Here’s how to adapt your training in the heat to tackle the UK heatwave safely, according to expert Sean Murphy
Words: Sean Murphy | Images: Shutterstock
You’re not losing fitness. It really does feel harder, and there’s a straightforward reason for it: your body is doing two jobs at once.
In cool conditions, most of your blood goes to your muscles. In the heat, a large share is redirected to the skin to cool you down, leaving less available for the muscles and forcing the heart to work harder to meet both demands. The same session, at the same effort, costs more. Your heart rate climbs, your perceived effort rises and your output drops – not because you’re less fit, but because your body is running two systems with one blood supply. This is sometimes called cardiovascular drift, and it happens to everyone, regardless of how well trained they are.
The good news is that it’s temporary. The body adapts to heat quickly, and when it does, the benefits extend beyond simply coping with summer conditions. But the first few weeks of warm weather are always the hardest, and understanding why can make them easier to train through.
Why training in the heat feels harder
The cardiovascular demand is the biggest factor. When core and skin temperature rise, the heart has to push blood to the skin for cooling while still supplying the muscles. Heart rate can climb five to 15 beats above what you’d expect for the same effort in cooler weather. That’s not a sign of poor fitness; it’s your body managing heat.
On top of that, even a small rise in skin temperature is enough to increase perceived exertion before anything else has changed. Your brain registers the heat and dials back your willingness to push, so you feel worse before the physiology has shifted.
If you’re in the second half of your menstrual cycle, it can feel harder still. Resting core temperature rises by 0.3 to 0.5℃ during the luteal phase, which means you’re starting from a higher baseline before the heat even plays its part.
None of this means your training is going backwards. It means the same effort is costing more – and that’s a temporary problem, not a fitness one.
Your body adapts, your fitness benefits
Heat acclimation takes around 10 to 14 days of regular exercise in the heat. During that period, your blood plasma volume expands, giving the heart more to work with. Sweating starts earlier, so the body gets ahead of heat build-up rather than reacting to it, and heart rate at the same effort comes down. The work starts to feel more like it used to.
Early summer is the worst because the heat has arrived but the adaptation hasn’t. By mid-summer, the same effort will feel measurably better. Training through those first uncomfortable weeks isn’t just surviving, either. The adaptations your body makes to cope with the heat also improve your performance in cooler conditions, so for as long as summer lasts, you’re training with an advantage you wouldn’t have built indoors.
Hydration supports this process, and it’s the variable most within your control. Even a modest fluid loss is enough to impair performance and compound the cardiovascular strain you’re already dealing with in the heat. For longer sessions in sustained heat, don’t rely solely on thirst to tell you when to drink, because by the time you feel it, the deficit has already built up.
The simplest check is urine colour before you train: pale straw generally means hydrated, while darker urine means you need more fluid. Drink consistently throughout the day rather than simply before a session, and remember that sweat contains sodium and not just water. An electrolyte drink or sodium-containing foods around training can help the body retain the fluid you take in, because water alone replaces the volume but not the electrolytes lost through sweat.
UK heatwave: how to exercise safely in hot weather
Train by effort, not by numbers
When it’s hot, pace drops, output drops and the watch tells you you’re slower. That’s the correct response, not a sign of decline, and chasing the same numbers in heat as in cool weather forces the body past the point where the session is productive.
Training in the heat should be judged by how the effort feels, not what the data says. If perceived effort is high, the session is still doing its job, regardless of what the pace or power number reads. The numbers come back as the body acclimatises, and adding more volume or intensity to compensate for feeling slower is the mistake that leads to overtraining rather than improvement.
The same applies to progressive resistance training. A session in a warm gym may feel harder, and the weight might need to come down slightly, but that’s fine. What matters is that each set finishes close to muscular failure, because effort per set is the variable that drives adaptation, not absolute load. A hard set in the heat is still a hard set.
Don’t let summer be the reason you drop your gym sessions. Outdoor activity doesn’t give the body a reason to maintain muscle, and that signal only comes from progressive resistance training. Keep the foundation in place, adjust your expectations outside, and let the adaptation do its work. By mid-summer, the heat feels more normal – and the fitness is still there.
Sean Murphy is global chief personal training officer at Ultimate Performance (U.P.), with more than 15 years’ experience as a leading authority in the fitness industry. He oversees more than 300 trainers worldwide and sets the standards that have made U.P. the global benchmark in strength training since 2009. A senior member of the Executive Team, Sean leads the development and execution of the U.P. Method, with responsibility for trainer education, exercise execution, gym-floor performance and clients’ results. Visit www.ultimateperformance.com
