Training in the heat: push on, scale back or postpone?

You head out on your usual loop. Same route, same plan, same mindset. Yet your heart rate sits ten beats higher, your legs feel heavy after twenty minutes, and you finish more drained than after your last long run in May. None of that means you've lost fitness.

In warm weather your circulation has two jobs at once: supplying your working muscles with blood and releasing heat through the skin. Both draw on the same cardiac output. So your heart rate climbs at the same output, the effort feels harder and performance drops. On top of that, you lose water and salt through sweat.

The number on a thermometer in the shade isn't what matters. Weather services use the felt temperature, which accounts for radiation, humidity, wind and your body's own heat balance. Europe's national weather services publish heat warnings in yellow, orange and red on that basis, and Meteoalarm collects them for the whole continent. The thresholds behind each colour are set nationally, so check the warning for the place where you actually train.

Most summer advice stops at "get out early and drink plenty". That doesn't help much at four o'clock on a Tuesday afternoon, when the only question is whether you go or not.

So here is a traffic light. It uses the felt temperature rather than the thermometer reading, and it is our editorial rule of thumb, not a medical verdict or an official warning level. As a rough guide, air temperature plus high humidity plus direct sun quickly adds up to a felt temperature well above the measured value.

The amber column is what a typical European heatwave day looks like, and it's where most mistakes happen. Running your spring interval session at 30 °C doesn't make you train harder. You just build up fatigue that comes back three days later as a bad session.

Two rules make the difference. First, steer by perceived effort. If you can talk in full sentences, you're in the easy zone. If you can't, it's too much for a 31-degree day. Second, postponing is no defeat. A session you move to the next morning isn't a lost session.

"Two litres a day, half a litre an hour when you exercise." Figures like these sound practical. They don't hold up.

The scientific consensus in sport is clear: sweat rates vary enormously between people and situations. Clothing, fitness, intensity, humidity and wind can change your needs several times over. A blanket figure fits almost nobody. You end up either under-hydrated or with too much fluid sloshing in your stomach.

A better approach is to find your own number. It takes a single training session.

Two per cent of 75 kilograms is one and a half kilos. That sounds like a lot, and on a humid day you reach it quickly. What matters is the direction, not the decimal point: you want to know whether you're a heavy or a light sweater. Everything else follows from there.

This is the part that lasts beyond summer. The real question isn't how you get through July. It's whether at 70 you can still do the mountain hike you managed at 40.

For that, muscle is the hardest currency. From around the age of 30 we lose it steadily unless we do something about it. The WHO recommends that adults do muscle-strengthening activities on at least two days a week, on top of aerobic exercise. In observational studies, regular strength training is linked to a lower risk of cardiovascular disease and all-cause mortality. That's no promise of a longer life, but it is a very good reason.

A hot summer is no obstacle here. It's an opportunity. Strength training moves easily to a cool basement, a shaded courtyard or the terrace in the evening. It replaces the run you were about to cancel anyway.

Two rounds a week are enough to start. Choose a weight where the last two reps are hard but still clean. And train in the coolest hour of the day, not the most convenient one.

The night counts as much as the day. If it stays above 20 °C after dark, which meteorologists call a tropical night, you sleep worse, never fully cool down and start the next day already in deficit. Two or three of those nights in a row and your training quality drops, even though you've done nothing wrong.

The consequence is simple. After a bad night, the hard session becomes an easy one. That's arithmetic, not an excuse. Load without recovery doesn't build fitness. It only wears you down.

The good news to finish: heat isn't a fixed opponent. It's a training stimulus, and your body responds to it surprisingly fast.

If you expose yourself regularly and in measured doses to warm conditions for about one to two weeks, measurable adaptations follow. Plasma volume rises, heart rate at the same effort falls, and you start sweating earlier, with less salt in your sweat. The first effects appear within a few days. Full adaptation of the sweat glands takes longer.

The key word is measured. Adaptation comes from easy sessions in the warmth, not from racing the thermometer. An official heat warning doesn't turn this into a challenge. The red column still applies.