If you've been training this summer and wondering why your heart rate suddenly seems out of control at a pace that felt comfortable just a few months ago, you're not alone.
And more importantly—you are not getting weaker.
This is one of the most common conversations I have with athletes every summer.
"Adam...is it just me?"
No.
It's not just you.
What's happening is completely normal, and once you understand the physiology behind it, those frustrating summer workouts start making a lot more sense.
Your Heart Has One Job
During exercise, your heart has one primary responsibility:
Deliver enough oxygen-rich blood to your working muscles.
The amount of blood your heart pumps every minute is called cardiac output, and it's determined by two factors:
- Stroke Volume – how much blood is pumped with each heartbeat.
- Heart Rate – how many times your heart beats each minute.
Think of it like this:
Cardiac Output = Stroke Volume × Heart Rate
In cool weather, these two variables stay in balance. Stroke volume remains fairly consistent, heart rate accurately reflects your effort, and your training zones behave exactly as you'd expect.
Then summer arrives.
What Heat Does to Your Body
The moment you start exercising in hot conditions, your body is forced to solve two problems at once.
1. You're Losing Blood Volume
When you sweat, you're not just losing water.
You're losing plasma—the fluid portion of your blood.
Even if you're drinking throughout your workout, research shows plasma volume can decrease by 10–15% during a moderate training session.
Less plasma means less blood returns to the heart.
Less blood returning means less blood pumped with every beat.
Your stroke volume decreases.
2. Your Body Prioritizes Cooling
At the same time, your body begins redirecting blood toward your skin so heat can escape.
This process—called cutaneous vasodilation—helps prevent dangerous increases in core temperature.
It's incredibly important.
But it creates another challenge.
Now your cardiovascular system is trying to do two jobs:
- Deliver oxygen to your muscles.
- Keep your body cool.
Those two demands compete with one another.
So Why Does Your Heart Rate Keep Climbing?
When stroke volume drops, your heart has only one option.
It beats faster.
This phenomenon, often called cardiovascular drift, is well documented.
Research published in Medicine & Science in Sports & Exercise found that during moderate-intensity running in the heat:
- Stroke volume decreased by 15–20%
- Heart rate increased by 17–19% between minutes 15 and 45 to compensate
Nothing about your fitness changed.
Your body simply had to work harder to maintain blood flow.
Here's the Part That Frustrates Everyone
Your pace starts falling.
Or...
You stubbornly try to hold your normal pace, and suddenly what should be an easy run feels like race pace.
This is where so many athletes start questioning their fitness.
But here's what's really happening.
Although your heart rate is climbing, a larger percentage of your blood is now being sent to your skin instead of your muscles.
Your watch may read 155 bpm, but your legs aren't receiving the same oxygen delivery they would have received at 155 bpm in cooler weather.
In other words...
Your heart is working harder.
Your muscles are getting less.
That's why the pace you comfortably held in April suddenly feels impossible in July.
The relationship between pace and heart rate has changed—not your fitness.
You're Not Losing Fitness
This is probably the biggest takeaway.
Heat changes the relationship between:
- Pace
- Heart rate
- Perceived effort
It does not automatically mean you're getting slower.
Your body is simply allocating its resources differently.
It's trying to fuel your muscles while simultaneously preventing you from overheating.
That's physiology—not failure.
How Should You Adjust Your Training?
Slow Down
This is the hardest adjustment for most athletes.
The pace that was Zone 2 in April probably isn't Zone 2 in July.
Run by effort.
Run by heart rate.
Leave your ego at home.
Start Every Workout Hydrated
Once plasma volume starts falling, you're already behind.
Showing up well hydrated won't eliminate cardiovascular drift, but it can delay it and reduce how severe it becomes.
Let Heart Rate Guide Easy Days
Trying to force spring paces during summer heat often turns an aerobic workout into a threshold workout.
That's not productive.
Remember:
Easy training should still be easy—even if the pace looks slower than you're used to.
The Good News
Your body adapts remarkably well to heat.
With consistent exposure, you'll begin to:
- Expand plasma volume
- Sweat earlier and more efficiently
- Lower your core temperature at the same workload
- Improve your tolerance for hot conditions
These adaptations don't just help you survive summer.
They often translate into stronger performances when cooler race weather arrives.
Final Thoughts
Summer training can be humbling.
Your watch says you're working harder.
Your pace says you're getting slower.
Neither tells the full story.
Trust the process.
Respect the conditions.
Remember that your body is solving a much bigger problem than simply running fast.
Sometimes the smartest athlete isn't the one forcing April paces in July.
It's the one willing to slow down today so they can race faster this fall.
Ready to Train Smarter This Summer?
At SuperFly Coaching, we help endurance athletes understand the why behind their training—not just what to do.
If you're preparing for a fall race and want a plan that works with your physiology instead of against it, we'd love to help.
Visit superflycoaching.com to learn more or schedule a free discovery call.