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the science behind the latest interpretation of the WorldTour: GravelBikes.Online

When Dan Bigham broke the world hour record in August, clocking 55.548km at the Grenchen Velodrome in Switzerland, the Briton did much of the heat acclimatization work he had carried out in the build-up. “When it came to the thermal side, I carried two Core sensors and also ingested a core temperature pill,” he told GravelBikes.Online shortly afterward.

The subsequent lowering of Filippo Ganna’s record to 56.792 km involved similar heat acclimatization work to manage the track temperature. “Core is one of our team partners and we’re working hard with them to build this comprehensive thermal model,” Bigham continued. “Chris [Blomfield-Brown, Core product manager] he lives an hour away from the Grenchen velodrome, so he was present at practically every training session we did there. He’s a very helpful guy.”

As it turns out, he’s a pretty handy guy, as we tracked him down to find out if a metric known for heat-related work has any benefit during a long, cold(-ish) winter.

Popular in the peloton

The core body temperature sensor is a square-shaped portable device that is placed next to the torso. It is ideally attached to a chest strap or bra, although the adhesive patches can be used for sleeping or everyday activities. If you look closely enough, you’ll see its outline across the crests of several riders in a hot race like the Tour de France. It works by measuring the transfer of thermal energy moving to or from the body. It’s then processed by an algorithm to calculate real-time core body temperature and transmitted to a smartphone or Core-twined device like a high-end Garmin bike computer.

This is unlike an electronic pill that is swallowed and then directly measures core body temperature, a tympanic thermometer that measures the core through the ear, or a rectal measurement. Costs and impracticality mean that all three have their flaws when cycling. Here, for example, Core, is where they come in. Its offer is discreet and precise. How accurate we can’t verify yet. We know of a Slovenian study that applauds its reliability but questions its validity.

For Core, their work with athletes around the world has led them to conclude that “our accuracy is an average absolute deviation of 0.21°C compared to the e-pill. However, our focus is to keep improving our technology, so it will continue to be an ongoing process of gathering more data in the aggregate to get even better measurements.”

What is unmistakable is that it is ubiquitous on the WorldTour. “What teams do we work with?” asks Blomfield-Brown, mirroring our question. “Well, I can’t say the unofficial ones, but officially they are Bora-Hansgrohe, Quick-Step, Movistar Team, Trek-Segafredo, Ineos Grenadiers… On the women’s side, Canyon-SRAM and Ceratizit WNT. If you take unofficials into account, there’s probably not a team in the WorldTour that we’re not involved with to some degree.”

Image by Wout Beel

Again, we can’t verify the unofficial claims, of course, but what we are aware of is that people like Bigham and his team at Ineos have spent many hours poring over the raw data and comparing it to e-pills. The fact that they still use it suggests that it has its scientific merits.

Not just because of the heat

It’s an impressive list. And one that is apparently more than just marketing. WorldTour teams are regularly buzzed with the latest and greatest high-tech transformative tool to gain a competitive edge. But this one has gained momentum. Because? More specifically, how do teams use this Core sensor?

“One of its uses is the well-known one, which acclimatises to the heat,” explains Blomfield-Brown. “As you spend more time training in the heat, your body stimulates numerous adaptations that promote better performance in that heat. One is that your body improves its ability to balance the amount of blood it sends to the skin to cool down but still sends enough to working muscles.

“Another interesting thing that happens when you train in the heat is that your body produces more plasma. Basically, it says you need to sweat more. And the whole sweat mechanism starts working much more efficiently. Before his hour record, Dan would spend hours sitting on a turbo trainer in a heat suit. He’d spin his legs at just 50 watts and he’d already have sweat all over his hands. His body is anticipating what’s to come.” This is important because using the Core sensor, you can calculate when a rider can acclimate.

This is all very well, you may ask, but those of us in northern Europe are entering winter. We are looking to acclimatize to cope with the cold, not tame the heat. Is the Core simply a summer adventure?

“Not one bit,” says Blomfield-Brown. “When your body produces more plasma, ultimately it also produces more hemoglobin. That’s why you can enjoy performance gains in the cold by training in the heat.”

In theory, this is a big win. Hemoglobin is a protein in red blood cells that carries oxygen throughout the body. By comparison, higher hemoglobin levels could mean a greater ability to deliver oxygen to working muscles and therefore cycle faster, stronger and longer.

The Blomfield-Brown bias is supported by independent research. Take a study 2020 by an outstanding team of Scandinavian sports scientists, including Bent Ronnestad and Carsten Lundby. In their study, five weeks of heat training increased the hemoglobin mass of 23 riders, resulting in an increase in lactate threshold power and 15-minute average power. The disadvantage centered on the number of heat sessions required to obtain this significant effect, which was five one-hour heat chamber sessions per week.

