inhale and exhale more air in order to get the same amount of oxygen as in normoxia (i.e. seaside). What happens in the heat? Ventilation increases as well. There is emerging evidence showing that respiratory muscle training improves exercise capacity at altitude. This could mean that some of the performance improvements in hypoxia that come as a result of hypoxia acclimation could be attributed to improved function of respiratory muscles. Guess what happens during training in the heat? Ventilation is increased as well.

Molecular responses

As a result of training we get better. We all know that. But in order to get better, numerous things need to occur in our body. For instance, mitochondria need to grow in number and/or improve their function. For this to happen certain genes need to be activated. Interestingly, it looks as both hypoxia and heat exposure activate similar pathways, including Hypoxia-Inducible-Factor-1 (HIF-1) and Heat Shock Proteins (HSP).

HIF-1α induction with various stressors and potential relationships between HIF-1and systemic acclimation responses to heat and hypoxia. Italics indicate a potential relationship between HIF-1 and acclimation that has not yet been experimentally examined. Taken from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4977168/

While a lot is known about hypoxia, much less is known about exposure to the heat so this area of research still requires a lot of investigation. However, at least in theory, heat could have similar effects as hypoxia.

Red blood cell mass

In hypoxia, most attention is paid to increasing the haematocrit and especially the total number/mass of red blood cells in the blood. The ability of the blood to transport oxygen through the body is very important from a performance point of view. It is so important that professional cyclists used to inject erythropoietin (EPO) and even previously stored blood (i.e. blood doping). EPO is a hormone that stimulates the production of red blood cells. It is known that exposure to hypoxia induces the production of EPO and thus has the ability to increase red blood cell mass.

Before we start the discussion on heat, let us first briefly address the misunderstandings related to the haematocrit. In the past, haematocrit was used to see how “good the blood” is in relation to physiology. The haematocrit is basically the percentage of the volume that the red blood cells make up. It is believed to represent the oxygen-carrying capacity of the blood. However, this is not necessarily true, since a higher or lower haematocrit after a training intervention is not always accompanied with a higher or lower red blood cell count. The reason for this is that we first need to know the total amount of blood in the body before we can assess the effectiveness of a particular intervention. For example, the haematocrit increases after a few days when ascending to altitude. But this is due solely to a reduction in the total volume of blood – the plasma volume, to be precise. Only after a few weeks do we see an expansion of the real blood cells. The opposite occurs after a few days of exposure to heat. The amount of plasma increases and at the same time the haematocrit decreases. This is the reason why many athletes believe that heat “destroys” red blood cells, when in reality the quantity of red blood cells remains unchanged.

These misconceptions are mostly a result of complicated methodologies for measuring total blood volume. Only a few labs around the world have an ability to accurately measure red blood cell volume.

Back to the heat…

While reading exercise physiology textbooks and in them chapters mentioning heat acclimation you will read that after a few weeks of heat acclimation plasma volume decreases and haematocrit returns to baseline values. However, this data is based on the studies that didn’t properly assess blood volume but rather looked at haematocrit changes. They thought that red blood cell mass would simply not change as a result of heat exposure. However, it looks as low haematocrit might actually be a signal for red blood cell production. So instead of reduced plasma volume, it could be that red blood cell mass increases as a result of heat exposure.

So, based on sound theoretical knowledge, we decided to take the risk and get Tim to train in the heat to get ready for altitude.

We were aware of studies that do not support heat training. However, we thought that the design of these studies was not optimal. But let’s not go into so many scientific details in this article.

Interestingly two studies have been published recently showing that training in the heat can indeed increase red blood cell mass. Study 1. Study 2.

tIM’S TRAINING for granfondo stelvio santini

Project Stelvio – that is how we named Tim’s project of getting ready for the Granfondo Stelvio Santini started on the first of March and ended on second of June when the race took place. In this time there were 106 training sessions and 172 hours on the bike. On average Tim trained 12.3 hours per week. Apart from 3 training sessions, all of them were done indoors. A bit crazy, but this is how it was.

Training was simple. It was completely polarised. 3 weeks of hard work – 3 high intensity interval training sessions weekly and the rest was performed in moderate exercise intensity domain. Each three week long block was followed by a week of testing and easy riding.

This is how training volume in each training zone looked like on a weekly basis:

Tim’s training volume. Blue represents time in moderate exercise intensity domain and red represents time in severe exercise intensity domain. Training was strictly polarised as there was almost no training in the heavy exercise intensity domain.

And this is how training distribution looked throughout the training period across the three exercise intensity domains – moderate, heavy and severe:


In the last training block we used the heat acclimatisation in the climate chamber. The temperature in the climate chamber was 40ºC and the humidity 40-55%.

Guidelines for heat adaptation recommend athletes to ride at an intensity that elicits a core temperature of at least 38.5ºC. However, it is Tim’s opinion that this is insufficient.

It is true that evidence is indicating that reaching core temperature of 38.5ºC is enough to get adaptations to the heat. However, when I measured core temperature while doing 8 minute long intervals, my core temperature reached more than 39ºC and I did not get heat adapted. To me, this means that some other conditions have to be met. Perhaps it is the temperature in the muscle, the skin-core temperature gradient (or lack of it), elevated temperature in the brain or high sweat rates. Who knows.

Given that it is very easy to get to a core temperature of 38.5ºC we opted for a big higher temperature, so 39ºC. A common approach to heat acclimation is to start a session with high intensity intervals to rapidly increase the core temperature. However, this was simply not sustainable due to a very high stress.

The second thing we tried was to train at the same intensity as outside the chamber. An example of such session is seen below. Tim started at 200W and heart rate quickly increased above the 1st lactate threshold as well (the idea was to train polarised so this wouldn’t work). In addition to that, Tim’s core temperature very soon reached very high values that required him to slow down for safety reasons and cycle at an intensity as low as 90W or on some occasions even stop. We thought this wouldn’t be ideal either as we wanted a combined stress of a constant power output combined with a heat stress.

Blue – power; black – hear trate

In the end we found the best approach to be that Tim starts at 200W for 15 minutes after which the intensity was reduced to 65W for a couple of minutes for the core temperature to stabilise and HR to drop. Then, intensity was clamped at the power that enabled Tim to maintain his HR at 135-145 BPM (~150W) which surprisingly also meant stable core temperature at around 40.3ºC.

Blue – power; black – hear trate

Heat sessions were usually preceded with an interval training session so Tim trained twice a day when doing heat acclimation. The aim was to do 3-4 sessions weekly for 4 weeks.

It was actually Kickr that got the treatment of the fan and not Tim.

Comparing power achieved in the chamber with the power achieved in the temperate conditions at the same heart rate can be seen below:

Diff in Power versus HRTemperateHeat
 Power [watt]225142
Heart Rate [bpm]135138

As you can see, the power in the chamber and thus mechanical stress was much lower. However, even though some might argue that training stress score was lower due to lower power output and adaptations should be smaller, this was not the case.

Below are FTP test results and the results at the time to exhaustion test at 360W. In between the second and third measurement heat acclimation took place. The increase in FTP was pretty substantial, wasn’t it?

In the end Tim finished the event on third place and subsequently the winner was found to be positive on the doping test. It can be said that the heat acclimation worked, right?

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