This is a topic I have written about in the past, but is something that all runners should have a good understanding of. With temperatures starting to rise, so do the questions of if it is beneficial to train in the heat, or if it should be avoided.
Some runners argue that any challenge is a beneficial challenge, so training in the heat is a positive stimulus. Other runners argue that the heat takes away from their ability to push themselves, increases the risk of dehydration, and as a result has a detrimental impact on training. Both of these points of view intuitively make sense, so we’ll have to go to the science behind heat training for our answer.
While this answer is complicated, what researchers DO know is that training in the heat has a unique impact on your physiology and makes you faster in a way that training at colder temperatures cannot.
Take, for instance, this 2010 study. In the study, a group of cyclists underwent a 10-day heat acclimation process. Basically, they rode pretty easy (50% of their max) in a hot environment (40C).
Their change in V02max (maximum ability to consume oxygen), time trial results, cardiac output (how much blood the heart can pump per unit of time), and lactate threshold (the effort level when you start producing lactic acid) were measured.
To no surprise, the athletes showed an improvement in all parameters when re-tested in a hot environment.
However, when re-tested in a cool environment, the athletes amazingly showed significant improvements in all categories: V02max (5% improvement), time trial results (6% improvement), cardiac output (9.1% improvement+/- 3.4%), and lactate threshold (5% improvement).
Simply put, these athletes had more efficient circulatory systems, and went faster because they rode in the heat. The best part is they didn’t even have to push hard in the heat- all they did was follow a protocol that kept them at 50% of their max effort.
Why does it work?
Why is there this spike in performance without actually working that hard? Researchers believe the answer comes down to that increase in plasma volume. This review article does a great job of summarizing the topic.
An increase in plasma in our blood is not related with an increase in red blood cells, so why does the oxygen transporting capacity, and more importantly our performance, improve with a plasma increase?
When we become dehydrated, our bodies quickly learn to adapt to the stress they are being put through. As a result, we retain excess fluid in our blood (more than we otherwise would), a state which is logically named hypervolemia.
The authors explain, “Hypervolaemia serves to minimize cardiovascular stress by preventing significant reductions in mean arterial pressure, central venous pressure, and cardiac filling, thereby maintaining or improving stroke volume.”
In short, the added fluid volume in our blood vessels helps us to pump more blood through our circulatory system. It also helps to maintain a healthy blood pressure as we hammer away.
Don’t always train in the heat
Is there a benefit to training in the heat? Without a doubt, yes. It will help to increase the volume of our blood and improve oxygen transportation to our oxygen hungry running muscles. However this is not a reason to ALWAYS train in the heat.
There is a large body of research out there showing that heat has a detrimental impact on our ability to reach and maintain high levels if intensity while training. As our body temperature rises, scientists now know that our CNS tries to slow us down (i.e. our muscle/cardiovascular physiology isn’t the limiting factor as we heat up, our nervous system is). Not only that, researchers also know that we will often slowdown in anticipation of overheating. If we know its hot out, we will actually slow down before our core temperature rises!
The take home message: In order to get the full benefit of your higher intensity runs, it is crucial to do at least some of them in colder temperatures (early morning, later at night) in order to get the full set of physiological benefits.
The bottom line of this research is consistent with what is common sense for most of us- we train hard and beat ourselves down as much as we can (provided we can recover and avoid injury), and our bodies will adapt and perform better as a result. However, just like mileage and intensity, there are limits to training in the heat (which you can very easily figure out, as I wrote about here). Don’t push the envelope and become dehydrated. Heat stroke, and heat exhaustion are very real threats that need to taken seriously.
-Do 1-3 easy runs/week in the heat (research shows you get the benefit of heat training at low intensities, so why push it?)
-Keep your higher intensities runs to times when temperatures are cooler (to avoid the dreaded heat-induced slowdown)
-Make sure you do not lose more than 3% of your body weight during your heat runs
-Use the heat training exclusively in a 4 week period leading up to your A-race (the impact of heat training is rather quick, and chronic training in the heat is not required)