July 14, 2026 · 6 min read

Power-to-Heart-Rate Ratio (EF — Efficiency Factor) — How to Track Aerobic Base Progress

Efficiency Factor (EF) is normalized power (NP) divided by average heart rate for a session or interval — it tells you how many watts you "buy" per heartbeat. A rider with an NP of 180 W at 135 BPM has an EF of 1.33; when after 8 weeks of base the same heart rate delivers 195 W, EF rises to 1.44 — hard proof the aerobic base has genuinely grown. It's the simplest metric of aerobic progress: a rising EF on comparable sessions = the engine runs more efficiently.

EF makes sense because of the physiology: heart rate reflects the internal cost of an effort, power the external output. Base adaptations (bigger cardiac stroke volume, more mitochondria, better fat oxidation) make the same power cost fewer beats. FTP often stands still during the base period — while EF climbs, because it measures something other than maximal capacity. That's why it's the first-choice metric for tracking the effects of aerobic riding.

How to compute and compare EF

• Formula — EF = NP / average heart rate. For the whole session or a chosen steady segment (e.g. 60 min of Zone 2).
• Typical values — amateurs usually 1.0–1.5; the higher the value at the same relative intensity, the stronger the base. The absolute number depends on your HRmax, so don't compare with a friend — compare with yourself.
• The condition: comparable sessions — EF only makes sense between similar workouts: same zone, similar duration, trainer vs trainer. You can't compare interval EF with Zone 2 EF.

How much of an EF rise means real aerobic base progress?

A trend of +5–8% across 6–8 weeks of comparable sessions is the typical, real effect of a base block — e.g. from 1.30 to 1.38. Single jumps of 3–4% between two rides are noise (sleep, caffeine, temperature, stress). Look at a rolling average over several sessions of the same type, not a single result.

What corrupts an EF reading

Heart rate responds to everything, not just fitness — so EF demands measurement hygiene:

• Heat and dehydration — raise heart rate by 5–10 BPM at the same power and depress EF. On a trainer without a fan, EF will always look worse. Compare sessions in similar conditions.
• Cardiac drift — on a long ride heart rate rises despite steady power; first-hour EF will beat second-hour EF. Zone discipline matters — set the grid per Coggan's power zones and read EF alongside the drift.
• Caffeine, short sleep, stress — shift resting and exercise heart rate. One bad night can "eat" 3% of EF.
• Changing the power source — pedals vs trainer differ by 2–4%, so always compute EF from the same meter — see trainer vs pedals.

EF or decoupling — which tracks the aerobic base better?

Both, because they measure different things: EF shows the level of efficiency, decoupling its durability over time. A rising EF with shrinking drift (<5%) is the textbook picture of a maturing base. An EF rising alongside high drift suggests you're riding too hard and the improvement is illusory.

Summary

EF is the cheapest "fitness test" there is — no test to exhaustion, just a heart-rate strap, a power meter and comparable sessions. Compute NP/HR for steady, repeatable Zone 2 workouts, watch the 6–8-week trend and read it alongside cardiac drift. A 5–8% rise means the base block is working even if FTP hasn't moved. And when EF stops rising despite the volume — that's the signal the base has matured and it's time to add intensity instead of grinding more identical hours; the PMC chart will tell you how much load headroom you have.

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