In a nutshell 💡
- At exactly the same level of exertion, heat causes your heart rate to rise by 10 to 20 beats per minute within thirty to forty-five minutes. You aren’t going any faster; your heart is working harder.
- The cause isn’t your physical fitness—it’s the decrease in the volume of blood ejected with each beat, which the heart rate compensates for.
- Dehydration and hyperthermia combined have a much greater impact than the sum of their individual effects.
- Cooling your skin reliably affects how you feel. This is helpful, but it’s not the same as reducing the actual physical strain: your alertness remains intact.
- The temperature you see accounts for only 10% of the official risk index. What really matters is the evaporation of your sweat, and the sun’s impact is twice as significant as the thermometer’s reading.
You aren’t going any faster. Your heart, however, is.
This is the observation that confuses athletes the most during heat waves: the same workout, at the same pace, on the same route, yet a heart rate that climbs session after session even though nothing in your training has changed. Many conclude that they’ve lost their fitness. This is almost always wrong.
What has changed isn’t your physical condition—it’s the extra work the heat imposes on your circulatory system. Your body must supply your muscles with oxygen and, at the same time, send blood to your skin to dissipate the heat generated. These two tasks compete for the same blood flow, and it’s your heart that acts as the arbiter.
This article will explain exactly how much the heat takes out of you, the precise mechanism behind it, and—most importantly—what cooling your skin can do for you, as well as what it cannot do. This last part is rarely discussed, yet it’s the one that matters most for your safety.
How much does your heart rate increase in extreme heat?
This phenomenon has a name: heart rate drift. At a constant intensity in a hot environment, the heart rate rises on its own as the effort continues.
Reference measurements, taken on trained cyclists pedaling at 60% of their maximum oxygen consumption at 35°C, provide a clear order of magnitude. Between the fifteenth and forty-fifth minutes, at exactly the same power output, the heart rate rises from 151 to 169 beats per minute. That’s 18 additional beats—a 12% increase—without the athlete increasing power by a single watt.
In a broader population and during moderate exertion, the commonly observed range is 10 to 20 additional beats—or 7 to 15%—after thirty to forty-five minutes. These values describe athletes trained under specific conditions: they are not a universal rule, but they provide a good frame of reference.
See also: Heart Rate and Heat: Why Does Your Heart Race, and How Can You Protect It?
Why Does Your Heart Beat Faster for the Same Amount of Work?
The answer lies in a simple equation, and it explains everything else.
The blood flow your body receives is the product of two factors: heart rate and the volume of blood ejected with each beat. When this second factor decreases, the first must increase to maintain blood flow. As mentioned above, stroke volume drops by 16% while heart rate increases by 12%—yet blood flow remains largely unchanged. Your heart isn’t racing; it’s compensating.
Four factors combine to reduce this stroke volume: decreased venous return, increased blood flow to the skin, plasma volume loss through sweating when it isn’t compensated for, and tachycardia itself, which shortens the filling time between beats.
Of these four causes, only one can be addressed externally: the portion of blood flow diverted to the skin. This is exactly where skin cooling comes into play, and it is also the limit of what it can do. A cooling towel wiped over your forehead or forearms at every water stop is the most immediate solution: it re-moistens in a matter of seconds and can be applied wherever and whenever you need it. It lightens some of the load placed on your skin’s circulation to dissipate heat. However, it has no effect on your plasma volume—which is a matter of hydration—nor on the other two causes.
Heat and dehydration multiply rather than simply add up
This is the point that athletes underestimate the most. Taken individually, moderate hyperthermia or moderate dehydration each reduce stroke volume by a few percent. Combined, they have a much worse effect.
At a 4% loss of body weight combined with a high core temperature, stroke volume drops by about 20% and cardiac output by 13%, whereas each of these factors alone results in a loss of only 7 to 8%. The warning threshold is much lower than commonly believed: as little as a 2% loss of body weight is enough to have measurable effects on performance and cardiac workload.
In practical terms, for a 70-kg runner, 2% represents 1.4 kg—an amount that can be lost during a single one-hour run in intense heat. Sweat loss during exercise in a hot environment averages about 1.5 liters per hour, with an actual range of 0.8 to 2 liters depending on the individual.
| Situation | Effect on stroke volume | What changes during exercise |
|---|---|---|
| Hyperthermia alone | approximately −7 to −8% | Higher heart rate at the same pace |
| Dehydration alone | approximately −7 to −8% | Same effect, feeling of heavy legs |
| Both together | approximately −20% | Cardiac output decreases by 13%, marked performance drop |
To learn more: Nape of the neck, wrists, temples: Which areas of the body should you cool first?
Why is humidity more important than the displayed temperature?
A temperature reading of 30 °C says almost nothing about the actual heat stress. The benchmark used to regulate sporting events—the WBGT—does not measure a single factor: it combines three, and air temperature is only the smallest component.
