FTP vs Lactate Threshold

Cyclists use “FTP” and “threshold” as if they were the same word. They are not the same measurement, and the gap between them is large enough to change how you read a lab report. Enter your FTP to see the zones it produces, then read what that number is and is not:

Two different kinds of number

Lactate threshold is a physiological measurement. A rider performs a graded exercise test while blood is sampled at each stage, and the threshold is identified from how lactate concentration rises — commonly at a fixed 4 mmol/L, sometimes at the first non-linear increase. It requires a laboratory.

Functional threshold power is a performance estimate. It is defined as the highest average power sustainable for roughly an hour, and in practice it is inferred from a shorter field test: 95% of a 20-minute effort, or 90% of two 8-minute efforts. It requires a power meter.

One describes what your blood is doing. The other describes what you managed to hold on a road.

How far apart they actually land

Nine junior road cyclists were tested both ways — a field FTP from an uphill 20-minute time trial and a full laboratory workup on the same athletes:

Measure	Power
Field FTP	269 ± 34 W
Critical power	236 ± 24 W
Power at 4 mmol lactate threshold	233 ± 23 W

The field FTP significantly exceeded both laboratory measures — by roughly 15% over the lactate threshold value. At the same time it tracked them closely across riders: the correlation with the 4 mmol threshold was r = 0.94. That combination is the whole story of this comparison. FTP ranks cyclists almost exactly as the laboratory does, and reports a systematically higher wattage while doing it.

Why FTP reads high

Two reasons stack up. The first is definitional: the 5% discount applied to a 20-minute test is a population average, and reviewers have shown that 95% of 20-minute power tends to overestimate true 60-minute power. The second is situational: a field test on a real climb, with the rider’s own bike and no laboratory ergometer, produces a higher effort than the lab does — in the junior study, even peak oxygen uptake was higher outdoors than indoors.

What this means in practice

For everyday training, very little. Zones built from FTP are internally consistent: if your FTP is 5% optimistic, every zone is 5% optimistic with it, and the workouts still land in the right relative places. The distinction bites in three situations:

• Comparing across methods. A lab report saying your threshold is 233 W does not contradict a power meter saying your FTP is 269 W. They measured different things.
• Comparing across riders. Two cyclists tested by different protocols are not on the same scale, which is also why watts per kilogram figures should be read with the protocol attached.
• Believing the physiology. A 2022 review of power profiling puts it bluntly: FTP lacks a physiological definition, and the limits of agreement with critical power and with maximal lactate steady state are too wide for the terms to be interchangeable.

Use FTP for what it is good at — a repeatable field number that anchors a training plan — and retest it the same way every time. The FTP training zones calculator turns it into watts, and the Coggan zone reference explains what each band is meant to develop.