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.
Exercising 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 won’t change much before you can 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. With a 1°C increase in core body temperature, 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, being well-hydrated, acclimating to heat and humidity, and having high aerobic fitness, you can decrease the effects of heat stress on your body and your sport performance. This helps explain why a ride at the end of August can feel a lot different than a 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 beat heat stress disruption in your training?
Top Ways to Mitigate Heat Stress
1. Fluid Ingestion
Drink cold fluids before, during, and after a hot ride. Cold fluid ingestion is especially helpful because it keeps you hydrated, maintains blood volume, and lowers core temperature simultaneously. With fluids you need to have a good mix of water and electrolytes. When too much water or sugary drink dilutes blood sodium levels, your body is headed toward hyponatremia, a dangerous and potentially fatal condition. This is especially important for women in peri and postmenopause and women in the luteal stage of their cycle, as they have a lower volume of water in their body and a more rapid rise in core temperature during the early stages of exercise.
To avoid hyponatremia, you will need to practice the concentrations of your drinks, everyone is different. I can’t just say drink to thirst, as peri/postmenopause women have a decreased thirst drive even though they need more fluids, and for everyone else you are not all the same, nor do you require the same amount of fluids. I can say that according to Dr. Stacy Sims, the leading researcher on all things womens physiology and exercise, recommends to use a mix of 4% carbohydrate (about 7-9.4 grams), Sodium 180 to 225 milligrams, and Potassium 60 to 75 milligrams per 8 ounces of water. If you notice the amount of sugar is much lower than many off the shelf sports drinks, they come in at about 5 to 9% sugar, and some even higher. The reason we avoid this high concentration is that it can cause gastric distress. The intestines will not be able to allow the passing of your drink through your intestinal walls with that concentration so then fluid will just sit there. Sitting there feeling heavy and causing your core temp to rise, and performance to decrease, all the while inching closer to heat stress that is dangerous.
Drink cold fluids before, during, and after a hot ride. Cold fluid ingestion is especially helpful because it keeps you hydrated, maintains blood volume, and lowers core temperature simultaneously. With fluids you need to have a good mix of water and electrolytes. When too much water or sugary drink dilutes blood sodium levels, your body is headed toward hyponatremia, a dangerous and potentially fatal condition. This is especially important for women in peri and postmenopause and women in the luteal stage of their cycle, as they have a lower volume of water in their body and a more rapid rise in core temperature during the early stages of exercise.
To avoid hyponatremia, you will need to practice the concentrations of your drinks, everyone is different. I can’t just say drink to thirst, as peri/postmenopause women have a decreased thirst drive even though they need more fluids, and for everyone else you are not all the same, nor do you require the same amount of fluids. I can say that according to Dr. Stacy Sims, the leading researcher on all things womens physiology and exercise, recommends to use a mix of 4% carbohydrate (about 7-9.4 grams), Sodium 180 to 225 milligrams, and Potassium 60 to 75 milligrams per 8 ounces of water. If you notice the amount of sugar is much lower than many off the shelf sports drinks, they come in at about 5 to 9% sugar, and some even higher. The reason we avoid this high concentration is that it can cause gastric distress. The intestines will not be able to allow the passing of your drink through your intestinal walls with that concentration so then fluid will just sit there. Sitting there feeling heavy and causing your core temp to rise, and performance to decrease, all the while inching closer to heat stress that is dangerous.
2. Aerobic Fitness
A 2019 meta-analysis*** on heat stress concluded that aerobic fitness was a highly effective strategy for heat stress mitigation. This speaks worlds about your base fitness levels, and what that means 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 efficiently. This is a great reminder that aerobic fitness (Zone 2 training) is important year-round, not just in the off-season.
3. Pre-Exercise Cooling
You don’t need a fancy, special-made ice vest like what you may see from a World Level Athlete 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, drinking ice cold water, or putting an ice sock down the back of your shirt. My favorite cooling technique is to freeze a handheld water bottle the night before a run/ride. When running you get to carry it in the palm of your hand which feels great. When running you can hold it to your neck as needed, or stick it down your shirt for cooling. That same handheld bottle can go in the back jersey pocket while riding. You can also buy some cheap pantyhose, fill them up with ice, tie them off, and shove them down the back of your jersey before the start of a training session.
4. 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.
Athletes should aim for easy sessions (~50% VO2max) lasting 60-90 minutes in hot temperatures 80+F. Depending on where you live, you might be able to go outside for an easy ride or run 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 windows closed.
You don’t need to be doing heat training rides every day, however. Just two to three workouts in the heat a week over the course of the 2 weeks is enough, in most cases, to build the adaptations. You’ll quickly realize that even at an easy Zone 1 power output, your heart rate and RPE can climb into the upper Zones. On the days when you are doing a much harder workout, try to do it at a cooler time of day. We all know this isn’t always possible, so understand that if you do have to do your hard workout in the peak of the heat, your performance will be lower than you know you can do, until you have acclimated to the heat. To be safe it’s best to back off on either the intensity, or the number of intervals when you haven’t had the time to properly acclimate.
5. Skin Protection
This isn't heat acclimation per se, but damaged/burned skin can have a detrimental affect on your performance in the heat. When our skin burns it loses it's ability to function properly and thus both cooling and heating will be affected. This happens because burns can damage...
- Blood vessels, which can make it difficult for the body to control temperature
- Sweat Glands, which can make it difficult for the skin to cool through sweating
- Skin surface area, burns reduce the amount of skins surface area that can release heat
All that is to say protect your skin from sun burn damage. Use UVA and UVB sun blocks, use special clothing to protect you. Do whatever you can to protect your skin from damage.
To wrap things up, remember to have plenty of fluids on board before, during and after your workouts. Give yourself some Zone 2 training in the heat, and make sure your Zone 2 is strong. Be patient as the adaptations can take up to 2 weeks or longer to start feeling stronger at the higher intensities. Don’t forget when going hard, you need to keep your core temp down, precooling and during exercise cooling are your friend.
If you are interested in deeper science stay tuned for a follow up to this blog post.
References
*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/
**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
***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/
****Lorenzo, S. et al. (2010). Heat acclimation improves exercise performance. Retrieved from https://pubmed.ncbi.nlm.nih.gov/20724560/
