Heart rate drift during long rides — normal or a problem?

On my long Z2 rides (3+ hours), my heart rate gradually drifts upward even though I'm keeping power constant. I'll start at 130bpm at 180W and by hour 3 I'm at 145bpm at the same 180W.

Is this normal cardiovascular drift? Or does it indicate a fitness/hydration issue?

Paincave is showing my aerobic decoupling at around 8-10% on these rides. Is that too high?

This is cardiac drift and it's completely normal. It happens to everyone on prolonged exercise and is caused by:

1. Plasma volume loss — You sweat, blood volume decreases, so your heart has to beat faster to maintain the same cardiac output.
2. Thermoregulation — Blood is diverted to the skin for cooling, reducing the volume available for the muscles.
3. Substrate shift — As you deplete glycogen, your body relies more on fat oxidation, which is less metabolically efficient.

What's normal?

• Aerobic decoupling (Pw:Hr) of <5% = excellent aerobic fitness
• 5-10% = good, room for improvement
• >10% = your aerobic base needs more work, OR you're under-fueling/dehydrating

Your 8-10% is okay but suggests room for improvement. Two things to check:

1. Are you eating/drinking enough during these long rides? Dehydration amplifies drift significantly.
2. How long have you been doing consistent Z2 training? Cardiac drift decreases as your aerobic base improves over months.

Great explanation from Bjorn. I'd add one practical tip: track your decoupling over time.

When I started structured Z2 training, my decoupling was ~12% on 3-hour rides. After 4 months of consistent base work, it dropped to ~4%. This is one of the most reliable indicators of aerobic fitness improvement.

Paincave shows this on the ride evaluation, which makes it easy to track. A decreasing trend = your aerobic engine is getting stronger, even if your FTP hasn't moved yet.

Thanks both. I've been fueling with about 40g carbs/hour on my long rides — sounds like I might need to increase that based on the nutrition discussion above.

Going to start tracking decoupling week over week. If it's still above 8% after a month of consistent Z2 + better nutrition, I'll know it's a fitness issue rather than fueling.