Summer rides mean dripping sweat, overloading on fluids, and contemplating cutting the sleeves off your jersey. But why is riding in the heat so hard, anyway? In this article, we’ll look at the science behind heat and humidity, how it affects your performance, and what you can do to mitigate heat stress so that you perform well in the heat.
What is Heat Stress?
Cycling in the heat causes a number of physiological changes that seriously affect your performance, such as an increase in heart rate, core body temperature, skin temperature, sweat rate, and blood lactate — none of which is beneficial to performance. When your heart rate rises, for example, your body has to work harder to push the same amount of power into the pedals.
Riding in the heat isn’t just hard — it can also be extremely dangerous. Your core body temperature and hydration status are two internal barometers that cannot sway much before you become incapacitated. A study from the Journal of Applied Physiology looked at the effects of dehydration and hyperthermia as they put trained cyclists through a series of four max-effort cycling tests in varying conditions. Just a 1°C increase in core body temperature (i.e., hyperthermia) caused a 16% reduction in VO2max, in addition to reduced oxygen uptake and a significantly elevated maximal heart rate.
Another study showed that cyclists can experience a 4-5°C increase in core body temperature during a sub-hour time trial effort in the heat. In all of these studies, heat stress and power output are negatively correlated. In other words, as the body heats up, power output goes down.
However, by acclimating to heat and humidity, being well-hydrated, and having higher aerobic fitness, you can decrease the effects of heat stress on your body and your cycling performance. This helps explain why a hot ride in August can feel a lot different than a hot ride in June. By the end of the summer, your body has had time to adapt to the hot and humid conditions, therefore improving your performance in the heat. So what can you do to dampen these effects on race day?
Top Ways to Mitigate Heat Stress
1. Aerobic Fitness
A 2019 meta-analysis on heat stress concluded that aerobic fitness was the most effective strategy for heat stress mitigation. Specifically, the authors said that “AF [Aerobic Fitness] was found to be the most effective in terms of a strategy’s ability to favorably alter (body core temperature), followed by (heat acclimation), (pre-exercise cooling) and lastly, (fluid ingestion).”
It’s a fascinating finding and one that speaks to the importance of overall fitness. Your body is extremely strong and resilient when it is aerobically fit — oxygen is flowing, mitochondria are operating at full capacity, and your heart is pumping blood to the muscles needed for power. Remember that aerobic fitness is important year-round, not just in the off-season.
Here’s a good base building plan that will help you target your aerobic fitness: Base Season Fitness FTP Builder 12 weeks
2. Heat Acclimation
One study showed that just 10 days of heat acclimatization improved time trial performance, power output, plasma volume, and maximal cardiac output in both hot and cool conditions. Most studies find that 9-12 days of heat acclimatization training is the minimum amount necessary to earn physiological and performance adaptations.
Cyclists should aim for easy sessions (~50% VO2max) lasting 60-90 minutes in 104°F (40°C) conditions. Depending on where you live, you might be able to go outside for an easy ride in these conditions. But for those in cooler climates, you can simulate hot and humid conditions by riding indoors without a fan or even with the heat cranked up. Keep the windows closed, and soon you’ll feel like you’re riding in a sauna. For the ultimate heat training experience, you can set your trainer up in the bathroom after someone takes a hot shower to mimic the added effects of humidity. Read about more heat acclimatization strategies here.
You certainly don’t need to be doing heat training rides every day. You’ll quickly realize that even at an easy Zone 1 power output, your heart rate and RPE will climb into Zone 2, then Zone 3, and maybe even Zone 4.
3. Pre-Exercise Cooling
You don’t need a specialty-made ice vest from a WorldTour cycling team to stay cool before your race. Pre-exercise cooling includes anything that you do to cool your body. This can be as simple as moving from the sun to the shade during your warm-up or putting an ice sock down the back of your neck. My favorite cooling technique is to buy some cheap pantyhose, fill them up with ice, tie them off, and shove them down the back of my jersey before the start of the race.
4. Fluid Ingestion
Drink cold fluids before, during, and after a hot ride — it’s as simple as that. Cold fluid ingestion is especially helpful because it hydrates, quenches thirst, and lowers core temperature simultaneously. Just don’t overdo it. When too much water dilutes blood sodium levels, your body is poised for hyponatremia, a dangerous and potentially fatal condition.
To avoid hyponatremia, drink to thirst. Don’t force fluid down your throat if you’re not thirsty. If you’re looking for more concrete numbers, aim to drink 500 mL every 30-45 minutes in hot and humid conditions. And don’t just drink plain water. Experiment with different drink mixes and electrolytes to see what works best for you — just not on race day!
Alhadad S.B. et al. (2019). Efficacy of Heat Mitigation Strategies on Core Temperature and Endurance Exercise: A Meta-Analysis. Retrieved from https://pubmed.ncbi.nlm.nih.gov/30842739/
Arngrímsson S.A. et al. (2003). Relation of heart rate to percent VO2 peak during submaximal exercise in the heat. Retrieved from https://pubmed.ncbi.nlm.nih.gov/12391114/
Lorenzo, S. et al. (2010). Heat acclimation improves exercise performance. Retrieved from https://pubmed.ncbi.nlm.nih.gov/20724560/
Nybo, L. et al. (2001). Effects of marked hyperthermia with and without dehydration on VO(2) kinetics during intense exercise. Retrieved from https://pubmed.ncbi.nlm.nih.gov/11181620/
Racinais, S. et al. (2020). Core temperature up to 41.5ºC during the UCI Road Cycling World Championships in the heat. Retrieved from https://hal-insep.archives-ouvertes.fr/hal-02569281/document
Sawka, M.N. et al. (1993). Physiological Responses to Exercise in the Heat. Retrieved from https://www.ncbi.nlm.nih.gov/books/NBK236240/
Tatterson, A.J. et al. (2000). Effects of heat stress on physiological responses and exercise performance in elite cyclists. Retrieved from https://www.sciencedirect.com/science/article/abs/pii/S1440244000800808