Sweat drips from every pore and my sorry excuse of a fan isn’t cutting the mustard.
I’m 40 minutes into the first zone-two endurance workout of a new training plan and a combination of a few months off the bike and without turbo training in the early days of summer means my complexion isn’t the only thing in the red. And things are about to get much harder.
“Pause the workout and add your clothing layer… If you’re using fans, turn them off,” instructs my screen.
I do as I’m told, donning a decorator’s boiler suit and woolly hat before remounting. What was a trickle turns into a torrent and I’m soon completely drenched as my body attempts in vain to cool itself down.
As the workout’s intensity rises, so does my heart rate, and I’m forced to delve deep into my mental reserves not to throw in the (increasingly sodden) towel.
After 15 minutes that feel like hours, I’m done and peel off my extra layers for my cool down. ‘Easiest’ session of the training block complete, things are only going to get harder from here.
I’m layering up on the turbo trainer to put the Core 2 thermal sensor through its paces.
Used by the majority of WorldTour and Women’s WorldTour teams – both officially via partnerships and, in the case of Tadej Pogacar, paid for out of his team’s pockets – the device’s updated iteration is claimed to ‘transform human performance’ via the training protocol of heat adaptation.
What is heat training?
When it comes to heat training, the adage ‘no pain, no gain’ certainly rings true.
First touted as a training technique back in the 1970s and 80s, the current vogue for adding temperature into the equation began in 2010 when a study that was published in the Journal of Applied Physiology showed that heat acclimation improved VO2 Max, time-trial performance and lactate threshold in both cool and hot environments, and had comparable benefits to altitude training.
Since then, more research has shown an intensive five-week block can increase haemoglobin mass – the protein in red blood cells responsible for carrying oxygen to muscles – with greater volumes equaling a better aerobic capacity.
But why does it have this effect?
“Our body always tries to keep homeostasis – a constant core body temperature – but when you're generating forward movement or power, meaning muscle activation, you're producing heat,” explains Core product manager Tobias Schmid.
“Our body is highly inefficient, and only around 20% of the calories we turn into energy are used for forward movement or power production on the bike.
"The other 80% is excessive heat, which is first stored in your body. We need to get rid of it and that's where the core temperature increases, and your thermoregulatory system starts to get rid of this heat via different mechanisms, such as sweating.
“The cool thing about our bodies is that we can train that system – same as we can train our muscles, our lungs and hearts – and that can lead to quite interesting performance improvements.”
Schmid explains that consistent heat training generally has two outcomes – short-term heat adaptation that can help improve performance in hot conditions and long-term haematological changes.
The former can “diminish the performance loss in the heat by around 26%” by training your thermoregulatory system to become more efficient at cooling your core temperature – meaning you start sweating more quickly – which, in turn, enables you to maintain a higher power output in hot conditions or give you more of a buffer in cooler climates.
The latter, meanwhile, means your body can transport oxygen more efficiently to your muscles thanks to a greater haematological mass.
“For thermoregulation, a lot of your blood is diverged through the skin to get rid of the heat, and there is less blood available for the muscles,” says Schmid.
“But over the long-term, your body adapts and makes more blood available for the muscle contraction.”
Achieving these adaptations requires you to regularly elevate your core temperature to 38.5-39°C, along with an elevated skin temperature for sustained periods of between 45 and 80 minutes.
This can be done actively (warm-weather training, or overdressing while riding on the turbo trainer) or passively, such as sitting in a sauna post-exercise.
So, how long would you have to do this for?
“If you want to push it, you can get fully heat adapted in two weeks – it would be five to six sessions a week,” says Schmid, before adding a noticeable increase in VO2 max would require three to five sessions a week for four to six weeks.
Putting your body under thermal strain is a delicate balancing act, though – as anyone who has suffered from overheating or sunstroke will know.
Historically, hitting the adaptation sweetspot and attaining an accurate reading of your core temperature safely has required ingestible temperature sensors, or more invasive techniques (rectal thermometer, anyone?).
The wearable Core sensor is aimed at overcoming these issues and making heat training more accessible in the process.
How does the Core sensor work?
“Core temperature or heat strain is now measurable with Core,” says Schmid. “It's a new training tool, like a power meter for your thermal power.”
The new iteration – the Core 2 – is 48% smaller and 30% lighter than its predecessor, has a modular design that makes it easier to mount to a heart-rate strap, and now features a low-battery indicator, but its internal technology hasn’t changed and functions in the same way as the first version.
The stamp-sized device slots onto a heart-rate monitor strap and sits directly on the skin. It has one sensor that measures skin temperature, while another is used to calculate your core temperature via thermal power.
“The heat flux sensor measures the thermal transfer through the sensor – from the skin to your environment – and that's thermal power; if you're working harder, your thermal power is higher.
"We have built a model that uses those signals to calculate your core body temperature.”
While Core doesn’t claim its accuracy is medical-grade, it’s adoption, and use by the world’s best athletes and coaches is an indication that the data it produces is of value.
How does its application work in the real world?
On its accompanying app, it displays both skin and core temperature while the device is worn, but without a background in sports science, this raw data is like being given a power-meter reading with no context.
This is why Core has also introduced a heat strain index (ranked from 1-10) and heat zones (ranked from 1-4), which can both be displayed on a head unit or smartwatch.
“The heat strain index says how hard your body is working to keep cool, or how high your heat strain is on the body using a combination of the core and skin temperature,” says Schmid.
“It's not a linear formula though because the heat strain on the body isn’t linear depending on skin and core temperature.
"At the moment, it's universal – for some, a three feels like five or six for others – but we’re looking into individualisation.
“We’ve also developed heat training zones, which are similar to power or heart rate zones,” he adds.
Made up of four zones, they can be used to guide training and racing.
“On race day, for optimal performance, you want to stay in heat zone one or two. Heat zone three is where, according to the literature, the adaptations in the body happen. The fourth is a danger zone, so you shouldn't spend a lot of time there.”
After a session, your ride data contributes to your overall heat training load and heat adaptation score.
The former shows how much that day’s training contributed to your heat adaptation, with it highlighting if you’re at risk of overdoing it – ”prolonging recovery and draining you” – while the heat adaptation score tracks your progress from thermal rookie (0-24% adapted) to heat accustomed (25-49%) through to heat adapted (50-89%) and eventually heat champion (90-100%).
Why are they used by pro teams?
While this data is a useful addition to the training toolbox for amateurs looking to add a completely new strain on their body and therefore unlock potentially untapped performance gains, how does the Core benefit professional cyclists, who often live and train at altitude and therefore could have reached their haemoglobin mass ceiling?
“Many cycling teams do heat training at sea-level for the heat [preparation] and to keep those altitude gains, but also to prepare for altitude camps they're then doing,” says Schmid. “So they're cycling between heat, altitude, heat, altitude.”
The data from the Core therefore enables them to prepare for extreme conditions such as those experienced at the Tour Down Under and La Vuelta and retain haemoglobin mass when at sea-level with regular maintenance sessions.
Is the Core sensor worth it?
After eventually cooling down, I check the Core app to find my first training session has only made a small dent in my thermal journey.
While the potential performance benefits are clear, and it’s a more accessible, convenient, and at £264.95, affordable route to these gains than altitude training or sleeping in an altitude tent, the mental load required to get to the end of one session is comparable to an FTP test.
And given the number and frequency of the workouts required, you’d probably need to be a competitive age-group athlete (or masochist) for the Core sensor to truly ‘transform’ your performance.
