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More riders than ever have turned to indoor training in recent months as the world continues to battle with COVID-19. While riding indoors offers much of what outdoor riding does, there are clear differences. The thrill of descending, the joy of being out in nature, the simple pleasure of going somewhere — none of this is possible indoors. And no matter how good your indoor fan is, the wind in your face inside can never really replicate the real thing.
As it turns out, the amount of available cooling while riding indoors might just affect your cycling performance. Which raises an interesting question: Can you adjust your indoor riding conditions to give you the edge in your next e-race?
Jason Boynton is a PhD candidate in exercise and sport science at Edith Cowan University in Perth, Western Australia and has conducted research on the effects environmental conditions have on endurance performance and training outcomes. Here’s his take on what the science says and how to get the best out of your indoor riding.
Heat and exercise
The relationship between temperature and endurance exercise is complex and unique. At the most basic level, temperature has an effect on the rate of biological chemical reactions (i.e. metabolism) in our cells. It’s very much a Goldilocks and the porridge scenario: an increase in temperature can improve the rate at which these cellular processes occur, but if things get too hot then the components of the cell’s machinery begin to break down and processes start to fail.
The complexity of this relationship is further increased when you consider exercise itself increases muscle, and subsequently, body temperature. This is because approximately 75% of the energy released in your working muscles (i.e. what comes from breaking down food) is lost as heat energy. This quantity of excess heat increases as your metabolism increases during exercise.
Fortunately, the human body has a number of physiological responses that make it exceptional at dissipating this potentially detrimental byproduct, allowing you to maintain your power output when exercising. Paradoxically though, these responses themselves can potentially have a pernicious effect on cycling performance.
The best mechanism we have for dissipating heat waste is our ability to sweat. But as you’ve almost certainly experienced, excessive sweating can lead to a state of dehydration, which can cause your cycling performance to drop off rapidly. Your physiology also responds to increases in body temperature by shunting more blood to the skin in an effort to dissipate excess heat to the environment. This means less blood is available for the working muscles for a given heart rate; or conversely, that heart rate increases for a given power output. This latter scenario is known as cardiac drift.
You can actually run your own cardiac drift ‘experiment’ on the trainer. Try riding at a set power output (e.g. 200 W) for 45-60 minutes. Notice how your heart rate creeps up and your level of comfort decreases as you get hotter, all while riding at a steady, uniform wattage. The take-home message here is that your physiological response to increasing body temperature during exercise can have an increasingly detrimental effect on your performance as you exercise longer.
Endurance Athletes are Thermoregulation Anomalies
Compared to other animals, humans have an amazing ability to regulate their body temperature when exercising, and endurance athletes as a subpopulation are an exceptional case. With cyclists, the ability to throw down the watts for long periods of time leads to increased metabolic heat production. This in turn increases the need to remove excess heat.
Luckily, the endurance athlete’s ability to thermoregulate has been ratcheted up to 11. With a high cardiovascular ability, smaller body size (and therefore large skin-surface-to-volume-ratio), low body-fat percentage, and improved sweat ability, the endurance athlete is phenomenal at losing heat to their environment.
Of course, not all of us cyclists fit into the mold of the stick-thin, 3% body fat, VO2max of 80+ mL/kg/min thermoregulation god that is your average Grand Tour GC rider. However, the closer you get to that ideal, the better you will be able to thermoregulate. And a number of those characteristics are under your control — for example, body-fat percentage and aerobic fitness.
Environmental Conditions and Cycling
It should come as no surprise that endurance exercise performance is substantially reduced in extreme hot and cold environmental conditions. If we were to graph this relationship, with environmental temperature across the bottom axis and performance (e.g. power output) on the vertical axis, we would see an upside-down U-shaped curve (see image below). The shape of this curve implies there is an optimal temperature range in which to perform endurance exercise tasks.
However, before we discuss this favourable temperature range for cycling, let’s take a closer look at riding in the heat and other less-than-ideal conditions, because this is important for understanding how to optimise indoor riding conditions.
Thermal energy (i.e. heat) naturally moves down a gradient from areas of high heat to low heat. This transfer of energy becomes easier when the difference in temperature between two areas is greater. When cycling in hot environmental conditions (or in a hot room), it becomes more difficult to offload your heat waste from exercise to the surrounding environment. This effectively causes the detrimental side-effects of cardiac drift, discussed above, to occur sooner and at a larger magnitude than they would normally.
Not a good situation when the name of the game is “watts”.
The concept of a gradient is important for other environmental factors that also affect exercise performance, like humidity and convection (i.e. the flow of air moving past the skin). When the humidity is high it reduces the rate in which water can become a vapour (i.e. evaporate). Under these conditions your most effective response for dispensing of heat waste, sweating, becomes impaired.
Conversely, increasing the rate at which air moves past your skin improves the body’s ability to lose excess heat to the environment. This is an important factor to consider when training on an indoor trainer as the massive amount of airflow provided during normal outdoor riding is effectively reduced to nothing if a fan isn’t provided.
Optimal Trainer Performance
Sport scientists and environmental exercise physiologists have performed a number of experiments to determine the optimal temperature for endurance exercise. The methods for these experiments have ranged from lab-based studies with subjects doing performance tests at multiple temperatures, to studies retrospectively analysing marathon results versus weather conditions.
