At the 2025 Tour de France there was an open secret being whispered through the peloton: most stage winners were wearing a thumbnail-sized sensor clipped to their heart-rate strap.
The device, made by Swiss company Core, measures a rider’s core body temperature in real time – a hidden layer of physiology that can tell whether a rider is thriving or overheating. According to Core, 17 of the 21 stages were won by riders using their sensor-prompting speculation that it offered a winning edge.
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Temperature sensors
In the case of real-time core-temperature monitoring, it’s something that has moved from Olympic labs to everyday cycling. The concept is simple: detect rising internal heat early enough to adjust pacing, cooling or hydration before performance drops. The aforementioned Core device combines skin temperature, heart rate and heat flux (the flow of heat from body to skin) to calculate core temperature and a heat-strain index.
Validation studies comparing Core with ingestible thermistors show a mean error of just 0.23°C- impressive for a non-invasive, ‘stick-on’ device. For an expert view on whether wearable thermometers can really make a winning difference, we turn to Professor Yannis Pitsiladis of Hong Kong Baptist University.
“Core temperature alone tells only part of the story,” says Pitsiladis, explaining that the data has to be interpreted within a broader context. “Heat strain and performance decline stem from the interplay of temperature, cardiovascular load, and environmental stressors. Without that integration, the data can be misleading.”
Pitsiladis is at pains to explain that real-time temperature monitoring is not, on its own, a game-changer. “Thermoregulation involves metabolic heat, cardiovascular redistribution and environmental exchange. Seeing that process unfold second-by-second can reveal how workload and environment interact.”
Professor Mike Tipton, an expert in human thermoregulation at the University of Portsmouth, urges riders to start with a question: why measure temperature at all? “If you can’t define what a meaningful change means for you, the number is just decoration,” he cautions. “Most devices estimate core temperature from skin or heat-flux signals, which are distorted by wind, rain and sunlight. Even if the reading were perfect, you still need to know your personal critical limit before it becomes actionable.”
Tipton warns that over-reliance on readouts can erode self-awareness. “Cyclists have evolved excellent internal sensors – perception of effort, breathing, sweat. Chasing digital precision can dull that intuition.”
A 2023 Journal of Sports Sciences study backs his caution, reporting mean outdoor errors of 0.5-1.0°C in popular wearables – large enough to make “heat limit” thresholds unreliable. “Directional trends are useful,” Tipton concedes, “but absolute numbers outside controlled conditions remain questionable.”
Pitsiladis agrees that validation is the dividing line between science and sales. Ingestible thermistors-pill-sized sensors swallowed to measure core body temperature from within the gastrointestinal tract – remain the gold standard; most commercial devices still estimate rather than measure.
“The technology must evolve to meet biological complexity, not the other way round,” says Pitsiladis. Until then, cyclists should treat temperature data as a compass, not a map: a helpful guide to how the body copes with heat, but far from an infallible performance predictor. In short, today’s sensors can show when you’re getting hot – but not always how or why it truly matters.
Body sensors can pump out raft of real time data – but is it useful?
(Image credit: David Lyttleton)
Oxygen and ventilation monitoring
Where Core focuses on thermal load, other systems aim or claim to capture real-time oxygen use and ventilation – essentially the holy grail of endurance physiology: how efficiently you use oxygen to generate power. Garmin smartwatches, for instance, offer wrist-based Pulse Ox tracking alongside cycling metrics including heart rate and respiration. Similarly, the Oura Ring tracks overnight oxygen, temperature and recovery for daily readiness, while the Wellue O2Ring provides continuous oxygen and heart-rate monitoring with alerts.
Professor John Dickinson from the University of Kent has spent two decades studying respiratory performance in elite athletes. He says the data can be insightful, but the current crop of wearables still fall short of lab precision.
“In the lab, we can directly measure oxygen consumption and ventilation – that’s gold-standard,” he explains. “But most wearables estimate it through proxies like heart rate, wrist blood flow, or chest strap strain. They’re decent for trends, but not precise enough for elite decision-making.”
Recent research backs that up: smartwatch VO2 max estimates can be off by 15%, often overestimating unfit users and underestimating trained ones. Breathing frequency straps were found to perform slightly better within 10% of actual rate – but lack the ability to measure tidal volume (the amount of air per breath).
“Without tidal volume, it’s difficult to interpret,” says Dickinson. “Heart rate still tracks oxygen use well, so right now, a good heart-rate monitor and a proper lab test may tell you as much as these new devices.”
Staying on top of your hydration
Hydration sensors are the next frontier – patches and wearables that claim to monitor body water or electrolyte loss in real time. Some of the best smartwatches allow users to manually log fluid intake and estimate hydration status based on activity and conditions. The idea is seductive: never guess your salt balance again.
