Aerobic decoupling is measured as the percentage change between the first and second halves of a steady-state effort. Under 5% = good aerobic fitness for that duration. Over 5% = you've ridden beyond your sustainable aerobic ceiling. It's one of the best indicators of aerobic base development: a rider can increase FTP without improving decoupling, but they can't increase race durability without it. Targeted 90-minute to 3-hour rides at upper zone 2 are the primary stimulus for reducing decoupling. TrainingPeaks auto-calculates it on every ride.
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