Why is my heart rate climbing even though I'm running at the same pace?
Aka you’re worried that your easy pace suddenly feels difficult.
You're halfway through an easy run.
Your breathing feels controlled. Your shoulders are relaxed. Your effort hasn't changed.
Then you glance at your watch. [cue svu opener]
Your heart rate has climbed 15 beats.
Cue the internal panic.
Am I getting less fit? Am I overtraining?
Why does this suddenly feel harder?
Chances are, you're experiencing cardiac drift.
Cardiac drift (also known as cardio drift) is the upward climb in heart rate over the course of a workout without any change in intensity. It's a normal physiological response, not necessarily a sign that something is wrong.
In hot or humid conditions, it happens sooner and to a greater degree.
Here's what's happening:
Your muscles produce heat.
Your body redirects blood toward your skin to cool you.
Less blood is available for your working muscles.
Your heart compensates by beating faster.
Stress hormones like epinephrine and norepinephrine also rise during exercise, nudging heart rate higher.
None of this means you're getting less fit.
It means your body is managing multiple jobs at once. (Any other soulpreneurs out there FEEL this?)
Remember what we talked about in my summer running guide? Heat is already one stressor. Cardiac drift is one of the ways that stress shows up.
I coach almost exclusively by effort in the summer (regardless of your “athleticism”) because the weather doesn't care what pace your training plan (and tbf, neither do your hormones!) says.
I can't tell you how many runners have messaged me saying: "My heart rate is way higher than normal. Should I be worried?"
Ninety percent of the time?
They're running in South Florida in July at noon. I’m looking at you, MC.
I know, I know, we love outsourcing information to our watches, our rings, our phones for body data.. However…. This is exactly why I coach to your Rate of Perceived Effort (RPE) aka the story behind the numbers.
Here’s the breakdown of the difference:
Objective Metrics/External Load:
heart rate (HR), power output (i.e. watts on the rower or cals on the bike),
speed or pace,
weight lifted, or
reps performed.
Subjective Metrics/Internal Sourcing: This is how it feels to you and is rated on a scale (such as 1-10). Factors, like what make us human, influence RPE such as: fatigue, stress, sleep, heat, hydration, mental state, and emotional or invisible backpack. For instance, you may feel like a 7/10 on a run even if you’re “slower” because you didn’t sleep well.
RPE
Breathing
Mood
Muscle Fatigue
Think of driving a car.Your speedometer says 65 mph.
But are you driving uphill into a headwind with the AC blasting?
Or downhill with a tailwind?
Same speed. Completely different effort.
That's the difference between pace and RPE.
Actual Effort: What your car is doing on the road (speedometer)
RPE: How the drive feels, whether it’s uphill, windy, low on gas, so it feels harder than the number says.
But Coach, why don't pace and effort always agree?
Because you're a human, not a robot.
Heat.
Humidity.
Poor sleep.
Work stress.
Dehydration.
Your menstrual cycle.
A fight with your partner.
All of those can make an "easy" run feel much harder without changing your actual fitness.
3-4 - Easy, conversational, controlled
5-6 - Steady, focused, short phrases
7-8 - Working, uncomfortable AND sustainable
9-10 - Hard, race effort, only a few words or single word at a time if that
A gradual increase in heart rate during a long run is expected. What concerns me isn't a normal drift.. it's when heart rate spikes unexpectedly alongside symptoms like dizziness, chest pain, or fainting. You can read more about that here from a medical perspective.
Summer training isn't about proving you're tough.
It's about becoming adaptable, flexible.
The strongest runners aren't the ones who force the pace, or rather, force anything.
They're the ones who know when to adjust it.
That's exactly what racing asks of us, too.**
** If you've ever wondered why marathon pacing becomes so challenging late in the race, cardiac drift is one piece of the puzzle. Even if you hold a perfectly steady pace, your body is working progressively harder to maintain it.
