Last year mid 90 degree heat brought me to my knees and significantly impacted my performance on the second day of the Fat Dog 120. After two days in these temperatures and running for over 24 hours I felt like I could barely move under the oppressive blanket of heat. I had to lay down on the side of the trail to try and recover, catch my breath, and compose myself. I was in the last 20 miles of the gruelling 120+ mile race, but I just couldn’t push myself any harder to get to the finish. In the end I dropped back from 6th place to finish in 32nd. Others didn’t seem to be affected as badly. Many thoughts went through my head at the time. Could it be that heat affected me differently? Did I push harder than others the day before and was paying for it now? Did the relatively cool temperatures in Seattle not prepare me for this? What should I have done differently? How do you run in the heat?
Since that race I’ve been thinking about incorporating heat training into my own race preparation. I’ve also heard of people using heat training to prepare for altitude or even to improve their performance in cooler temperatures. I’ve heard a lot of anecdotal accounts, but I was curious what the research has shown. With this paper I have set out to determine what proof there is for heat training’s benefits, and how best I might incorporate it into my training plan and those of our athletes.
What I found
The impact of heat on running performance can be significant. Around 40 degrees fahrenheit is optimal, and performance quickly declines starting at only 50 (http://fellrnr.com/wiki/Impact_of_Heat_on_Marathon_Performance ). On average, a 4 hour marathoner at 40 degrees can expect to run about 10 minutes slower at 50 degrees, and almost 40 minutes slower at 80 degrees! Blood vessels expand, causing a reduction in available blood for muscles and added stress on the cardiovascular system. Also, dehydration and electrolyte loss beyond a certain point starts to cause additional negative performance impacts.
Heat acclimation training has been shown to have multiple benefits for endurance athletes. Becoming acclimatized to heat will reduce the negative affects of racing in the heat, and has also been shown to improve performance in cooler conditions and at altitude.
Here’s a list of the various specific benefits I’ve found throughout the articles I’ve read. I’ve included all of my primary sources for this information at the end of the article if you’d like to read more. This is an impressive list of benefits, and as I pulled it together I began to think that heat training might be a valuable addition whether or not I had a hot race coming up.
- Plasma and red cell volume increase (up to an 8.7% and 8.1% increase, respectively)
- Vo2Max increase (average of 5% in cool, and 8% in hot conditions, primarily due to blood volume increase)
- Left ventricle changes increasing oxygen delivery to muscles
- Reduced blood lactate
- More effective perspiration
- Reduced electrolyte loss through sweat
- Better heat regulation and cooler resting body temperature
- Higher volume of “heat shock” protein cells (HSP72)
- Psychological preparedness
In summary, heat training boosts your blood and body for endurance and makes you more effective, both physically and mentally, in hot conditions. See this figure from a study published in The Journal of Physiology for a great demonstration of the benefits, both in hot and cool conditions compared to a control group.
How long do the benefits last? Some physiological adaptations have been shown to disappear after around 3 weeks post heat training, although there is some evidence that the performance benefits last longer.
Older athletes tend to naturally do worse in hot conditions, which also means they’ll see the biggest benefit from heat training. The disparities in age disappear after training.
Comparison to and combining with Altitude Training
As I listed the benefits of heat training above, it was clear to me that there were some similarities to altitude training. I’ve also heard anecdotal accounts of people effectively using heat training to prepare for altitude. That was intriguing, and I was curious how I should choose between them, and possibly if it would be good to combine them and boost the benefits even higher.
Research shows that the principle of specificity in training (i.e. training for the specific sport or conditions you’ll be competing in is better) applies in deciding to use heat vs altitude training. What conditions will there be in the event you’re training for? If you’ll be competing at altitude in cool temperatures, then Altitude Training is your best bet. In this case natural altitude is the best, followed by Hypoxic (reduced oxygen to imitate altitude) methods. If you’ll be in hot conditions at a low altitude, then heat training will be best. But what do you do if your event has both altitude and heat? That’s a question best answered by a coach with knowledge of the race and your strengths and weaknesses. If forced to pick based on the available research, I’d lean toward heat training. Heat training produces many of the same benefits, some that altitude doesn’t produce, is easier and typically less expensive to incorporate, and has fewer negative impacts. Another thing to keep in mind is that heat training has been shown to have positive effects in elite athletes, while artificial altitude training hasn’t (https://www.ncbi.nlm.nih.gov/pubmed/19203133 ).
