Tobias Johannessen of Uno-X Mobility is on the rollers beside brother Anders against the virtual backdrop of Zwift. Nothing unusual there, except the brothers are melting in bib longs, long-sleeve jackets and, for good measure, an extra nylon jacket with heat-retaining hood. A week later, just prior to the Tour, Tobias dropped another video, this time of him riding outdoors. “25 degrees outside, 5 layers of jackets, body temp 39.6, 2.5 litre fluid loss and one bike rider from Norway that is a bit more adapted,” he wrote. Johannessen neglected to mention his warming booties. Johannessen’s posts lit up Instagram and highlighted how heat training has become a hot topic in professional cycling. But why are riders cranking up the dial? Rouleur gets a sweat on to find out…
Learnings from triathlon
One of the drivers behind Tobias’ novel layering system is Olav Aleksandr Bu. The Norwegian is regarded as one of the most progressive brains in elite endurance sport, coaching Kristian Blummenfelt and Gustav Iden to Ironman World Championship titles in 2021 and 2022, respectively, as well as steering Blummenfelt to the Olympic triathlon title in 2020. In a coup for the Norwegian cycling team, Bu joined Uno-X Mobility as head coach at the end of 2024.
Bu is one of the figureheads of the Norwegian Method, a highly scientific, structured and individualised approach to endurance training that’s almost evangelical about metabolic control, frequent lactate testing and polarised training zones.
In the build-up to Blummenfelt’s gold-winning performance in the sultry conditions of Tokyo, Bu ensured heat training played a pivotal role. Come the race itself, the extent of the heat protocols reached a wider audience due to the Norwegian’s see-through tri-suit, not surprisingly picked up by the Daily Mail. While the Mail editors raised their eyebrows, the suit kept Blummenfelt’s core temperature stable and run speed high.
“We undertook extensive research into heat leading up to Tokyo, but the plan is to make more significant changes with Uno-X into 2026,” says Bu. “When it comes to heat, the team have defaulted to the recent research, parts of it coming out of Norway and Tokyo, where the advice is to ride for 50 minutes at moderate intensity five or six times a week, ensuring core temperature rose above 38.5°C.”
That was down to the work of noted exercise physiologist Bent Ronnestad of the Inland Norway University of Applied Sciences, who also freelances for WorldTour outfit Visma-Lease a Bike. He showed that five 50-minute heat sessions a week for five weeks stimulated a near 3% increase in haemoglobin mass. This refers to the total amount of haemoglobin (protein in red blood cells that binds to oxygen) in the body and is a variable strongly correlated with a rider’s maximal oxygen uptake or VO2max. That 3% gain is similar to oxygen-boosting gains from a three-week altitude camp.
To dig a little deeper, repeated exposure to heat results in an increase in blood plasma volume, which elicits a lower heart rate, reducing the strain on the cardiovascular system, so a rider can keep working hard despite the extra stress of the heat. Heat training also improves the sweat response, kicking in sooner so the rider more readily maintains a core equilibrium. That’s the perfect physiological acclimatisation platform to peak in the heat.
In many ways, all of this heat training is linked to dehydration. That’s because when you dehydrate, water is pulled out of the bloodstream through sweating. Blood plasma volume decreases so less blood is pumping around the body, leading to less oxygen reaching the muscles. With repeated heat exposure, the body compensates by boosting this plasma, which helps a rider maintain a higher intensity than at the start of the heat block. It’s why professional riders often hypohydrate through these 50-minute sessions. That’s where you strategically dehydrate, to place greater stress on the body and ultimately crank up the chances of larger performance gains in the heat.
The Pareto Principle
Uno-X Mobility riders have applied this dehydration strategy, albeit as Bu stressed earlier, things are set to evolve into next season. “This year, we identified lower-hanging fruit to grab first,” he says. “But heat, like everything at this level, should be about quality and applicable to the end point, which is racing. It’s the Pareto Principle. For these riders, they have such a high base that 80% of performance gains you make from this far in their cycling lives come from 20% of precise training interventions. When it comes to heat, we’re shifting toward extending the volume of accumulated heat.”
Why is down to specificity, Bu says. “When we’re at altitude, we’re not just training for 50 minutes because that not only doesn’t match what the riders experience in races but is insufficient exposure for the adaptations we’re after. It’s the same with heat. If you expect to perform in a race or stage that lasts for four to five hours in extreme heat, 50-minute sessions don’t really prepare you sufficiently. They’re better than nothing, of course, but longer is arguably better.”
