Heart Rate and Heat: Why It Spikes and How to Adjust Your Training

Use RPE alongside heart rate rather than replacing one tool completely. If your heart rate is high but your breathing is controlled and the run feels easy, you may be seeing heat-related drift. If your heart rate is high and the effort feels rough, slow down, shorten the run, or change the session.

How to Adjust Your Training in Hot Weather

Hot weather doesn’t mean all training has to stop; it just means your plan may need to be adjusted.

The goal is to get the intended training effect without adding unnecessary stress. Your body counts the heat as part of the workload.

Slow the Pace

This is the simplest adjustment and often the hardest one for runners to accept as we can have a hard time distinguishing our running pace from our worth. Ridiculous, yes. But I know you know what I’m talking about.

On hot days, your easy pace should usually be slower. That might mean 15, 30, 60, or more seconds per kilometre slower depending on the temperature, humidity, sun exposure, terrain, and how acclimated you are.

Let the effort guide the run and don’t worry about pace too much.

Shorten the Session

A 60-minute easy run in mild weather may become a 40-minute easy run in the heat. A long run may need to be trimmed, split, or moved.

This is especially important if heat arrives suddenly before you’ve had time to adapt. Heat acclimation can improve sweating response, plasma volume, cardiovascular stability, and perceived tolerance, but those adaptations take repeated exposure over time. 

Move the Workout

Early morning is usually better than midday or late afternoon. Shade, trails, waterfront routes, and loops near water fountains can also help.

If the workout is high intensity, consider moving it indoors, adjusting your calendar and swapping in an easy run, or doing a shorter version. There’s no prize for forcing intervals in terrible conditions and then ruining the rest of your week’s plan because you need three extra recovery days afterwards.

Adjust the Goal of the Day

Some days are no longer about hitting the exact pace.

Instead, the goal might be:

• Keep the run easy
• Maintain good hydration
• Practise fuelling in warm conditions
• Stay relaxed
• Finish without overheating
• Learn how your body responds to heat

That last one is good to keep in mind if race day falls in the summer!

Hydration Helps, But It’s Not Magic

Fluid intake is obviously important in the heat, but hydration doesn’t make you immune to heat stress.

When you sweat, you lose fluid. As fluid loss increases, plasma volume can drop, heart rate may rise, perceived effort can increase, and your ability to maintain pace may suffer.

For endurance exercise, performance decline is more consistently seen once dehydration reaches around 2% of body mass loss, especially in warm or hot conditions. That means a 70 kg runner losing about 1.4 kg, or 1400ml of sweat, during a long run or race may already be at a level where performance can be affected. Even mild dehydration, around 1–2% body water loss, may also affect mood, attention, and cognitive performance, which matters when you’re pacing, fuelling, navigating aid stations, or making decisions late in a race. 

Still, more water is not always better. Overdrinking can be dangerous, especially during long events. Your goal is to start well hydrated, drink according to thirst and conditions, and consider sodium if you’re running long, sweating heavily, or racing in hot weather.

If you’re a salty sweater, cramp-prone, or unsure how much sodium you lose, sweat sodium testing can help personalise your hydration strategy.

What About Race Day?

Race day is where runners can get into trouble, because the plan feels set.

You paid for the race entry, trained for a goal pace, and tapered perfectly. 

But then the forecast changes.

The BMO Marathon weekend in Vancouver is a classic example of this. Held on the first Sunday of May each year, runners have trained through the winter, and can still be finishing their final long runs in cold rain or even snow. Then our famous “false summer” arrives, and race day is all of a sudden 25+°C. 

When race day is hot, your pacing plan needs flexibility.

Start more conservatively than planned. Use RPE early, not only after things start falling apart. If your heart rate is unusually high in the first few kilometres, take that seriously. A small adjustment early is usually better than a major blow-up later.

For marathons and ultramarathons, heat is especially important because exposure time is long. You’re not only managing pace. You’re managing core temperature, hydration, sodium, fuelling, and decision-making while your body is under stress.

Use aid stations. Drink appropriately. Pour water on yourself if available. Use cooling towels, ice, or shade when possible. Most importantly, don’t ignore signs that something is wrong.

Watch for Heat Exhaustion and Heat Stroke

Training adjustments are important, but heat safety comes first.

Heat exhaustion can include dizziness, weakness, headache, nausea, heavy sweating, cool or clammy skin, rapid pulse, and feeling unusually unwell.

Heat stroke is a medical emergency. Warning signs can include confusion, altered behaviour, collapse, very high body temperature, severe headache, vomiting, loss of coordination, or hot skin. Sweating may still be present, so don’t rely on “not sweating” as the only sign. Exertional heat stroke requires immediate emergency care. 

Call 911 right away if heat stroke is suspected.

Don’t try to be a hero. No training run, race, or finish line photo is worth risking your life.

When VO2 Max Testing Helps

VO2 max testing won’t stop your heart rate from rising in the heat, but it can help you understand your normal physiology.

A proper test can identify your VO2 max, heart rate max, ventilatory thresholds, and individualized training zones. That gives you a better baseline, so when heat, fatigue, illness, stress, or dehydration changes your heart rate response, you have more context.

VO2 max testing can help answer questions like:

• What are my actual training zones?
• Is my watch estimating my max heart rate incorrectly?
• Where are my aerobic and threshold intensities?
• What should easy actually feel like?
• How should I adjust when conditions change?

Hot weather still requires judgement, but better baseline data makes that judgement easier.

The Bottom Line

Heart rate often rises in the heat because your cardiovascular system is supporting both running and cooling. This is normal, but it changes how you should interpret your data.

On hot days, use RPE, adjust pace, shorten sessions when needed, hydrate appropriately, and respect warning signs of heat illness.

Your watch can still provide handy information, but it’s important to listen to your body.

Key takeaways

• Heart rate usually rises in hot weather because your body is sending blood to both working muscles and the skin for cooling.
• Cardiovascular drift means heart rate can increase over time even when pace stays the same.
• Your normal heart rate zones may be less reliable in heat, so use RPE alongside heart rate.
• Adjust training by slowing your pace, shortening sessions, moving workouts earlier, and reducing intensity when needed.
• Hydration affects both heart rate and performance. Around 2% body mass loss is where endurance performance decline becomes more consistent, especially in warm conditions.
• Sweat sodium testing can help runners who sweat heavily, finish runs covered in salt, cramp often, or struggle with hydration during long runs and races.
• Heat stroke is a medical emergency. Call 911 if it is suspected.
• VO2 max testing can help runners set accurate baseline heart rate zones and understand how their body responds to training.

Looking for some accurate data to guide your training?

You can book in for a VO₂ Max test or Sweat Sodium test in Port Moody to give you personalized performance and hydration data to get the most out of your training.

You can also subscribe to my newsletter or follow me on Instagram for more evidence based tips for runners.

Frequently Asked Questions about Heart Rate and Heat