If heat training is so good, then why not combine it with altitude training and get twice the benefit? There is limited research on this, and so far studies indicate that the positive effects may even be negated by doing so (https://www.ncbi.nlm.nih.gov/labs/articles/27787334/). This may be due to the increased physiological stress of heat and hypoxic training reducing the running specific training load or recovery possible in those conditions. This is combined with the fact that altitude training has been shown to reduce blood plasma levels, thereby negating one of heat training’s positive effects. The main differential benefit with altitude compared to heat training is an increase in hemoglobin levels, and while this was shown to occur in the combined study cited above, the performance improvement anticipated did not occur. This chart in the Journal of Medicine and Science in Sports and Exercise shows clearly that on average the combination of heat training and altitude tracked very closely to doing nothing at all, while heat training alone resulted in significantly faster time trial performance. This appears conclusive, but it’s also interesting to note the wide disparity in individuals (the lighter grey lines). I think there is room for more research here, and there could be methods that do take advantage of both. Until it’s tested further I’d recommend caution.
I found two primary methods for heat training.
The first, which sounds horrible to me, is to wear a heat suit or to run in hot conditions for some of your runs each week. A heat suit is just a bunch of extra clothing to contain the body heat you produce during your run. Yuck! That sounds both painful, and like a huge inconvenience, not to mention I’m sure you’d be running a lot slower! If you live in a hot area it would be much easier to run in the midday heat, and while this is an effective method of heat acclimation it will negatively impact your running specific training. Think of it as making a tradeoff of giving up a bit of running fitness or neuromuscular gains to get the heat specific benefits.
The second, and in my eyes preferred method, is to sit in a sauna a few times each week after you run. That sounds doable, and like it wouldn’t have too much of an impact on the running specific training you do each week. Great! This could be challenging if you don’t have easy access to a sauna.
Here is a summary of each method:
Quick – wear heat suit while training for 60-100mins, or run in the middle of hot days (3-4 times per week)
Results in 5 days-3 weeks
Gradual – 20-30min dry sauna sessions after running (3-4 times per week). Allow the body to naturally and gradually regain equilibrium after session.
Results in 3-12 weeks
Obviously there are risks with either method. Heat stroke is no joke, so don’t push yourself beyond your limits!
Is heat training right for you? If you have a coach I’d discuss with them first. There appear to be significant benefits, but it wouldn’t be worth reducing the time or intensity of your running specific training unless you have a race with very demanding conditions (such as Badwater). If you have time and recover well enough from your workouts as is, then give it a try!
I’ve already started to integrate the gradual method into my own training. My sweat appears less salty, but it’s too early to share any other benefits I’ve seen for myself.
I’m starting with 3-4 sauna sessions per week. Once I feel adapted (measured by time to pre Sauna heart rate after leaving the Sauna) I’ll reduce that to 1-3 to maintain the adaptation.
I setup a “Sauna” setting on my Garmin watch to aid with and keep track of my heat training. To do this I created a new ‘other’ workout type, disabled GPS, and configured a training screen with time, temperature and heart rate (the temperature isn’t very accurate, and may not be available on your watch, so isn’t necessary). I stay in the sauna for 20-30 minutes and hit the lap button when I leave. This allows me to keep track of how long it takes to recover (feel better and reduce my heart rate) and track my adaptation over time.
I’m careful to let my body cool naturally after leaving the sauna, and not to take a cool shower or drink until I’m recovered. This is a critical part of the process that causes your adaptation to heat. Your body is forced to practice recovering naturally, which is similar to what it will have to do during a race. One measurable aspect of this is dehydration. We usually think of dehydration as a bad thing, but it has been shown to improve race performances at lower percentages, and stresses your body enough to help create some of the adaptations we’re looking for. I target around 3% dehydration between my run and the sauna. I weigh myself before I go for my run, then again after leaving the sauna. I’m 150lbs and target losing around 4-5 lbs (4.5lbs is 3% of 150). It’s important not to push yourself much beyond that point, as it could be damaging or require longer recovery.
There isn’t a ton of research out there on heat training at this point, so I’m excited to see what comes from new studies. Here are a few questions I was unable to answer in my research.
- What about adding a sauna session before or in the middle of a running workout? On the downside I could see this being more impactful to the quality of your run. It could also provide more realistic conditions for your body to cool itself, and would utilize systems differently, such as the increased air exchange from breathing harder.
- How can someone best measure their adaptation at home? Blood tests can be expensive and inconvenient, so it would be good to have a method to measure the adaptation at home. Measuring time to recovery post heat exposure (i.e. return to near pre-exposure heart rate) should be a good indicator, but I haven’t seen anything called out in research.
- Why is a dry sauna better? I found clear preference for dry sauna vs wet sauna or steam room, but no comparison on the difference in benefit. How does this translate into humid outdoor conditions? It would be good to understand the details of these differences to make better training decisions.
- Could alternating between heat and altitude avoid the negative effects of combining them at the same time, and potentially boost performance higher?