Living in Sweden makes it hard to train and cycle outside all year round.

With most of our (intensive) training hours inside for 5 months a year, we’re reluctant to include even more training on the smart trainer with additional heat training sessions on top of our "normal" training. Having heat training protocols that include both passive and active heat training sessions makes the heat training block more enjoyable.

What is passive heat training?

Active heat training is using the body, layered in clothes, to generate extra heat from the inside. Passive heat training is being in a hot environment, like a sauna or a jacuzzi bath, and warming the body from the outside. Being a Swede and neighbor with Finland, it is almost mandatory to have a sauna nearby.

Check out the passive Heat Training Load feature in the CORE App.

How to grade and estimate the heat strain

Since CORE advises against using the CORE sensor in the sauna/steam/bath for safety and accuracy reasons, you will have to check and estimate your feeling. A fever thermometer in the ear kind of works, but my experience shows that the measurements are not reliable. Sweat in the ear/on the thermometer makes the measurements unreliable and measure inconsistently.

My experience is that RPE and heart rate (HR) is lower while doing passive heat training compared to active heat training, but that does not have to be a bad thing. According to the Heat Strain Index (HSI) chart down below, a higher skin temperature demands a lower core temperature to be in the same HSI zone. For example, a core temperature of 37.6°C and a skin temperature of 38°C reaches 3.0 HSI, the same HSI as a core temperature of 38,5 and a skin temperature of ~34°C.

An easy thing to remember: Active heat training means you want to be in Heat Zone 3 (HZ3) in the middle, with a core temperature of ~38,5–39,0°C and a skin temperature of ~35–36°C.

Passive heat training you want to be at the top, with a high skin temperature (~37–39°C) and lower core temperature (~37.5–39°C)

Why is that?

The skin is our biggest organ of the body, and is responsible for releasing sweat so that we don't overheat while exercising. We keep cool easier in cooler climates and can therefore raise the core temperature more while losing hardly any performance. But as soon as our environment gets hot, we can’t cool our skin down as well, forcing our bodies to send more blood to the skin to try and cool it down, and therefore not using that blood in the working muscles. This results in performance decrease and higher HR. It is therefore better to keep the skin cool while training and racing, to keep both RPE, HR and HSI as low as possible given the work output you give.

The benefits of passive heat training

Passive heat training lets you focus on training while training, and heat training afterwards. You will not have to find an extra 5-7 hours of training in a week, nor change your normal training sessions to heat training sessions.

And it can be mentally easier to not have to do active heat training every day for your heat training block.

But the extra benefits come from combining the two. Generating the heat yourself while pedaling your bike or running on the treadmill with extra clothes IS the best option, since that is what you do when you race and your body will adapt to that. But only doing active heat training may fatigue you unnecessarily.

And you don't have to sit in the sauna for as long as you would have to ride on your trainer. 30 minutes in the sauna can almost equal 60 minutes on the trainer, since it takes around 20-30 minutes to reach up to at least 3 HSI on the bike. That means you “only” get around 30 minutes in HZ3, while you almost get the same time in HZ3 in half that time in the sauna, especially if you sit directly after a normal training session when the core temp already is elevated.

The negatives of passive heat training

The negatives…well the negatives come from not riding your bike or running. Active heat training does still give the best adaptation to you.

And you would have to find a sauna or jacuzzi near you so you don’t “waste” extra time just to get to that place.

How to schedule a heat training block including passive heat training

How I schedule heat training blocks differs a bit from athlete to athlete, but I like to work with 10–14 days of heat training every day, followed by maintenance heat 2–3 times a week for some weeks before repeating heat again. Between active and passive sessions, that should keep your Heat Adaptation Score between 90–100%.

A 4-week heat block can look like this:

Tracking your Passive Heat Training Load

CORE’s Passive Heat Training Load calculator now makes it easy to track your passive heat sessions. You simply enter the following:

1. Starting core temperature
2. Duration of your passive sessions
3. Type of session (sauna, steam room, hot bath, etc)
4. Temperature and humidity.

With a model backed by substantial scientific research, the calculator then estimates your Heat Training Load and adds it to your Heat Adaptation Score. The calculator is now available in the CORE app which makes keeping track of your passive training load very easy.

Conclusions

Combining active and passive heat training is beneficial for most people, especially us up in the northern hemisphere who ride the stationary bike too much in the winter already. I recommend having around 2/3 of heat sessions as active sessions, and 1/3 as passive. Preferably the passive sessions are directly after your normal training, when the core body temperature is already elevated.