Training in the heat: how to train safely and well in hot weather

Why does the same pace feel harder in the heat?

Working muscles produce heat, and to get rid of it your body sends blood to vessels near the skin and sweats more. In hot, humid air that cooling becomes harder. Your heart is now pumping for two jobs at once: carrying oxygen to the muscles and cooling the skin. Fluid lost in sweat also reduces blood volume. Each heartbeat moves a little less blood, so the heart beats faster to do the same work.

That is why a pace that feels comfortable on a cool day pushes your heart rate noticeably higher on a hot one and feels hard. In long sessions, heart rate keeps creeping up over time even when pace stays the same. This is not lost fitness; it is the body's normal response to heat. Humidity makes it worse, because sweat that cannot evaporate does not cool you, it just drips off.

In the heat, train by effort, not pace

The most common mistake on a hot day is forcing yourself to hit your cool-weather pace or wattage. That can turn an easy session into a medium-hard one, and a hard session into a dangerous one. The better approach is to hold the target by perceived effort (RPE) and heart rate and accept that your output will drop. Most of the training effect comes from the load your body experiences, not the speed on your watch.

In a threshold workout, for instance, shortening the repeats or lengthening the recoveries is worth more than clinging to pace and abandoning the session halfway. On easy days it is normal for your speed to drop noticeably; an easy day should stay easy. On a very hot day, moving a hard session to a cooler day or indoors, onto a bike trainer or into the pool, is also a sensible choice.

When should you train in the heat, and how should you think about fluids?

In summer the easiest gain is picking the right time. Early morning is usually the coolest part of the day; evenings ease after sunset, though pavement and concrete keep releasing stored heat for a while. Shaded routes, waterfronts and breezy roads make a difference. Light-coloured, thin, sweat-wicking clothing, a cap and sunscreen are small but effective measures.

In the heat you lose both water and electrolytes such as sodium in your sweat. Not starting a session already dry, carrying fluid on longer sessions and listening to thirst are a general starting point. How much and what kind of fluid you need varies widely between people, depending on sweat rate, duration and weather. Drinking too much plain water can also cause problems. For a plan that suits you, especially for long events, talk to a health or nutrition professional.

Does the body get used to heat? How acclimatisation works

Yes. As you train regularly and carefully in the heat, your body adapts: you start sweating earlier and more freely, blood volume rises and heart rate stays lower for the same work. The general view is that most of the adaptation comes within a week or two. The way in is to start with shorter, easier sessions in the heat and build duration and intensity gradually, not to open with a long, hard session on day one.

If you are racing somewhere hot and live in a cooler climate, planning how you will gain this adaptation pays off. Arriving at the race location a few days early and doing short, easy sessions in the warmer part of the day are common approaches. The adaptation fades within a few weeks if you stop. If you have a heart, blood pressure or other health condition, talk to a health professional before training in the heat.

How do you pace a race in the heat?

Chasing a finish time you set for cool weather on a hot day is the fastest way to pay for it in the second half. Deliberately start more calmly, listen to what your heart rate and breathing are telling you in the first part, and save any surge for the end. In the heat, athletes who start slow and finish strong often pass those who went out fast and ended up walking. Think of your goal as an effort, not a time.

Find out where the water and sponge stations are, try the drink you will use in training, and try nothing new on race day. Pouring water over your head and neck helps you cool. Follow the organiser's announcements about the weather; in extreme heat the course or start time may change.

What are the warning signs of heat illness, and what should you do?

Heat-related illness can start with mild symptoms and become serious quickly. If you notice any of the signs below, stop the session or race at once, get into shade or somewhere cool, loosen your clothing, cool your body with water and, if you are fully alert, sip fluid slowly. Confusion, odd behaviour or collapse is an emergency: call emergency services immediately. This article is not medical advice.

• Dizziness, light-headedness or unsteadiness on your feet
• Nausea, vomiting or a severe headache
• Muscle cramps and unusual weakness
• Heart rate that keeps climbing even as you slow down
• Goosebumps or feeling cold despite the heat
• Confusion, slurred speech or odd behaviour — an emergency

Frequently asked questions