Johannessen has used heat training for a while (Image: Zac Williams / SWpix.com)
Vis-à-vis that Insta hit Johannessen rolled out on a long outdoor training ride. Will we see more of that from the Norwegian and wider peloton in general come 2026? Maybe, says Bu, albeit fundamental physiology says indoor heat training will remain the preferred method.
“It’s harder to heat train cycling than running because of the windchill from cycling at higher speeds,” he says. “Even when running, it’s hard to heat train effectively because humans are arguably the greatest cooling animal around.
“Our primary method of dissipating heat is through the skin, which is the largest organ in the body. Dogs, on the other hand, primarily lose heat through panting. It’s why our ancestors were so successful at hunting animals. We’d basically outrun them by not overheating. Sweating is an extremely powerful way of cooling the body.”
“You can show your friends how effective this heat transfer is with a party trick,” Bu adds. “Next time you’re on the beach, take your warm bottle of drink and wrap it in a wet towel. Leave it in the sand and miraculously it cools. That’s down to water evaporating from the towel. It’s all to do with thermodynamics!”
So, Uno-X riders, like many riders at all levels, will often over-dress and train indoors where the only cooling chill comes from a fan. This, they might do for a set heat block. It’s an effective strategy for prepping for summer races, as highlighted on the EF Education-EasyPost team website where the team’s head doctor, Jon Greenwell, undertook rider heat testing in the labs at Loughborough University.
“We’ve got really good data within the team that shows how well the riders acclimatise,” he said. “If your core temperature and the ambient temperature are high, you can maintain a given power output for a much longer period of time if you’ve done a block of heat training.
“We’ve got data from some of the riders on our team after we put them in a heat chamber in a really controlled environment and ran tests in extreme temperatures. And the improvements after a block of heat training are profound. If you are climbing Alpe d’Huez or the Galibier on a standard day in the summer, it can send you 20 minutes further up the climb before the heat really gets to you.”
Perma-heat training?
Much of this focus is about acclimatising to race strong in the heat, though raising haemoglobin mass levels is potentially conducive to improved performance whether you’re riding in the hot or cold, sea level or up high. It’s why some riders heat train all-year-round. That’s an effective strategy, albeit maybe not for the elites. According to research by Julien Périard, strategic and periodised heat training is more effective than doing it continuously throughout the year. Why is that excessively long blocks of heat training can reduce training quality, lead to maladaptation and even lead to chronic fatigue. It’s a school of thought Bu subscribes to.
“Ultimately, this comes down to philosophy and what you believe,” he says. “You have plenty of riders who live and train at altitude almost all-year round, despite there being little evidence that makes you a better cyclist. It’s the same with heat, that applying heat all the time makes you a better cyclist.”
“At the elite level, it’s really quality over quantity,” Bu continues. “For the professionals, it takes time to improve the neurological system, musculoskeletal system and cardiovascular system to adapt to a change in stimulus. If you mix these signals, you don’t extract the best from your athletes.”
Bu and his team periodise heat training, as they would altitude – at certain times of year in the build-up to key, hot races – though if you are a committed amateur athlete who is heat training all-year-round, Bu says you don’t need to dial down the thermostat yet.
Pauline Ferrand-Prévot at the Tour de France Femmes 2025 (Image: Tornanti.cc)
“There is more evidence that heat training can be used less strategically by amateur athletes, probably because they’ve generally more room to grow. This is true for many training and nutritional strategies. Basically, what works for the elite works for everybody, but what works for everybody doesn’t necessarily work for the elites.”
That, says Bu, includes training tools. He’s an advocate of the Core temperature sensor that is become ever-present in the professional peloton these past couple of seasons with official users including Soudal-QuickStep, Movistar Team, Canyon-SRAM zondacrypto and FDJ Suez. Many others use it, including riders at Uno-X and amateur riders, plus its outline has been spotted through Tadej Pogacar’s polyester apparel.
For the unfamiliar, Core is a square-shaped wearable device that is positioned on the side of the torso. Ideally, it’s clipped to a chest strap or bra strap. It works by measuring the thermal energy transfer moving from or into the body. This information is then processed by an algorithm to calculate real-time core body temperature and relayed to either a smartphone or a Core-twinned device like a Garmin bike computer.
