How to run in the heat: hold the effort, let the pace drop

Heat changes what a pace costs

Take a runner in Manila at 33°C and 80% humidity, and another at 18°C, both doing the same session at the same pace. The first is working substantially harder: on the heart and circulation, on temperature control and on metabolism.

Race results show the same pattern. A study of marathon results by Ely and colleagues found that performance slowed steadily as the wet bulb globe temperature (WBGT, a heat measure that takes humidity into account) rose from 5 to 25°C. The effect was larger for slower runners than for the fastest.

The Athlevo heat rule

On hot, humid days, the same effort at a lower pace gives the same useful training effect. A pace target written for cool weather turns an easy day into a hard one.

Athlevo adjusts every session when it is above 30°C with humidity above 65%. A plan written for mild conditions does not apply as written.

Adapting to heat

A review from the Gatorade Sports Science Institute says heat acclimatization usually takes about 7 to 14 days of regular heat exposure. Most of the fall in heart rate comes in the first 4 to 5 days and is essentially complete after 7. Core temperature during exercise is lower, sweating starts earlier and sweat rate rises. Fit athletes can adapt in about half the time. The gains fade: they last about a week after exposure ends, and about 75% are lost by about 3 weeks.

That is why Ground running does double duty for runners who train outdoors in a hot climate. It builds the aerobic base and it keeps heat adaptation going.

You may read that heat training raises VO2max even when you race in cool weather. One 2010 trial of trained cyclists found a gain of about 5% in VO2max, measured in cool conditions, after 10 days of heat training. The review above notes that such findings are not universal. Athlevo treats heat adaptation as how you cope with heat, not as a promise of a higher VO2max.

Know the warning signs

Heat exhaustion can bring headache, nausea, dizziness, weakness, irritability, thirst, heavy sweating and a raised body temperature. Stop, get out of the heat, take off extra clothing, cool down with cold water or compresses, sip cool water and get medical care.

Heat stroke can bring confusion, slurred speech, seizures, loss of consciousness, a very high body temperature, and either hot dry skin or heavy sweating. It can be fatal if treatment is delayed. Call your local emergency number straight away and cool the person quickly while you wait.

This is general information, not medical advice. If you are unsure, stop and get help.