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

You set off on your standard loop. Same route, same plan, same feeling in your head. Except your heart rate sits ten beats higher, your legs are heavy after twenty minutes, and you finish more wrecked than after your last long run in May. None of that says your fitness has gone.

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

What matters is not the number your thermometer shows in the shade. Weather services work with the felt temperature, which accounts for radiation, humidity, wind and your body's own heat balance. Europe's national services publish heat warnings in yellow, orange and red on that basis, collected for the whole continent on Meteoalarm. The thresholds behind each colour are set nationally, so check the warning that applies where you actually run.

Most summer advice stops at „get out early and drink plenty". That is little help at four in the afternoon on a Tuesday, when the question is simply whether you go or not.

So here is a traffic light. It works off 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 rough orientation: plain air temperature plus high humidity plus direct sun quickly produces a felt temperature well above the measured value.

The amber column is what a European high summer actually looks like, and it is where most of the mistakes happen. Running your spring interval session at 30 °C is not training harder. It banks fatigue that comes back three days later as a bad session.

Two rules make the difference. First, steer by perceived effort. If you can speak in full sentences, you are in the easy zone. If you cannot, it is too much on a 31-degree day. Second, postponing is not a defeat. A session you move to the following morning is not a session you lost.

„Two litres a day, half a litre an hour when you exercise." Numbers like that sound practical. They do not hold up.

The scientific consensus in sport is clear: sweat rates differ enormously between people and situations. Clothing, fitness, intensity, humidity and wind shift the requirement several times over. A blanket figure therefore fits almost nobody. It leaves you either short or sloshing.

Better: find your own number. It takes a single training session.

Two per cent of 75 kilograms is a kilo and a half. That sounds like a lot and is reached quickly on a humid day. What matters is the direction, not the decimal place: you want to know whether you are a heavy sweater or a frugal one. Everything else follows from that.

Here comes the part that outlasts the summer. Because the real question is not how you get through July. It is whether you will still be doing the mountain day at 70 that you did 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 adults do muscle-strengthening work on at least two days a week, on top of aerobic activity. In observational studies, regular strength training goes together with a lower risk of cardiovascular disease and all-cause mortality. That is no promise of a longer life, though it is a very good reason.

A hot summer is no obstacle here. It is the opening. Strength work moves into the cool basement, the shaded courtyard, the terrace in the evening. It replaces the run you were about to cancel anyway.

Two rounds a week is enough to start. Pick a weight where the last two reps are hard but still clean. And put the session in the coolest hour of the day, not the handiest one.

The night counts as much as the day. If it stays above 20 °C after sunset — meteorologists call that a tropical night — you sleep worse, never properly cool down, and start the next day already in deficit. Two or three of those in a row and your training quality drops without you having done anything wrong.

The consequence is unspectacular: after a bad night, the hard session becomes the easy one. That is not an excuse, it is arithmetic. Load without recovery buys you no progress, only wear.

The good news at the end: heat is not a fixed opponent. It is a training stimulus, and your body responds to it surprisingly fast.

Expose yourself regularly and in measured doses to warm conditions over roughly one to two weeks and the adaptations show up in the numbers. Plasma volume rises, heart rate at the same effort falls, sweating starts earlier and carries less salt. The first effects appear within a few days. Full adaptation of the sweat glands takes longer.

The word that matters is measured. Adaptation comes from easy sessions in the warmth, not from racing the thermometer. An official heat warning does not turn this into a challenge. Then the red column still applies.