Running in 35°C heat while stubbornly holding your usual pace feels virtuous. It feels hard, so it must be working. The problem is that “feeling hard” and “training effectively” are not the same thing, and months of grinding through sweltering runs can quietly undermine your fitness rather than build it. A sports scientist would likely tell you the same thing I eventually had to hear: pace is the wrong unit of measurement in extreme heat, and clinging to it is the most common mistake recreational runners make.
Key takeaways
- Your GPS watch becomes unreliable in extreme heat—but there’s a better metric that actually works
- A sports scientist exposed the surprising reason why slower summer runs aren’t a failure
- Heat exposure triggers a hidden physiological transformation that improves performance year-round
What Actually Happens to Your Body at 35°C
The physiology is unforgiving. Heat puts your cardiovascular system under enormous extra stress: blood gets redirected to the skin to cool you down, which means your heart must pump faster just to deliver oxygen to your muscles. You can expect your heart rate to rise 10 to 15 beats per minute higher than usual, before you’ve even started pushing. That’s a significant tax on your system before you’ve covered a single kilometre.
Core temperature climbs faster than most runners realise. Once air temperature hits around 29°C, your body can heat up by 2 to 3°F in just half an hour of running, and it doesn’t take long to approach the danger zone of 39°C or higher. A peer-reviewed study measuring recreational runners completing a 10km at 35°C found that participants had consistently greater perceived exertion compared to running at 25°C, and that translated into an average of 14 additional minutes to complete the same distance. That extra time isn’t weakness, it’s biology.
The single most important factor governing heat production during exercise is metabolic rate, essentially how hard you’re working. Less than 20% of the energy we create during exercise actually goes towards moving, the rest creates heat. So the faster you run in the heat, the more heat your body generates internally, while simultaneously being bathed in hot external air. The double whammy is why the scenario presenting the greatest risk is when air temperature and humidity are high, sun exposure is constant, and there is little to no breeze.
The Mistake: Chasing Pace Instead of Effort
Here’s what the data says clearly: your GPS watch is lying to you in summer. Environmental conditions such as heat, humidity, and wind all affect pace, and in those conditions it is far more appropriate to gauge effort based on RPE (rate of perceived exertion) or heart rate instead. RPE is the 1-to-10 scale of how hard you subjectively feel you’re working. Unlike pace or heart rate, RPE is based entirely on how hard you feel you’re working at any given moment, and it’s adaptable to all conditions, including hills, heat, and fatigue, without requiring any additional equipment.
The practical implication is simple but counterintuitive. In summer heat and humidity, your pace will slow down, and training by RPE helps you avoid overexertion when hitting pace targets simply isn’t realistic. A simple alternative to using target heart rate when prescribing intensity in hot conditions is to use rating of perceived exertion, a subjective measure of intensity. If your easy run is normally a 3 out of 10, it should still be a 3 out of 10 in a heatwave — even if your kilometre splits look embarrassing by comparison. The slowdown is the correct response, not a failure.
Research on pace penalties is sobering for those who care about their times. Faster runners averaging around a 5:45 per-mile pace slowed by approximately one second per mile for each degree Celsius above the optimal temperature, while runners in the 7:25 to 10-minute-per-mile range slowed by as much as four to four-and-a-half seconds per mile per degree Celsius. At 35°C, roughly 20°C above the optimal running temperature, the arithmetic becomes quite stark.
Why Structured Heat Exposure Actually Works
The paradox is that running in the heat, done correctly, is one of the most powerful Performance/”>Performance tools available. The key word is “correctly.” Within just five to nine days of repeated heat exposure, your body undergoes profound physiological changes. The first adaptation occurs within three to five days: plasma volume expansion, which increases blood volume and improves cardiovascular function. More blood plasma means your heart can deliver oxygen more efficiently and manage temperature regulation at the same time.
Studies show that heat-acclimated runners experience 5 to 8% improvements in VO2max in hot conditions, and 5 to 6% improvements even in cool conditions. That cool-weather benefit surprises most people. Ten days of heat acclimation can provide considerable ergogenic benefits in cool conditions, in addition to the expected performance benefits in hot conditions. In other terms, training properly in summer heat can make you faster in the autumn. The physiological adaptations include muscle glycogen sparing, reduced blood lactate at a given effort level, plasma volume expansion, and improved myocardial efficiency.
The sweat system gets a meaningful upgrade too. Your sweat response transforms: you start sweating earlier, produce more sweat, and lose fewer electrolytes per litre, with acclimated athletes able to reduce sweat sodium concentration from 60 mmol/L down to just 10 mmol/L, dramatically improving fluid balance. That efficiency matters enormously over long runs or races.
For a proper acclimatisation protocol to take hold, at least 10 to 14 consecutive days of active heat exposure are needed, where core temperature reaches 38.5°C or above, with profuse sweating induced for at least 60 minutes, in conditions similar to or exceeding the expected competition environment. Active protocols typically involve 60 to 90 minutes of moderate-intensity exercise at around 60% VO2max in 30 to 35°C environments. That’s a controlled, deliberate stimulus, not a defiant death-march at your winter 10k pace.
Running Smart in the Heat: What to Actually Do
Ditch the pace target for summer training. Use effort instead. Unlike pace, RPE automatically accounts for changes in terrain, weather, and training fatigue, meaning your sessions stay physiologically appropriate regardless of what the thermometer says. On a 35°C day, a genuinely easy run might be nearly two minutes per kilometre slower than your usual jog. That’s fine. Accept it, record it, move on.
Hydration warrants real attention. Water alone isn’t enough in extreme heat, sodium, potassium, and magnesium all need replenishing, and electrolyte tablets or sports drinks become necessary during longer or high-intensity sessions. A simple check: weigh yourself before and after your run — for every pound lost, rehydrate with 16 to 24 ounces of fluid.
Cooling strategies during the run also help. Splashing water on your head, neck, and wrists cools you quickly and can help lower core temperature fast, which is why you see elite runners dousing themselves at every drinks station in hot-weather races. Aerobic exercise training in temperate climates can modestly improve exercise capability in warm-hot conditions, but such training programmes alone cannot replace the benefits of heat acclimatisation. The heat itself has to be part of the equation, just managed intelligently.
One often-overlooked detail: aerobically trained athletes can induce heat acclimatisation more rapidly, as much as 50% faster, and retain its benefits longer than athletes with lower aerobic fitness. If you’re already a solid runner, your body is primed to adapt to heat more efficiently than you might expect, which makes the case for structured heat training even more compelling for anyone with a decent base.
As always, if you have any underlying health conditions or concerns about exercising in extreme heat, please consult your GP before changing your training regime.
Sources : ncbi.nlm.nih.gov | ncbi.nlm.nih.gov