Aerobic Efficiency: What watts per heartbeat really reveal
Same pulse, more watts: that's the most honest fitness indicator your sensor can deliver.
If you train regularly with a power meter and a heart-rate strap, one of the most valuable endurance-sport metrics is sitting right in front of you, and most people ignore it: Aerobic Efficiency, the ratio of power to heart rate.
The idea in one sentence
If you push 200 watts at 145 bpm today and 215 watts at the same 145 bpm in two months, you've gotten fitter. Period. No extrapolation, no VO2max estimate, no model assumption. Just: same pulse, more output.
How is Aerobic Efficiency calculated?
In its simplest form: EF = Normalized Power / average heart rate. A session with NP 220 W and 155 bpm has an EF of 1.42. For running: EF = (speed in m/s × 60) / HR.
Important: EF is only meaningful for aerobic, steady sessions. Sprint intervals or hill repeats distort the value heavily. trace automatically filters the sessions that are useful for EF tracking (typically endurance to tempo, > 45 minutes).
What the trend reveals
A rising EF over weeks is one of the most reliable signs of aerobic adaptation, even when your FTP tests are stagnating. Many athletes go through phases where max power stays flat while EF slowly climbs. That means your aerobic engine is improving, you're saving the top-end refresh for later.
A suddenly falling EF, on the other hand, is a warning. Common causes:
- An incoming infection. Heart rate reacts first.
- Persistent fatigue (matches a strongly negative TSB).
- Heat or dehydration during the session.
- Sleep deficit over the last 2 to 3 nights.
Decoupling: the second EF value
Closely related is aerobic decoupling: how much does your heart rate rise in the second half of a long session while power stays the same? Below 5 % counts as a sign of good aerobic endurance; above 10 % suggests your base isn't yet sufficient for the intensity you're riding.
Why this is more interesting than VO2max estimates
VO2max in apps is usually interpolated from your best efforts, an estimate built on top of an estimate. Aerobic Efficiency, by contrast, measures what's actually happening: two values that come straight from the sensor, in a direct ratio. That makes it less prone to model error and more meaningful over the long term.
How to use this in practice
In trace you see your EF as a time series over the last weeks and months. The questions to ask:
- Is the line rising across a training block? Adaptation is working.
- Stagnant despite hard training? Probably a too-monotonous stimulus.
- Falling unexpectedly? Check sleep and health first, then training.
The beauty of the metric: no test, no lab, no effort above your comfort zone required. EF arises naturally from every well-executed endurance session. And it often tells you weeks before the next FTP test which direction your form is heading.