What it is
Cardiac decoupling measures how much your heart rate drifts relative to your power (or pace) during a ride. It compares the second half of the ride to the first: if you held the same power but your heart rate crept up 8%, your decoupling is ~8%. It's one of the best simple indicators of aerobic durability.
What's a good number?
Below 5% — solid aerobic fitness for that duration; the classic benchmark.
5–10% — some drift; normal for longer rides, heat or the tail end of a training block.
Above 10% — significant drift: the duration/intensity outran your current aerobic base, or something else stressed the system (heat, dehydration, poor sleep, illness).
Decoupling only means much on steady endurance rides of about an hour or more. Interval sessions, group rides and stop-start routes inflate it for reasons that have nothing to do with fitness — that's why LeCoach's chart has an 'endurance rides only' toggle.
Where to see it
Per activity in the day view stats, and as a trend in Analytics (Cycling and Running sections, with a 5% reference line). The value itself is calculated by Intervals.icu over the whole activity; LeCoach reads, tracks and coaches on it.
One bad number ≠ bad fitness
Heat and dehydration alone can add several points. Watch the trend across similar rides: decoupling steadily falling on your regular endurance route is base fitness being built. Ask your coach 'how's my decoupling trending?' — it will pull the right rides and interpret honestly.