Efficiency Factor

Definition

“Efficiency Factor” (EF) is the ratio of normalized power or normalized pace over a period to the average heart rate in the same period. The metric originates with Joe Friel and is meant to help recognise whether training is having a positive or negative effect on cardiovascular performance.

EF = NP [W] / HR_average [bpm]

In running, normalized pace in m/min is used instead of power.

Why it matters

If more power is produced at the same heart rate over weeks, the aerobic system is working more efficiently. EF makes this development visible without a test: comparing similar sessions is enough.

By comparing comparable segments, one can estimate whether an athlete’s endurance performance is rising, stagnating or declining. EF is therefore one of the few metrics that can be derived from ordinary base training.

EF is a field value. It says nothing about which substrate is being oxidised; only diagnostics show that.

How obseed measures it

obseed calculates EF for every recorded session with power or pace data and heart rate. For intervals and segments it is also formed per section, which allows direct comparison of base sessions over weeks.

Quality hinges on heart rate measurement. Wrist sensors produce outliers especially at the start of a session that distort the average; a chest strap is the more reliable source. Which source counts for heart rate data is a choice you make yourself in obseed.

EF also serves as the basis for aerobic decoupling, in which the first half of a session is compared with the second.

Example

An athlete rides a two-hour base session on the same route in March and in May.

March:  NP 185 W, HR 142 bpm  →  EF = 185 / 142 = 1.30
May:    NP 198 W, HR 141 bpm  →  EF = 198 / 141 = 1.40
Gain: (1.40 − 1.30) / 1.30 × 100 = 7.7 %

At practically the same heart rate she produces 13 W more. EF has risen by just under 8 %, a clear sign of improved aerobic efficiency. Whether fat oxidation has improved as well can only be checked in the next diagnostic.