Feeling the heat?

Ever noticed that a pace which feels comfortably sustainable on a cool morning suddenly feels surprisingly hard when the temperature rises?

You’re not imagining it — and it doesn’t necessarily mean your fitness has disappeared.

When we exercise in heat, our body has an additional job to do: get rid of the heat produced by working muscles. More blood is directed towards the skin to help lose heat, while sweating increases. As the temperature rises, and particularly when humidity is high, this becomes increasingly difficult.

Humidity matters because sweat only cools us effectively when it can evaporate. In humid air, evaporation is restricted, so you can be sweating heavily without getting the same cooling benefit.

The result is greater physiological strain — and a greater perception of effort.

Research examining temperature and humidity independently found that hotter conditions increased thermal strain and impaired endurance performance, while higher humidity, once temperatures are over 18 degrees C, also increased core temperature and perceived discomfort. 

So what happens to your running pace?

On a hot, humid day, your body is working harder simply to maintain its temperature. Heart rate may rise, sweating increases and your normal paces can feel considerably more difficult.

A sensible response is to slow down and run by effort rather then pace, and that doesn't mean the session has suddenly become less useful.

Is training in the heat still effective?

Yes — but its complicated....

Heat itself is an additional training stress. Repeated exposure can produce heat acclimation, including earlier and more effective sweating, changes in blood volume and cardiovascular adaptations that make it easier to exercise in the heat. This can take a few weeks to happen.

There is also evidence that some of the adaptations can improve performance when you return to cooler conditions. 

But there is an important distinction: If your aim is to practise a specific marathon pace, threshold pace or interval pace, the heat can make it difficult to achieve the intended physiological stimulus at the intended speed. For some sessions, therefore, it makes sense to run by effort or heart rate rather than pace — and to move key pace-specific sessions to a cooler part of the day if possible, or to the controlled environment of a treadmill in an air conditioned gym.

And what happens when it cools down?

You may feel slow and sluggish during a hot spell, then suddenly discover that your usual pace feels much easier when the temperature drops.

That's not necessarily a sudden improvement in fitness. Some of the apparent “loss of fitness” was simply the cost of thermoregulation.

If you've also accumulated some heat acclimation, you may find that you tolerate the cooler conditions particularly well. Heat acclimation has been shown to produce adaptations that can benefit performance outside the heat too.

Heat doesn't just affect your run

There is another reason I think it's useful to look beyond an individual session for the impact of heat.

A prolonged period of hot weather can affect sleep, recovery, appetite and therefore your overall ability to absorb training.

Sleeping in a hot environment can make it harder to sleep well, and there is evidence that heat exposure can reduce sleep efficiency and sleep quality, despite no difference in total sleep duration.

And sleep is a big part of recovery. If you're training hard, sweating more, sleeping poorly and feeling generally more fatigued, the cumulative stress can be considerably greater than your training plan alone suggests.

Appetite can also change. Exercise in the heat can suppress appetite in some circumstances, while heat stress can also affect gastrointestinal function and nutritional intake. A recent review specifically highlights alterations in appetite, carbohydrate and protein intake and gastrointestinal function during heat-acclimation training.

That matters for runners because under-fuelling during a period of heavy training can compound the effects of heat stress.

So if you're finding that you are unusually tired, sleeping badly, less hungry and struggling with paces during a hot spell, don't automatically conclude that your fitness has suddenly gone backwards. You may simply be dealing with a higher overall stress load.

The takeaway

During hot and humid weather: Run the effort, not the pace. A slower pace at the right effort can be exactly the right training session.

Heat can make running feel harder and can reduce the speed you can sustain. But when the weather finally cools down, don't be surprised if your normal paces suddenly feel much easier.