First, it accounts for 70% of the wet-bulb temperature: the reading from a thermometer whose bulb is wrapped in a wet cloth and exposed to the air and wind. This single value already incorporates humidity, heat, and wind—in other words, everything that determines whether your sweat evaporates or remains on your skin. Next comes 20% for solar radiation, measured by a thermometer placed inside a black sphere, and only 10% for air temperature—the reading you see on your weather app.
This has two practical implications, and the second is the one people tend to overlook. The first has to do with evaporation: when the air is already saturated with moisture, your sweat no longer evaporates. It runs down your skin, dehydrates you, and doesn’t cool you down. The absence of wind produces the same effect by preventing air from circulating in contact with your skin.
The second is easier to take advantage of: in this index, sunlight carries twice as much weight as air temperature. The same outing, at the same time and under the same weather conditions, isn’t equally taxing in the shade versus in direct sunlight. It’s the easiest variable to change, and it’s the one people talk about the least.
The High Council for Public Health has established a binding guideline: outdoor events should not be held when the WBGT index exceeds 32. This risk, it specifies, applies to all athletes, including elite athletes.
It is precisely in this saturated air that gear that does not rely on evaporation comes into its own. EVO PhaseCool™ cooling sleeves work through phase change at room temperature: they act through contact, where your sweat no longer evaporates.
What Skin Cooling Does—and Doesn’t Do
What Is Well Established: Cooling the skin during exercise lowers skin temperature by about 0.8 °C, improves thermal sensation, and reduces the perception of exertion. This is the most reproducible effect in the entire body of literature on the subject. And this is no trivial matter: your perception of exertion is what determines your pace, how long you last, and when you stop.
What has not been proven: meta-analyses do not allow us to conclude that skin cooling systematically lowers your heart rate or reduces your core body temperature. The results are contradictory depending on the protocols used. So we won’t write that.
The exact wording is this: cooling the skin reduces the perceived thermal stress and some of the heat dissipation work required of your skin’s circulation. It’s a real lever acting on a real link in the chain. It’s not an internal air conditioner.
This does not diminish its value—on the contrary: since your perception determines your pace and when you stop, influencing it truly changes your workout. A TechCool™ cooling sports headband works on the forehead—an area with a rich blood supply that is highly sensitive to temperature—without interfering with a helmet or glasses.
We prefer to describe what our products actually do. A cooling fabric makes your effort more sustainable; it does not eliminate the strain that heat places on your body. Confusing the two is risky.
— Édouard Castaignet, Co-founder of G-Heat
The paradox no one tells you about
A study on cooling the back of the neck during a run yields a finding worth noting. Runners wearing the cooling devices lasted five minutes longer—a 14% increase in duration. But upon stopping, their core body temperature and heart rate were higher than those of the control group.
The interpretation is crystal clear, and it deserves your attention. They didn’t keep going because the cooling had dissipated more heat. They kept going because the localized cooling dampened the warning signal that would have caused them to stop sooner.
A cooling accessory can therefore increase your ability to continue exercising while masking part of the actual strain. This is valuable when you want to stick with your workout. It’s dangerous if you conclude that you’re no longer at risk.
The area focused on in this study is the nape of the neck, and this is no coincidence: it’s one of the most effective areas to treat. For the same reason, it’s also the area that requires the most awareness. An accessory that covers the back of the neck, such as the Anti-UV PolyCool™ Saharan cap, works exactly where this study measured the effect: it will make your outing more bearable, while at the same time dampening some of the signal that would have told you to head back.
That’s no reason to do without it, and there’s even a simple solution: on this model, the neck guard is removable. You can keep it on when sun exposure is the primary risk—a long run in direct sunlight, for example—and remove it when your ability to sense your limits matters most, such as during an intense workout or in humid conditions.
In any case, don’t let your own feelings alone determine when to end the session. Set the duration and pace before you start, and use the warning signs listed below as criteria for stopping.
How can you gear up to lighten the load?
Three areas of the body offer the best ratio between cooled surface area and perceived cooling effect: the head, the back of the neck, and the forearms. These areas are highly vascularized, very thermosensitive, and easy to cover without hindering athletic movement.
The head and the back of the neck, first and foremost
This is where solar radiation strikes most directly and where the sensation of overheating sets in first. We discussed earlier why the back of the neck and the forehead deserve priority treatment, and under what conditions.
For sports activities themselves, the PolyCool™ cooling sports cap is the most suitable option: lightweight design, secure fit during movement, and cooling that you can reactivate in just a few seconds at a water source. It covers the scalp and forehead while leaving the back of the neck exposed. The range of cooling caps and hats covers other styles, and the UV-protective cooling products include those that provide sun protection.
The Torso: The Largest Heat Exchange Surface
Your torso generates and dissipates the most heat. A men’s cooling sports T-shirt or a women’s cooling sports T-shirt from TechCool™ works through technical weaving, without the need for pre-moistening, making it ideal for continuous activity where you don’t want to stop to reactivate your gear. The full range of cooling T-shirts offers various fits and levels of sun protection.
