Training and heat: Rouleur Performance

Rouleur performance takes a deep dive into the physiological challenges of training in heat, a serious challenge for Grand Tour challengers and turbo warriors alike

If you live in the Northern Hemisphere, are able to perceive temperature, and have a passion for cycling in daylight, chances are that riding outdoors is not currently an appealing option.

But it’s not all sunshine down in the Southern Hemisphere either. Well, actually, it is – and that’s sort of the problem. Many cities are experiencing heat wave conditions. Wherever you live, be it cold and miserable or unbearably hot, indoor cycling is an increasingly convincing alternative to battling wind, rain or snow. 

Improvements in smart trainer technology together with some online cycling platforms reaching maturity has made indoor cycling less of a chore while gamification elements can make pedalling in the cellar an almost fun experience.

However, despite these technological advances there is one obvious drawback. 

Riding indoors is hot. 

But it doesn’t have to be unbearable. With an understanding of how our bodies function in a hot environment, and an informed strategy to beat the heat, an indoor cycling session can be more effective, safer and certainly more comfortable.

We'll explain the science around heat and exercise a little later, but first we will outline a strategy to manage heat while training indoors.

Related – The best headphones for indoor training
Related – The best smart turbo trainers

How to stay cool

Body type

Firstly, while this is something you can't exactly change, this determines how seriously you need to take different cooling strategies. Certain body types retain heat, while others are efficient at heat removal. So if you have a more endomorphic body shape (short/stocky) then a really close adherence to all of these recommendations will deliver a significant benefit.


Most modern cycling attire is lightweight and breathable. However, leaving skin exposed to airflow is ideal – admittedly sometimes your location can restrict this. Men should try wearing their bib shorts with no shirt, while for women a sports bra under your knicks is a practical option.


Stay hydrated at all times. Even on days where you are not planning to train indoors. So how do you know if you are hydrated? Simply monitor the colour of your urine. This starts as soon as you wake up. At this early point in the day take note of your urine colour and compare it to the Urine Colour Chart which is presented at the end of this article. Then drink 500ml of water immediately. This is enough to overload your kidneys and force another urination within the next 30-60mins. Then check the colour of your second urination which should be close to optimal; 1 to 3 on the urine colour chart. Then keep it there (1-3) throughout the day by sipping small amounts of fluid when necessary.

Once on the bike it is a good idea to sip 150-200ml of fluid every 20 minutes. This can be water if you are riding anywhere from 30 to 60 minutes at moderate intensity. However, if you plan on riding harder or longer then a commercial sports drink is a better option.

Photo credit: Simon Wilkinson/

Training Room 

The aim is to discourage the accumulation of hot and heavy air in your training room. By heavy we mean air containing lots of water which increases humidity. If you can open a window at the front of your training room and leave a door open behind you this will encourage the recycling of air while you train. And remember: if you are left with a significant amount of sweat on your bike or floor then your training room is not optimised for evaporative cooling. Another tip is to leave all doors and windows open for 30 minutes before you start your ride.

Airflow = Cooling

This is the most important tip for making your indoor cycling sessions more enjoyable. Make sure you have a fan setup to achieve the benefits of evaporative cooling. The positioning of this fan is also important, though.

Since the majority of our body's sweat glands are located on the chest, forehead and back the airflow from this fan should be pointed at these areas. 

To target the sweat glands of the forehead and chest, place one fan directly in front of you. But an even greater evaporative cooling effect will be achieved by positioning a second fan behind you, to target the sweat glands of the back.

Try to avoid wearing a headband that covers the forehead. If you really must wear a headband make sure it is positioned high up on the hairline. In this position the sweat glands of your forehead are exposed to your fans airflow.

Discover Healey Cycling – bespoke coaching services in association with Rouleur Performance

Why we need to stay cool: The science behind training in the heat


To understand how a cyclist copes with training in a hot environment, it is worth looking at the limits of body temperature and how they are regulated at rest and while exercising. 

Resting core body temperature is tightly controlled and remains stable at around +/-37°C while skin temperature can drop to ~20°C if left uncovered in a very cold room. But once you have moved past the warm up phase of your indoor cycling session things begin to change. 

The temperature inside working muscles increases and can reach temperatures as high as 41°C while core temperature might only change a degree or two. It is worth noting that the safe limits of core temperature are 36°C to 37°C with hyperthermia (the opposite of hypothermia) being registered at 40°C and death by heat stroke at 42°C.

Now, as an indoor cycling room gets hotter and the temperature inside exercising muscles increases, the brain detects this stress and sweat production is accelerated. Under optimal environmental conditions (low humidity/good airflow) heat from exercising muscle is captured by the blood and delivered to the surface of the skin. 

Think of this response as a slightly more sophisticated car engine-cooling unit. The human heart replaces the mechanical pump, skin functions as the radiator and blood is the coolant. For a more enjoyable indoor cycling experience the aim is to optimise this system and your environment.


The relieving sensation of a cool breeze passing over your skin on a hot day is known as evaporative cooling. Evaporative cooling is very important for all cyclists when training outdoors, where physical exertion obviously generates a lot of heat. So what happens to performance if training is completed indoors? 

