Aerobic decoupling: what it is and how runners use it

Split a steady run into two halves. For each half, divide your average pace (as speed) by your average heart rate to get an efficiency ratio. Decoupling is how much worse the second half's ratio is than the first, as a percentage:

Decoupling % = (first-half ratio − second-half ratio) ÷ first-half ratio × 100

Cyclists do the same with power instead of pace (Pw:HR). For running on hills, use grade-adjusted pace, or the climbs will swamp the signal.

• Under 5%: heart rate and pace stayed coupled. The run was within your aerobic base. This is the widely used threshold popularised by coach Joe Friel.
• 5–10%: noticeable drift. Normal for a run longer or faster than you're used to, or in heat.
• Over 10%: the run was beyond your current aerobic durability, or something else (heat, dehydration, illness, poor fuelling) was going on.

• Only compare steady runs. Intervals and races decouple by design.
• Watch the trend, not a single run. Falling decoupling at the same pace and duration means your aerobic base is growing.
• Use it to set long-run pace: if you regularly decouple over 5%, slow your easy runs until you don't.
• Note the conditions. Heat alone can add several percent.

Relic computes decoupling automatically on steady runs of 40 minutes or more using grade-adjusted pace, leaving out the warm-up so early heart-rate climb isn't counted. It shows it next to pace fade and aerobic efficiency in your running analysis, so you can see when you started to slow and why.