Efficiency factor

Efficiency Factor (EF) asks one question across your runs: at a given heart rate, how fast are you going? It is the speed you hold for each beat of your heart. As your aerobic fitness improves, the same easy pace costs fewer heartbeats, so EF climbs. Tracked over weeks of similar runs, a rising EF is a quiet signal that your engine is getting stronger.

EF is a coaching heuristic, popularised by Joe Friel, not a peer-reviewed measurement. It is mechanistically plausible (it proxies the well-established fall in submaximal heart rate with aerobic adaptation), but the sections below are careful to say where the meaning comes from and where it does not.

The idea in one line

Divide your speed by your average heart rate, with speed measured so that going faster at the same heart rate reads as a higher, better number.

EF = speed ÷ average heart rate

We report EF as metres per heartbeat: speed in metres per minute (metres per second times 60) divided by average heart rate in beats per minute. The two "per minute" terms cancel, leaving the distance you cover per heartbeat (around 1.2 to 1.4 metres on an easy run). Multiplying speed by 60 first is just what keeps the number in that readable range rather than a tiny fraction. A single number in isolation says almost nothing. Its value is in the direction it moves across many like runs.

A trend, not a score

EF is a trend metric. One run tells you little, because terrain, weather, fueling, and how you slept all nudge the number. But line up a series of similar steady aerobic runs over several weeks and a rising trend becomes hard to fake: your submaximal heart rate is falling as you adapt, which is exactly what improving aerobic fitness looks like.

Worked example

Take two easy runs six weeks apart, in similar conditions (same route, similar temperature, same easy effort).

EF and aerobic decoupling

The two are close cousins built from the same efficiency factor, but they look in different directions: aerobic decoupling compares the first half of a single run against its second half, while EF compares whole runs against each other over time.

When it means something

EF only tells a clean story when you hold the conditions still. It is not comparable between athletes, and only comparable across like runs: the same terrain, temperature, workout type, and fueling. A flat easy run and a hilly tempo run will have very different EF numbers for reasons that have nothing to do with fitness, so comparing them tells you nothing. Line up runs that resemble each other, and only then read the trend.

Honest caveats

EF is sensitive to heart-rate artifacts: a dropout, a spike, or a strap that read high for the first minute can move the number more than a real change in fitness would. Worse, it can be misleadingly raised by fatigue, overreaching, or overtraining, which can suppress heart rate at a given pace. (Acute illness usually does the opposite, raising heart rate, so a run that suddenly feels hard with a high heart rate is its own warning.) A lower heart rate at the same pace usually means you are fitter, but sometimes it means you are run down, so a jump in EF alongside heavy legs and poor sleep is a warning, not a win.

The absolute value is meaningless. Only the trend, watched under controlled and comparable conditions and read alongside how you actually feel, carries a signal. Evidence tier: coaching heuristic, but mechanistically plausible, since it proxies the well-established fall in submaximal heart rate with aerobic adaptation.