Why Heart Rate Rises While Pace Drops: The Hidden Effects of Dehydration
Familiar situation: you're running your usual route, same pace, but heart rate is 15 beats higher. Legs feel heavy, breathing harder. Overtrained? Getting sick? Or is it simpler β and scarier?
There's an effect that's rarely discussed in running groups, but every sports physiologist knows about it. It's called cardiac drift. And it explains about half the cases when "something's off" during a workout.
What Happens Inside
When you run and sweat, water leaves your body. And not just any water β mainly from blood plasma, the liquid part where red blood cells float.
Blood becomes thicker. Like ketchup left with the cap off.
What does the heart do when it needs to pump thick blood at the same rate? That's right β it works harder. One beat pumps less blood (stroke volume drops), so more beats are needed.
Here are the specific numbers:
| Water Lost | Heart Rate Increase |
|---|---|
| 1% body mass | +3-5 bpm |
| 2% body mass | +8-10 bpm |
| 3% body mass | +15-18 bpm |
| 4%+ body mass | +20+ bpm |
For a 70 kg runner, losing 1% means 700 grams of sweat. In an hour of intense running in warm weather, you easily lose 1-1.5 liters. Draw your conclusions.
Why This Matters for Training
Let's say you have a tempo workout by heart rate: hold 160 beats per minute. You head out, first 20 minutes everything's great β pace 5:00/km, heart rate 158-162. Perfect.
But you're not drinking (or drinking little). After 40 minutes at the same pace, heart rate is now 168. After an hour β 175.
What does the obedient runner do? Slows down to bring heart rate back into the zone. Tempo becomes an easy jog. Training stimulus β out the window.
What does the stubborn runner do? Keeps holding pace. Heart rate 180+. They think they're "pushing through fatigue." Actually β they're driving their heart into the red zone at a load that doesn't require it.
Both options are suboptimal.
How This Affects Your Data
Here's where it gets interesting. All your smart watches, apps, and calorie calculators work based on heart rate. And when heart rate "lies" due to dehydration, the calculations lie too.
The real picture:
| Mass Loss | Calculation Error |
|---|---|
| >2.5% | ~10% |
| >4% | ~15% |
| >5.5% | ~20% |
| >7% | ~28% |
Lost 4% weight on a long workout? Your watch shows you burned 1000 kcal. Actually β about 850. Small difference? Over 20 workouts, that's 3000 kcal of error. Enough to break any nutrition plan.
How to Tell If It's Dehydration
Cardiac drift from dehydration is easy to confuse with:
- Fatigue / lack of sleep
- Oncoming cold
- Overtraining
- Heat (though heat and dehydration often go together)
Distinctive signs of dehydration:
- Thirst β yes, this is the main signal, don't ignore it
- Pace drops at fixed heart rate β or heart rate rises at fixed pace
- The feeling of "not moving" appears gradually, not from the first minutes
- After drinking, it gets easier within 10-15 minutes
If you feel bad from the first steps β it's probably not water. If degradation happens gradually after 30-40 minutes β very likely dehydration.
What to Do About It
Option 1: Adjust Heart Rate Zones
If you know your dehydration level (weighed before and after workouts), you can manually adjust target heart rate. Lost 3%? Your "160 target" is actually 175 on the watch. Hold 175 and don't panic.
Complicated? Yes. That's why there's option 2.
Option 2: Drink Enough
Revolutionary idea, right? But it works.
Practical numbers:
- 400-800 ml/hour β universal range
- In heat and high intensity β closer to 800
- In cool weather and easy workouts β closer to 400
No need to drink strictly by timer. Drink when you want to. But have water available so there's something to drink.
Option 3: Use the Calculator
The water loss calculator accounts for your intensity, temperature, humidity, and weight. It shows:
- Approximately how much you'll lose during the workout
- How much heart rate might "drift"
- What adjustment to make to calculations
It's not a replacement for common sense, but a good planning tool.
Practical Checklist
Before a long or intense workout:
- Check urine color β should be light yellow
- Drink 300-500 ml 1-2 hours before start
- Bring water or plan where to drink
During the workout:
- Don't ignore thirst
- If heart rate "drifted" after 30-40 minutes β try drinking
- Don't panic about high heart rate if you know you drank little
After the workout:
- Weigh yourself and compare with pre-workout weight
- Loss more than 3%? Drink more next time
- Record the data β over time you'll understand your pattern
The Main Point
Cardiac drift is not a disease or a problem. It's a normal physiological response. The problem is when you don't know about it and make wrong decisions.
Now you know. Drink water, pay attention to how you feel, don't trust heart rate blindly. And everything will be fine.