Running In The Heat

Why is running in the heat so difficult? How should I adjust my training when it’s hot and humid? How long does it take to adapt to the heat?

If these questions sound familiar, you’ve come to the right place. We brought on exercise physiologist Sean Langan, PhD, to explain more of the science behind heat training and how we can adjust to it as runners. If you want to learn about more interesting topics in sport science from Sean, check out his writing on Substack.

Why The Heat is Hard, How to Train in It, and How To Adapt

Intense training in hot weather (>80°F) is one of the most challenging things we can do because of how it alters our physiology. This is particularly true for the heart, but every system is strained a bit more when it’s hot. 

The optimal “performance temperature” is between ~45°F (marathon) to 60°F (5k), which comes over 1,200 Diamond League, Olympic, and World Championship races. Heat almost unanimously slows us down. It depends on how severe the weather is, the distance, and your fitness - more fit people slow down less. A 2:20 marathoner might be 6 minutes slower in 89°F vs 50°F conditions; for a 2:50 marathoner, 7-8 minutes slower might be expected.

What Makes The Heat Hard?

Aerobic capacity (VO_2max) and threshold decrease by roughly 5-10%, so for a given pace you’re working relatively harder. The heat adds an extra layer of intensity requiring more blood to be pumped from your heart, more carbs to be burnt in your muscle, more sweat (which dehydrates you), and higher perceived effort.

High humidity makes things worse because it amplifies all the physiological stuff and reduces sweat evaporation. Evaporated sweat is the main way we dump heat from our body. So, when you see sweat dripping off your body, it’s just dehydrating you without the cooling benefit.

Implications For Training

Your training can mostly proceed as normal when the hot weather starts to hit. The only time I would be conservative with the pacing is a long continuous tempo/threshold run, especially when you aren’t heat-adapted yet. This is where you’re most likely to cut the workout short due to feeling overheated. Otherwise, you don’t necessarily need to purposely adjust your pacing, although you probably will subconsciously anyway. 

Even if some runs are a little slower at first, if you focus on the same effort, the physiological response is like running 5-10% faster, and that still drives the adaptation. Plus, you’re getting all the heat-specific effects which will pay dividends later.

Overall, stick to your training plan but allow some flexibility. It’s OK to break up intervals, take extra rest, and slowly build into quicker paces. Don’t get discouraged if you can’t nail certain splits when it’s brutally hot. After ~5 sessions of heat acclimation, most can start handling race pace intensities.

For key workouts, beating the heat in the morning or using a treadmill is very useful. Body cooling strategies will also help you capitalize on performance.

If you’re competing outdoors in the summer and care about performance (and safety), you cannot afford to not be heat-adapted. The adaptations are substantial and develop quickly (after 5-7 sessions) and are maximized after ~12-14 sessions. Longer blocks (5 weeks) can even give you altitude-like effects on red blood cells. Altogether this improves your work capacity and threshold and can even restore the initial reduction in performance.

Heat Acclimation

The strategy is simple: get hot and stay hot. Honestly, it’s uncomfortable, so embrace it. You have to be okay with feeling hot for extended periods of time. 

Research study protocols do 60-90min/day at ~80-100°F, and this works well. But, in the real world it can create a dilemma where you need to also balance overall recovery, tapering for races, and hitting key sessions.

The total time (~90min) refers to heat exposure, so you don’t have to be running the whole time. In my experience, 20-30min of tempo running will get your temperature to an acceptable level. After that, 45-60min of intermittent walking and jogging can sustain it. But even 30-40 min workouts will begin to move the needle.

You can manipulate the system a bit by wearing extra layers to heat up faster. Cyclists are adding 50min indoor rides while wearing extra clothes to their normal training 5x per week, which can boost hemoglobin by about 5% after 5 weeks. Sauna or hot baths before/after exercise can also provide more “heat volume”, without additional running volume. 

How do you know your body temperature is high enough? There are 3 options:

• Low tech: Perceived “thermal” effort. If 10 is “unbearably hot”, aim to sit at 7/10.

• Medium tech: Heart rate. Do your heat sessions at a fixed heart rate range, somewhere between 70-80% of max. 

• High tech: Greenteg’s CORE device. This estimates core temperature, and the goal would be to target staying at or above 101.5°F.

For competitive athletes (especially the longer distances), an ideal strategy would be to build a “heat base” early in the season or ~1 month out from a race, consisting of 4-5, 60-90min sessions per week for 2-3 weeks. After that, 2-3x per week can sustain the adaptations. The initial base also allows you to re-acclimate faster later on, like adding a couple sessions during the taper when you’re closer to the race.

About The Author

Sean Langan, PhD, is an exercise physiologist and writer focused on understanding and optimizing human potential. He’s worked with many elite athletes and adventurists throughout his career, studying how exercise supports health and peak performance. Sean coaches athletes through his platform, SPL Systems, and writes as a contributing author for Sports Illustrated. He also has his own newsletter, The Performance Pulse, where he breaks down interesting research in sports science and its application to everyday living and training.

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