Whilst Nopinz founder Blake Pond is waiting to get in the heat chambers to thoroughly test his new SubZero kit, he asked Joe Beer, one of the UK’s leading multi-sport coaches, to write a case study about why it’s important to manage body heat when riding indoors...


Indoor training has undoubted merits – time efficiency, picking the perfect session for your goals, riding with friends (wherever they are in the world), and less and wear-and-tear.

From the early 80’s noisy wind trainers to today’s amazing direct drive trainers and the magic of Zwift, indoor training has been transformed. And with the 2020 lockdown, huge numbers have embraced it and many have taken to racing.

But there’s a problem… and it’s always been there…

The heat.

This causes thermal drift, or the increase of heart rate above that expected due to reduced cardiovascular efficiency. This wrecks your training accuracy and effectiveness. I’m not thinking about ambient temperature being a bit feisty, but rather the lack of airflow around indoor rides linked to poor fuelling and equipment choices affecting your performance, not to mention the sweat dripping all over your precious bike.

Bottomline: riders often lack a 360-degree approach to their clothing, nutrition and preparation for virtual training or racing.

Sweat Start Point

In your next session indoors, switch the Garmin/Wahoo to show calories then watch during your warm-up as to when you begin to sweat – call this the Sweat Start Point (SSP). Then do an outdoor ride and see how you feel at that same Sweat Start Point as measured by calories burned. I bet it feels much less sweaty because you’re nowhere near as warm.

The new marginal gains for indoor riders are to be found in the challenge of how to “beat the heat”. By beating the heat, the pitfalls of thermal drift can become a thing of the past and you can become a better rider.

Indoor Parcour

Some simple science: only 20-25% of work done goes into “work achieved” and around 75% is released as “heat” – we are not a super-efficient engines. We have to sweat to dissipate heat away from the body – and we have to produce a lot of it. As a very general rule of thumb we sweat around 100ml for every 100 calories used during cycling. Indoors, the problems of sweating heavily are exacerbated, impacting riding ability, mental focus, and training session effectiveness:

  • poor air movement takes less heat off the body so we have to sweat even more
  • core temperature rises, increasing the lactate level in the bloodstream
  • profuse sweating can affect the uptake of drinks which means poor fuelling and even further dehydration issues
  • there are interesting data showing that even a short effort of 20-60 minutes in a hot indoor environment can massively increase gut permeability which may result in increased infections and gut problems.

If you’re just training indoors, you might accept these problems as coming with the territory and work around them; more fans, easier efforts etc. But it’s still hard to keep thermal stress and heart rate drift down.

If you’re racing, it’s a different story. The high wattages and desire to be your best means thermal stress goes super high. And for those who take their Zwift racing seriously (and let’s face it, these numbers are growing rapidly), heat and the associated problems like cardiac drift and potential gut issues mean this “hot racing” scenario is a significant obstacle that can have a major impact on performance. Quite simply, as your core temperature rises, more effort goes into the body attempting to keep itself cool, and that means less effort going into the pedals. What you’re experiencing is the dreaded cardiac drift.

In most conditions, cardiac drift happens out on the road too, but its impact is usually marginal. Indoors, it can be far more significant and have a far more damaging effect on power output – not what you need when you go racing. The problem does not stop there: do this enough times and cyclists start getting what only runners tend to talk about, the dreaded “runners trots” – in other words gut heat damage that plagues the athlete and leads to use of medication and embarrassing scenarios that affect the person inside sport and during everyday life.

Be Positive

The good news, using science, it is possible to beat the heat:

Go high-tech with your clothing

You want your clothing to help air to move, wick the sweat and take the heat off the body. This explains why mesh fabrics have become so popular with the indoor community.

Nopinz are going one step further with their SubZero clothing. It has pockets for cooled gel packs, or even frozen energy gels (e.g. the Science in Sport TURBO+ version). The cooling packs help to drop skin temperature on critical areas such as the mid and lower back and the forearms, all major cooling points. And with a cool box on hand (or a very happy helper) you can even swap out the cooling packs on those longer sessions or more intense races.

Cardiac Drift

On the trainer, select a comfortable output, around 60% of FTP. Ride for 60 minutes at the target power but keep your eyes off the HR data. Oh and the big factor: no fan, no open doors or doing it on the patio – just you in a closed room. After the session, look at how the HR climbs for the same power over the hour. Don’t be surprised if you see a 15-beat increase, and at higher (i.e. race) intensity, the increase is likely to be even greater. That’s the thermal stress causing cardiac drift and it’s the enemy you need to beat.

Make your own wind tunnel

Moving air means fans. Big fans. You want to minimise ineffective dripping sweat and instead blow as much air at you from the front (one or two at head/chest height) and the back (pointing upwards from the floor).

There’s an art to having the fan at the right setting for the right effort. Some need to ride a bit before they can have a fan on, others like it windy from the start. Play around to see how best you tweak the wind to mirror your effort and the session goals. Don’t scrimp on fans – for less than the price of a couple of decent tyres you can build yourself a great cooling set-up.

Read “How much fan do you need when riding indoors?” for more on this topic >

Take hot baths

After a steady session in normal room temperature, research shows that hopping off the bike straight into a 40C bath for 15-20 minutes (building up to 30 minutes) 2-5 times per week will improve your heat control and reduce cardiac drift while exercising.

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Feed the machine

You can tweak the way you feed and prepare your metabolism to get more from sessions and also help beat the heat.

  • Hydrating well on low-intensity endurance rides will make your body more effective at taking fluid when you up the intensity.
  • Carbo-load before a hard, long effort. Caffeine beforehand and carbs during will all help to improve performance – and chilling drinks does act as a heat sink.
  • Well-fueled training will improve your capacity for work and can reduce heat build-up by making you more efficient.
  • Those having gut damage issues can supplement specific nutrients to help rebuild the gut and ensure resumed immune system effectiveness.

Indoor riding is an efficient, fun and varied training and racing format that is no longer considered only something for the few. But you really do not want thermal stress and cardiac drift which will result in reduced riding effort and possible internal gut damage. By stacking up the various training tips, nutrition tweaks and technologies to take control of the beat-the-heat challenge, you can simply power through.