From there, you can monitor your internal furnace and ensure that, like you would when monitoring power, it’s sustainable. The ranges of core temperature that lead to peak performance or heavy fatigue are part of the individual equation. A rise in temperature that might start to impair one rider’s power output might have no discernible impact on another rider.
“I’ve used it for years but, like the power meter, we ensure we never become slaves to data,” Bu says. “It’s often used in training so riders can match their feelings with the information coming out of the Core. But it’s also really effective at planning pacing strategies in different ambient environments.”
Lengthening altitude gains
Core is also a useful tool for tapping into another recent development in the professional peloton: extending gains made at altitude via heat training. A glimpse into this new strategy goes back to the annual Science & Cycling Conference, which takes place in the week before the Grand Départ in the same city. At the 2024 edition, Daniele Cardinale, head of the sport physiology and performance laboratory within the department of elite sport support at the Swedish Sports Confederation, presented on ‘novel approaches to altitude training’.
Many headlines and column inches were written about part one of his talk where the Swede revealed that carbon monoxide, used by teams for years to measure haemoglobin mass, could deliver similar gains to training at altitude if used more strategically. The uproar led to the UCI banning the repeated use of carbon monoxide rebreathing earlier this year.
Part two attracted far less attention in the media, but not with the teams, as Cardinale revealed that heat-suit training could preserve the gains forged at altitude, specifically maintaining elevated levels of haemoglobin mass. Altitude training is a common strategy to increase haemoglobin mass, stimulated by a number of factors including greater production of EPO or erythropoietin.
The problem is that these physiological gains are short-lived when returning to sea level with studies showing haemoglobin mass – and associated aerobic capacity – drop within 11 to 24 days of altitude training.
Cue Cardinale’s study, which had 18 cyclists (average VO2max of a lofty 76ml/min/kg) undertake a three-week altitude camp at around 2,100m above sea level. After the camp, the cyclists were split into two groups with one group replacing 150 minutes of low-intensity training with three weekly heat sessions of 50 minutes, while the other continued as normal. The heat efforts took place indoors with the riders dressed in woollen top and bottoms, a woollen hat, a down jacket, nylon rain jacket and nylon pants. What did the results show? That overdressing paid off, with the heat group maintaining the 4.1% increase in haemoglobin mass from the altitude camp, while the control group lost 3.3% of those gains.
Forward to 2025 and a smiling but sodden Jonathan Milan is celebrating his second victory of this year’s Tour de France, the Italian holding off Red Bull-Bora-Hansgrohe’s Jordi Meeus into Valence. Was it the textbook sprinter’s stretch that secured his sixth GrandTour win? Was it the impeccable timing of a sprinter learning and refining their craft year by year? Or was it down to his blood profile?
Milan winning stage 17 of the Tour de France 2025 (Image: Zac Williams / SWpix.com)
“Leading up to our altitude camp [two weeks in Sierra Nevada from 19 May], we did blocks of specific heat training, so it wasn’t a total shock to the system to be exposed to both heat and altitude at the same time,” says Lidl-Trek coach Mathias Reck. “Since the altitude camp came quite far from the Tour, we used heat training between the Dauphiné (which finished on 15 June) and the Tour (which started on 5 July) to preserve the altitude effect.”
The decision proved especially valuable for sprinters and support riders, who stayed at sea level to train for high-speed efforts, while still holding onto the altitude-induced performance gains. “Instead of doing another short altitude camp like the GC guys, we maintained those benefits with heat, while fine-tuning sprint performance,” says Heck.
Lidl-Trek are convinced of its altitude-extending merits. Bu, less so. “We have demonstrated, albeit it’s not yet published, that professional riders are perfectly capable of maintaining their haemoglobin mass without applying heat training after, but simply via a solid training programme. We must also remember that a high haemoglobin mass and high aerobic capacity are only part of the story. You don’t win a gold medal for having the strongest haematology.”
Too true. But the fact that the likes of the world’s finest rider, Tadej Pogacar, is a proponent suggests heat and haematological gains can play a small but potentially significant part in next-level performance. And that’s not just for the elite with the evidence suggesting recreational riders can potentially benefit with regular indoor heat training all-year-round. It’s time to update your Zwift or Rouvy subscription and bag yourself a year’s worth of disposable painter suits…