But Professor Glen Davison, also of The University of Kent, urges caution. “The concept is sound – in theory, these devices could track fluid balance in real time,” says Davison, “but accuracy depends entirely on the technology. Some measure sweat composition, others infer total body water through electrical or optical signals – and those don’t always reflect whole-body hydration.”
Even if these devices could perfectly measure dehydration, it might not, in Davison’s view, be meaningful data for most cyclists. “You can tolerate around 4% body-mass loss from sweat – nearly three litres for a 70kg rider – before seeing major performance effects,” he adds. “That’s a lot of sweat. For most, regular drinking strategies already cover that.”
Where these sensors might prove useful is in sweat composition – knowing how much sodium you lose can guide electrolyte intake during multi-day races or heat camps. Still, this concentration may vary from one ride to the next, and Davison stresses that data from a single patch site can’t represent the whole body. “If the only evidence comes from internal company data, we should be sceptical,” he adds.
(Image credit: David Lyttleton)
Real-time blood glucose
Continuous glucose monitors (CGMs) went through a brief boom-and-bust in elite cycling, which may serve as a warning to those of us lured by the promise of next-level tech-led physiology. Marketed as revolutionary tools for fuelling and recovery, they were banned by the UCI in 2023 after studies found minimal benefit and growing concerns over data misuse.
The CGM is a small, coin-sized wearable sensor that sits just under the skin, usually on the upper arm or abdomen, using a tiny filament inserted with a spring-loaded applicator. Once in place, it tracks glucose levels in the interstitial fluid continuously, typically for 10-14 days before needing replacement. Originally developed for diabetics, CGMs were increasingly adopted by elite riders as a performance and fuelling tool.
“Continuous glucose data isn’t a measure of performance. It’s a health metric.”
Dr Henry Chung
The sensor samples glucose levels every few minutes, building a 24/7 picture of glucose trends rather than single snapshots. Data is transmitted wirelessly to a smartphone app – and in some cases to a bike computer or training platform – where riders can see real-time graphs, alerts and trend arrows.
“It’s important to be clear: even continuous glucose data isn’t a measure of performance. It’s a health metric,” says Dr Henry Chung, lecturer in physiology and nutrition, University of Essex, “Cycling performance is driven by a mix of energy availability and physiological limits. Muscle glycogen is a major fuel source, alongside phosphocreatine, but performance is also shaped by factors including neuromuscular fatigue, time to exhaustion, cardiovascular strain, oxygen use and technical skill – none of which a CGM measures.”
Chung points out another common misunderstanding. “People assume blood glucose reflects what’s happening with muscle glycogen, but that isn’t really the case. There’s a time lag between glycogen being broken down, converted, and appearing as glucose in the blood. On top of that, muscle glycogen is regulated locally within the muscle.”
So blood glucose and muscle fuel can behave quite independently. “In real-world conditions, it’s extremely difficult to work out what’s actually happening in the body from this single number alone. In people without diabetes, glucose readings mostly reflect what you’ve eaten, what your liver is releasing, and how your body is using sugar overall – not how much fuel your muscles actually have available or how well you can perform on the bike.”
In other words, the relationship between real-time blood glucose and cycling performance is indirect, complicated and often not very useful in practice. For many riders, a CGM risks becoming a source of distraction and anxiety, providing a fluctuating number that is minimally useful in terms of guiding beneficial changes.
(Image credit: David Lyttleton)
“Real-time monitoring is a fascinating science,” concludes Professor Mike Tipton, “but for performance management in the field, it risks becoming data for data’s sake.” That’s a recurring concern among physiologists: data without interpretation. Real-time readouts, delivered devoid of context, can easily overwhelm riders or coaches, and don’t translate cleanly into actionable numbers.
Then there’s the cost and complexity. A full suite of validated monitors – core temperature, respiratory strap, hydration patch, CGM-may exceed £1,000, with data streams requiring custom analysis software. For amateurs, that’s a big outlay for insight that may still be at best ‘directional’ rather than precise.
That said, real-time gadgets are here to stay and, as Professor Pitsiladis points out, the technology is still in its early “brick-phone stage”. “It works,” he says, “but it’s not yet miniaturised or fully integrated. The next step is ecosystem-level sensing – multiple validated inputs communicating in real time.”
That vision – a fully synchronised network of sensors mapping metrics such as heat, oxygen, hydration and glucose simultaneously-sounds futuristic, but it’s already being trialled, with AI advancements fuelling the surge. Real-time monitoring may be of limited value right now, but it’s sure to get a whole lot better over coming years.
Would you like me to summarize the key expert opinions from Professor Pitsiladis and Professor Tipton regarding these sensors?