“In our experience, that’s not necessary, as indoor training is possibly a natural heat adapter because you’re not in the cold,” says Blomfield-Brown. “However, both WorldTour and recreational riders can benefit from banishing the fan a couple of times a week. To turn up the heat even more, we’re also about to launch Core suits. They’re like the ones Dan used in his acclimatization training for the indoor record, and they’re really effective.”

As you can see herethey look like a Tychem suit you wear to protect yourself when handling toxic materials.

“As long as you’re doing it safely, and that’s what the core is all about, we’d encourage training hot and running cold,” says Blomfield-Brown. “I had a friend who was competing in a Rouvy race. He trained a lot indoors with no fans. Then he runs with fans and his son put wet towels on him. He’s been riding for years, but he saw his normalized power increase by 4%. He said it was the cheapest performance gain he’d ever enjoyed!

Find your zones

So the evidence suggests that regular heat training will benefit you even if you’re outside in the cold. But what core temperature are you looking to hit to benefit from sensor feedback?

Image of Lotto Soudal

“It’s like heart rate training, as we all have different thresholds for what gives maximum performance. Thermal regulatory systems are individual,” says Blomfield-Brown. “What we know is, again like heart rate, if you train too low you won’t stimulate the necessary adaptations; if you train too high, the session will end too soon. I have seen pilots that can operate at 40.5° and have little performance degradation. Again, I’ve seen riders that can’t even get to 39.5° and it falls apart.”

Studies have shown that “normal” core body temperature can range from 36.1°C to 37.2°C, so like power meters and heart rate monitors, you’ll need to do some work to work out your individual tolerances, though to determine at what temperature your core starts to limit muscle power. The basic recommendation to carry out a heat ramp Review. Not only does this highlight where you start to leach power, but you also have the benchmark to see how cooling strategies can affect performance.

As this Heat Ramp link highlights, the Core app “allows you to track your heat training. For example, it shows you how much heat load you’ve accumulated each day, week, and month. This tells you if you’re doing enough to maintain your blood plasma gains.” Repeat the heat ramp Review every four to six weeks to monitor heat adaptation. “A heat ramp Review performed after a heat training block should result in a training zone of heat higher than the previous one; this shows that your body can handle a higher core temperature without losing power.”

Pacing is another area where the core could pay dividends, says Blomfield-Brown. “If you know you have a climb in 15km, do some active things to cool down your core, whether it’s ice packs or how much you drink. It’s like pre-cooling for a time trial. If one runner starts the bottom of the climb at 38.5°C and another starts at 38°C, the one who starts lower might have an advantage because their performance won’t degrade as much (depending on individual thermal bandwidth, of course). This happened in a high-profile race last year, but the team wouldn’t let me use it as a public example.”

At the recent Ironman World Championships in Kona, Hawaii, many triathletes modified their pacing strategy based on their core temperature, especially in the race where the lack of cold air means heatstroke is a very real problem.

I spoke to Kristian Blumenfelt’s trainer, Olev Aleksander Bu, a couple of months ago and he was almost fanatical about core temperature training, saying the sensor not only saved thousands of pounds on electronic pads, but also led to the creation of the slim, possibly X-tri-suit that the Norwegian wore to gold at the Tokyo 2020 Olympics. Its use extends to choosing clothing in the cold, too, he says. the core man, helping a rider empirically determine when to remove or add layers in a race like Flanders.

Today’s cyclist, whether he’s riding the Tour de France Femmes or the Fred Whitton Challenge, is loaded with numbers. Power, bpm, HRV…the list goes on. So will another metric help, hinder, or replace your current data set? For professionals, who have a team around them to digest and interpret the figures, and then advise, it is no problem.

For the recreational rider, well, this may be due to your interest in technology and your pursuit of the next big buck. For a training device, the sensor isn’t as fiscally prohibitive (£229.95) as other tools, but it’s still a significant investment.

Either way, we’ll certainly be interested in where this technology is headed over the next five to ten years, especially since global warming is clearly a very real thing.

Cover image by Wout Beel



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  1. I never thought Id see the day when gravel bikes become popular in the peloton. Mind blown! #CyclingRevolution

    • Gravel bikes in the peloton? Seriously? Its just a passing fad, mate. Stick to the classics and leave the gravel for off-roading. #KeepItReal

  2. Wow, the science behind gravel bikes is mind-boggling! Who knew it could be so fascinating? #GravelBikesOnline

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