Find out if your engine is getting strongerPro

Most of a run’s page describes that run. The two numbers under Advanced Insights describe you, in a way that carries from one run to the next: how much speed a heartbeat buys you, and how evenly you can hold an effort. Neither is exciting on a single run. Both are the earliest place a block of training shows up.

Where to find it

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Just under the metric cards, before Running Dynamics. It’s a Pro section, and it appears on any run that has both a route and a heart rate.

What you’re looking at

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• Efficiency Factor — your speed divided by your heart rate: how many metres per minute each beat produced, averaged over the run. Speed here is grade-adjusted, so a hilly route doesn’t read as a worse engine. Higher is better, and the interesting part is the direction over months, not the figure.
• Pace Stability Index — how evenly you held your pace, scored out of 100. A steady run scores in the high nineties; a run with surges, stops and a fade scores lower. It uses grade-adjusted pace too, so slowing for a hill isn’t counted against you.
• Potential Boost — a banner that appears only when this run raised one of your predicted race times, naming the distance and by how much. Most runs don’t have one, which is what makes it worth noticing when a run does.

Reading Efficiency Factor

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The trap is comparing two runs that were never comparable. Heart rate responds to heat, sleep, caffeine, the hour of the day and how hard yesterday was, so a single run’s Efficiency Factor moves around by a few percent for reasons that have nothing to do with fitness. What holds is the trend across the same kind of run:

• Easy runs, month over month. These are the ones to watch. Same routes, same effort, and the number drifts up as your aerobic base builds. This is fitness arriving before pace does.
• Not races against easy runs. A race is run at a heart rate where efficiency naturally reads lower. The half marathon above, at 1.17, says nothing about whether the runner’s easy runs at 1.30 are improving.
• Not summer against winter. Heat alone lifts heart rate at a given pace. Expect the number to sag in July and recover in October, and don’t read either as training.

The Trends screen charts Efficiency Factor over time and does the averaging for you. The monthly report carries the month’s figure with a change against the previous one, which is the level this number is honest at.

Reading Pace Stability Index

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This one is about the session, and it’s read against what the session was for:

• An easy run or a tempo wants a high score. A 94 on a half marathon is a well-judged race. A 94 on an easy run is normal.
• Intervals and fartlek are supposed to score low. The number is measuring exactly what you did on purpose. Don’t try to raise it.
• A steady run that scored low is the interesting case. Open the splits and find where it broke — a fade in the last third, a stop, a stretch that was faster than it should have been.

Between two race attempts at the same distance, the higher stability is usually the better-paced one, and often the faster one too.

Worth knowing

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• Both numbers are Pro, on every qualifying run, however old. Your history was scored on the day you first synced, so the Trends chart is complete without you opening a single run.
• Each split has its own pair, on the split’s detail screen, for finding which kilometre the stability went.
• The ⓘ beside the heading explains both numbers in the app’s own words.
• The web calculator gives you Efficiency Factor from any pace and heart rate you type in, useful for checking a run from another app against these.