Understanding FTP

Functional Threshold Power gets treated like a single source of truth — the number that decides your zones, your race pacing, even how fit you feel walking into a season. It's a useful number. It's also frequently misunderstood, and misused in ways that quietly cap how well it works for you.

What FTP actually measures

FTP is an estimate of the highest power you can sustain in a quasi-steady state for around an hour — the point where lactate production and clearance are closely matched, so fatigue builds slowly rather than suddenly. It's a proxy for your metabolic threshold, not a direct measurement of it. That distinction matters more than it sounds like it should.

Where the common tests fall short

The 20-minute test (take the average power, multiply by 0.95) and the ramp test are both popular because they're fast. They're also both estimates of an estimate. A 20-minute effort pulls on different physiology than an hour-long one, and pacing, motivation, and prior fatigue all shift the result — sometimes by a meaningful margin. That's before accounting for VLaMax: two athletes can post the same FTP with very different anaerobic contributions, which means the same "zone 4" prescription trains them differently.

What a proper test changes

A staged lactate test locates your actual threshold from the shape of your lactate curve, rather than inferring it from a single hard effort. It's slower to administer, but it separates your aerobic threshold from your anaerobic capacity — which is what actually determines whether you should be training more top-end power, more sustained threshold work, or both.

What this means for your training

FTP is still a useful single number for setting zones day-to-day — it's simple, and simple is valuable. But if your training has plateaued, or your zones have felt consistently too easy or too hard for months, the number itself might not be wrong so much as incomplete. That's usually the point where a full performance profile — lactate, power, and VO2 together — tells you more than another FTP retest will.