Running/training in the Heat: What New Research is Telling Us.

Heat training

When our summers give us a heat wave athletes split into two camps. One group starts avoiding the heat entirely, hiding on treadmills, chasing shade, turning the garage fan into a Category 5 hurricane. The other group leans into it. No fan. Midday runs. It can be argued that both groups are right.

Heat stress is very useful stress if applied correctly, but a training road block if not.

Training in the heat is no longer viewed as just the domain of runners who are going to do races in the heat. The newer research is pointing toward something broader. Its influence is in blood plasma volume and cardiovascular efficiency, amongst other things. This shifts heat training from survival strategy into actual performance tool. 

Let’s take blood plasma expansion as a starter. What does that mean?

Blood plasma is basically the fluid component of your blood. When you repeatedly train in the heat, your body starts increasing plasma volume as a protective mechanism, consequently you will have more fluid in your circulation and this means:

• better cooling capacity

• more stable heart rate

• improved stroke volume

• better blood flow to skin and muscles simultaneously

• lower cardiovascular strain at a given pace or power

In simple terms: your engine runs cooler and more efficiently. It changes how hard your heart must work.

What’s interesting in the newer literature is that plasma expansion appears to happen fast, often within 4 -7 days. There’s now evidence showing plasma expansion itself may improve performance because cardiac strain is reduced. That’s why some endurance athletes deliberately use heat training before ALL races, not just hot ones.

But training in the heat is stressful on the body and this is where you can get yourself into trouble. Heat stress increases overall training load massively, even when mechanical load stays low.

You can try to run easy outside and end up carrying recovery needs closer to a threshold session. The body doesn’t really care that the relative effort was low if the core temperature and cardiovascular strain were high.

This is why after training in the heat you may experience:

• elevated resting heart rate

• suppressed HRV

• poor sleep

• lingering fatigue

• unexpectedly dead legs

And because pace/power drops in heat, you can easily push yourself too hard and start forcing normal training outputs which accidentally turns aerobic work into survival work.

Hydration or rather dehydration really matters, but chronic under-recovery from repeated heat exposure is often the bigger issue for athletes.

The body can tolerate a controlled dehydrating session occasionally. What it struggles with is stacking heat stress on top of:

• hard intervals

• poor sleep

• calorie deficits

• travel

• already high training load

The endocrine and autonomic strain accumulates quickly. When we run in heat a lot of “summer overtraining” isn’t volume related. It’s ill-supported thermal stress that causes the problems.

So, the overriding message is use the heat to your advantage, but be wise, be aware of the strain you are putting yourself under and recover wisely.

In summary, heat training can be beneficial for these reasons:

• increase plasma volume

• reduce heart rate

• improve sweat response

• improve comfort in heat

• reduce perceived exertion

• improve cardiovascular stability

• potentially enhance temperate-condition endurance performance

• improve tolerance to long races with cardiac drift

But be mindful of the pitfalls, which are:

• doing every session overheated

• adding heat sessions on top of heavy intensity blocks

• confusing suffering with adaptation

• the practical issue that heat reduces training quality.

My advice is to train with moderate, repeatable exposure rather than extreme punishment.

• 30–60 minutes of controlled heat exposure

• keeping core temperature elevated but manageable

• avoiding full dehydration spirals

• wear a hat

• cool yourself by pouring water on your head