Aerobic efficiency — distance per heartbeat

The honest test of aerobic fitness isn't a hard day — it's what an easy day costs you. Aerobic efficiency is how much ground you cover per heartbeat: your speed divided by your heart rate. As your aerobic engine improves, each beat moves you further, so the number rises.

Its advantage over a race-time estimate is that it trends. It's computed from ordinary runs rather than maximal efforts, so every aerobic run is a data point and the line doesn't swing on whether you happened to race this month.

What's counted, and why. Only genuinely aerobic runs — efficiency falls at high intensity as anaerobic metabolism and cardiac drift take over, so mixing intervals in would track this week's *workout mix* rather than your fitness. Each week is taken as a median (so one hot or hilly run can't move it), then smoothed across three weeks so the line reads as a trend rather than noise.

What it can't remove. Heat, hills, fatigue and a bad night all move a single run's efficiency. That's exactly why it's smoothed and why a one-week dip is not a verdict — and it's why the trend is shown with the runs behind each point, so you can open a dip and see what it actually was.