As if by some stroke of luck, a number of both laboratory and retrospective studies have observed that endurance performance is optimised at around 10°C (50°F). However, there are some important caveats to this value when it comes to cyclists.
In our lab at Edith Cowan University we had well-trained cyclists perform intervals (5 x 4 minutes) all-out at 4°C (39°F), 13°C (55°F), 22°C (72°F), and 35°C (95°F) after a 10-minute warm-up at room temperature. We found that under these conditions, with fit individuals, performance (i.e. average power output) was not significantly different between the 4°C, 13°C, and 22°C sessions. And even in the 35°C session, power output did not decrease until the last two intervals when compared to other temperature conditions.
So why the large discrepancy between our results and previous literature? Given our study design it’s hard to determine for sure, but we have a few educated guesses. Firstly, on the low end of the temperature scale (i.e. 4°C), performance was probably assisted by the warm-up prior to entering the environmental chamber, plus the high intensity of the intervals. Both of these factors would have increased body temperature and helped attenuate the effects of exercising in the cold.
Secondly, we utilised a large fan that provided a wind speed of around 28 km/h (17 mph) during the work intervals. This may have provided enough cooling of the skin to allow for high performance in hotter temperatures. Last, but certainly not least, as mentioned above, well-trained endurance athletes are thermoregulatory anomalies. This potentially allowed them to perform close to an optimal level over a wider range of ambient temperatures.
What does this all mean for you riding your trainer at home? Well, the best guestimate we can make for the optimal temperature to perform in, based on the literature, is probably somewhere between 10-17°C (50-63°F) for the majority of individuals. This includes considerations for thermal comfort — because yeah, you could potentially perform just fine at 4°C (39°F), but your appendages probably aren’t going to feel great doing it.
Unfortunately, for the cyclist training indoors at home, room temperature for your typical household is somewhere around 23°C (73°F). This means riding on the trainer at normal room temperature without a fan is most likely at the high end of the optimal temperature range, if not outside it completely.
So when you’re looking to throw down the watts in that next e-race, maybe consider the power that is being drained out of you as you stew in that 23°C “furnace”, marinating in your own sweat, and barely feeling the breeze from grandma’s old desk fan.
(Of course, it’s also worth considering the thermal comfort of others in your household who aren’t exercising, and who mightn’t appreciate you setting the aircon to 10°C.)
Recurring Indoor Training
So we’ve determined the optimal temperature range for endurance performance — end of story, right? Just train in that range whenever you’re indoors and you’re good to go? More watts = better training outcomes, yeah?
Well, it depends. If there was such a thing as a Zwift standing long jump competition I’d tell you to set up your electronic jumping pad and computer somewhere with low gravity … like the moon. But as you were calling Elon Musk to see if you could hitch a ride up there, I’d have to remind you that subsequent training in that environment is going to be really bad for when you want to come back and compete again in Earth’s gravity. This is because it is important to train under the stresses experienced in competition, be it a 1 g environment or ambient temperature outside of the “optimal range”.
At one extreme, we know that if you have heat acclimated yourself in order to perform well in hot conditions (e.g. >35°C) you will lose much of that ability within a month if you are not exposed to sufficient heat stress somewhat regularly.
One of the physiological adaptations that occurs after regular exposures to heat during endurance exercise is a decrease in skin temperature. This improves the temperature gradient between the skin and the environment and allows for more body heat to be lost to the surroundings of the exercising athlete. However, we know that repeated exposures to cold/cool temperatures during exercise can stimulate the inverse adaptation, resulting in increased skin temperature during exercise.
As you might guess, increases in skin temperature are associated with decrements in aerobic performance, especially in warmer temperatures.
To be clear, I have yet to see experimental data from a training study showing regular exposure to cool temperature decreases performance in warmer conditions (i.e. room temperature). However, we do have sufficient physiological evidence for a convincing hypothetical mechanism that could produce this outcome. So take this section about repeated cold exposure during endurance exercise as more of a caveat than a hard thermoregulatory truth.
Lastly, I should note there are a number of studies that show regular aerobic exercise in the heat (i.e. heat acclimation) increases performance in temperate/cool conditions. This might make cyclists think they should crank the heat when training indoors. However, there is a lot of important nuance to this topic that is often lost in translation when carried over to the athlete. At the moment there is still a noteworthy debate around this topic in the literature and it is not as straightforward as it may appear on the surface. Indeed, this topic could well be the subject of an article in its own right.
In summary, previous research has demonstrated there is a particular range of environmental temperatures (10-17°C/50-63°F) where optimal endurance exercise performance is likely to occur. Typical room temperature (~23°C/~73°F) is likely outside this optimal range, especially when there is little to no airflow past the rider. In order to improve performance during e-races it may be worthwhile to decrease the temperature conditions of the room and provide oneself with a decent fan.
As a caveat, repeated exposures to cool environmental temperatures may potentially result in physiological adaptations detrimental to endurance performance at warmer temperatures. However, more research is needed to evaluate the certainty of this hypothesis.
About the author
Jason Boynton MS, is currently finishing his PhD in Sport Science at Edith Cowan University under the supervision of Associate Professor Chris Abbiss. His PhD thesis investigated the effects of environmental temperature on high-intensity interval training in endurance athletes. Jason is also a USA Cycling level 1 cycling coach and has been coaching endurance athletes since 2007. Wisconsin raised, he is loving the cycling and expat life in beautiful Western Australia.
You can find Jason on Facebook and Instagram.