You have probably experienced this yourself: indoor cycling ‘feels’ harder and evaporative cooling is part of the reason why indoor vs outdoor power data is so disparate. When riding outside the headwind generated by a cyclist's forward momentum is continuously recycling a layer of air next to the skin which in turn accelerates evaporative cooling. But when riding indoors this process is dramatically reduced and the heat loss from the rider to the outside environment is significantly less. 

Further to this, the humidity can also limit performance. If humidity is 75% or higher in your indoor training setting, the sweat coming off your skin is more likely to remain a liquid and drip onto the floor. So, if you think being soaking wet is an indicator of a good indoor ride, then think again... 

All that water was meant to evaporate into the atmosphere taking your body heat with it. Which means that puddle of water (and rusting headset screws) is confirmation your evaporative cooling was ineffective and exercise performance was increasingly compromised. 

Water and hydration

The ability to cool off by sweating and subsequent evaporative cooling is also dependent on the availability of water. When water lost via sweating is not replaced, the inevitable result is dehydration. 

Mild to moderate exercise intensity will yield sweat rates of 0.8 – 1.4L/h.  On the other side of the equation is fluid uptake. The maximum amount of fluid that empties from the stomach during exercise is 0.8-1.2L/h. Here we can see a clear difference of 200ml between water loss and replenishment rates. This is ground zero for the onset of dehydration. 

In a hot environment small to moderate levels of dehydration (2-4%) will lead to a decrease in VO2max of +10% with a corresponding drop in endurance capacity of more than 20%. In this situation plasma volume is also reduced, which increases the viscosity of blood. Thicker blood is harder to pump around the body, leading to a higher heart rate and ultimately less heat being lost from exercising muscles. So clearly, any strategies that promote optimal hydration will translate into better performance.

The kidneys are a key player in this water balancing act while kidney excretion (urine) can provide clues to your hydration status. During an indoor cycling session that produces a significant sweat rate, the kidneys will restore equilibrium in fluid balance within hours provided enough fluid is consumed during and after the ride. The kidneys achieve this rapid rate of rehydration via two hormones: Aldosterone which targets sodium and potassium balance and Antidiuretic Hormone (ADH) which controls water balance. 

Unless dehydration becomes extreme (exceeding 3% body weight) these two hormones work to maintain hydration within 1% of normal limits. Knowing this, it’s possible to deduce a lot around hydration level from the properties of urine. As a simple guide: urine should be dilute and slightly opaque when you are hydrated, and when you are dehydrated urine should be darker in colour.

This observation led to the development of the Urine Colour Chart. The chart is a colour coded scale from 1 (clear) to 8 (heavily concentrated) with an ideal target range of 1 to 3. It is a simple and effective guide to your hydration status and is used across all high performance sports. 


When evaporative cooling cannot take place performance will deteriorate and your enjoyment of indoor cycling will be diminished. If inappropriate clothing is selected for a ride then the evaporation of sweat is further reduced and core temperature will rise. 

In these days of modern cycling it is easy to find lightweight and breathable cycling attire. But this was not always the case. In days gone by it was not uncommon to see people exercising wearing multiple layers of clothing - often in the misguided assumption that more sweating was the sign of a better workout. In some extreme instances track suits were lined with plastic bags then taped at the wrists and ankles. 

That kind of impermeability is not only uncomfortable but genuinely dangerous when undertaking endurance exercise. In one extreme example a British Soldier wore a rubber diving suit for a training run. The sweat that came from his skin was retained within the suit which restricted his ability to lose heat by evaporation. The consequences, sadly, were fatal. 

Body type

Another important factor to consider when cycling indoors is your body shape and size which will determine your surface-area-to-volume ratio. In the animal kingdom small, skinny creatures lose heat faster than larger animals. A Hummingbird loses heat so fast that at night they find it difficult to stay warm. In contrast, large animals are in danger of overheating – it’s rare to see Elephants being too active in the midday sun. 

The same principles apply to the indoor cyclist. Certain body shapes and sizes are more efficient at storing or dissipating heat than others. Ethnologists have noted that the dimensions of the human body correlate with the ambient temperature in which the different races of mankind evolved. Evolution has shaped our bodies with precision to enable the human race to survive climatic extremes.

The Inuit of the Arctic are endomorphic: stocky with thick joints, short arms and legs. This design helps them conserve heat and is an example of a low surface-area-to-volume ratio. A very different somatotype is exhibited by those that evolved in hot and dry environments like the plains of central Africa. 

The Masai and Zulu are tall and slender with narrow joints and long limbs. Being tall and thin encourages heat loss as it provides the proportions for an ideal surface-area-to-volume ratio while low body fat limits the amount of heat that is stored internally. 

Like most elements of high performance sport – the challenges of training in heat can be considerable, but the answers are not complex. Staying cool may seem like a secondary consideration, but from a training, comfort and safety perspective, it's vital you don't overheat.

Controlling your training environment is only the beginning, of course. To find out more about science-based training, click here.

Images courtesy of Rapha, Assos and Offside/ Presse Sports

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