Heart Rate Drift and Aerobic Decoupling

Same 6:00/km long run: miles 1–5 average 142 bpm; miles 6–10 average 158. Pace never changed — the watch is showing cost, not a pace PR. Small cardiac drift is normal. Large drift or worsening decoupling signals heat, fueling issues, or an easy pace that was never easy.

Drift vs Decoupling

Cardiac drift: HR rises over time at roughly constant pace. Causes include rising core temperature, dehydration, glycogen drawdown, and starting too hard. Aerobic decoupling: how pace and HR diverge between early and late halves — often as a percent. Improving fitness usually reduces decoupling at the same easy effort.

Worked Measure: 90-Minute Zone 2 Long Run

Athlete: Zone 2 ceiling 148 bpm (from the Zone 2 Calculator). Cool day, chest strap. First 45 min: 6:05/km @ 140 bpm. Second 45: still 6:05/km @ 152 bpm. Drift ≈ (152−140)/140 ≈ 8.6% — acceptable early in base, watch fueling/heat next time. Same route next month at 6:00/km with second-half HR 145 (~3.5% drift) is the progress signal. Set alert bands with the HR Zones Calculator; sensor caveats: accuracy guide.

When Rising HR Means Adjust Today

• Drift above ~10% while pace is flat → slow down, hydrate, or shorten
• Chronic high drift in cool weather → easy pace still too fast
• Sudden new drift → illness, poor sleep, or under-fueling

Improve the trend with consistent Zone 2 volume, gradual long-run growth, and mid-run fueling on efforts past about 75–90 min.

Start Under the Ceiling

If early pace already sits at your Zone 2 ceiling, late HR has nowhere to go but Zone 3. Begin long easy runs 5–8 bpm under the cap so normal drift still finishes aerobic. Fuel after ~75 minutes even on “easy” days if drift has been creeping up in cool weather.

Decision Rule

If second-half HR rises more than ~10% above first-half HR at the same easy pace, slow until you are back under your Zone 2 ceiling — or end the run. If drift is ≤5% on a cool, fueled long run, keep the plan and track the same route monthly.