For the lower body, men’s cooling sports shorts or women’s cooling sports shorts complete the outfit using the same principle, with the full range of cooling shorts.
The forearms: the often-overlooked area
Few athletes think about them, yet they’re a major heat exchange surface, constantly exposed to the sun whether you’re cycling or running. The EVO arm sleeves mentioned above can be removed during your workout if the temperature drops—something you can’t do with clothing. This is the subtle advantage of cooling accessories over clothing: they can be adjusted while you’re exercising.
A must-read: Heatstroke or Sunstroke: How to Tell the Difference and React Quickly?
Why choose G-Heat for sports during a heat wave?
- We tell you what our products do—and what they don’t do. You’ve just read an entire section explaining the limits of skin cooling. This is the only way to make an informed choice when selecting your gear.
- Technologies that work regardless of conditions. Depending on the product line, the cooling effect comes from the fabric weave, direct contact, or phase change—allowing us to effectively cool you in both dry and humid air, where evaporation alone falls short.
- A line designed by body zone, not by product category. Head, neck, torso, forearms: you equip what matters for your activity, without buying what you don’t need.
- Equipment that doesn’t alter your movement. Lightweight, no cumbersome activation, compatible with a helmet or goggles: an accessory you take off after ten minutes doesn’t protect anyone.
- A brand born from a thermoregulation problem. G-Heat has been around since 2017 because one of its co-founders lived daily with a circulation issue caused by the cold. The issue of the body no longer regulating itself is at the root of the brand—not an argument added later.
Conclusion: What to Keep in Mind Before Your Next Run
In extreme heat, your heart works harder—a fact you won’t see on your watch except as an abnormally high heart rate. This extra effort isn’t a sign of poor fitness; it’s the cost of thermoregulation. Understanding this changes how you train: you adjust the intensity rather than pushing through, you plan ahead for hydration rather than reacting to the need, and you focus on the areas that offer the greatest benefit.
Cooling the skin has a real impact on how you feel—and therefore on what you’re capable of doing. It doesn’t eliminate the physical strain. Keep this distinction in mind, and you’ll be able to continue exercising in the summer without turning a great outing into an incident.
Discover the full range in the cooling apparel collection and among the cooling accessories.
Whatever the weather. G-Heat, in all weather conditions.
FAQ: Exercise and Extreme Heat
Is it dangerous to exercise during a heat wave?
It’s not prohibited, but it does require caution. The risk increases with intensity, duration, humidity, and the absence of wind. Official French recommendations advise limiting physical activity during heat alerts and avoiding intense exertion during the hottest hours of the day. When the WBGT index exceeds 32, outdoor events should not be held.
When is the best time of day to exercise in extreme heat?
Early in the morning or in the evening, avoiding the 12 p.m. to 4 p.m. time slot. Be mindful of humidity, however: a muggy evening can be more taxing than a hotter, drier morning, because sweat no longer evaporates.
How should you adjust the intensity and duration of your workout in extreme heat?
Reduce the intensity before reducing the duration, and accept that your usual pace will now correspond to a higher heart rate. Base your workout on how you feel or your heart rate rather than a stopwatch; break up long sessions into segments, and plan breaks in the shade. A shorter, well-managed session is better than a normal session pushed to the limit.
What warning signs should you watch for during exercise?
Headaches, nausea, dizziness, cramps, an unusual feeling of weakness, chills even when it’s hot, and especially any behavioral changes or incoherent speech. These symptoms require you to stop immediately and call 15.
How should you stay hydrated when it’s very hot?
Before, during, and after exercise—without waiting until you’re thirsty. The official guideline is 200 to 300 milliliters every 30 minutes. During prolonged exercise, a drink containing some sugar and sodium helps limit plasma volume loss, which is the direct cause of reduced stroke volume.
What clothes should you wear for exercise in intense heat?
Light-colored, loose-fitting, performance-oriented clothing that allows sweat to evaporate, along with sun protection for the head and neck. Cooling fabrics provide additional benefits for highly vascularized areas, particularly the neck, forehead, and forearms.
What is exercise-induced heatstroke?
It is a life-threatening emergency caused by exertion in a hot environment, characterized by a body temperature exceeding 40 °C and neurological symptoms. It is distinct from sunstroke, which is caused by direct sun exposure to the head. In both cases, the recommended course of action is to stop exercising, move to the shade, actively cool the body, and call for emergency assistance.
Sources and References
[1] “Cardiovascular Rehabilitation and Physical Activity During Hot Weather”, French Society of Cardiology
[2] “Paris 2024 Olympic Games: Heat Stress in Sports”, INSERM
[3] “Working in the Heat: Health Effects and Accidents”, INRS
[4] “Heat Waves: Advice for Organizers of Sporting Events”, High Council for Public Health
[5] “Sports and Heat Waves”, Ministry of Sports
[6] “Extreme Heat, Heat Waves”, Santé publique France
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