What FTP is, and how Peak Hybrid estimates yours

The short answer

FTP, functional threshold power, is the highest power you can hold for about an hour. It sets your power zones and the intensity of every ride with a power meter, so it shapes your training load too. You can test it with a hard 20 minute effort, or estimate it from the best efforts in rides you already did, and either way, a range of about 5% either side is a fair way to read it, since 20 minute power alone shifts that much with the warm-up.

What FTP measures

The original definition is your average power over an all-out 60 minute effort. Few riders want to do that often, so most use a shorter test. In 8 trained cyclists, 60 minute power averaged 309 W against 342 W for 20 minutes, about 90% (MacInnis 2019). The common rule takes 95% of a 20 minute test, and results like this one are why Peak Hybrid's factor runs from 0.90 to 0.95.

FTP is a practical number, and the research on how it lines up with your physiology is split. In 15 cyclists, FTP from a 20 minute test sat close to maximal lactate steady state, the highest effort where blood lactate stays level, with a typical error of 4.7% (Borszcz 2019). In 13 other cyclists, riding at FTP led to exhaustion in about 34 minutes, with blood lactate at 6.7 mmol at the end, and those authors conclude FTP is not a valid marker of the maximal metabolic steady state, a closely related boundary (Wong 2022).

How long a rider can hold FTP varies a lot. In 23 trained cyclists, 95% of 20 minute power was held for about 51 minutes on average, give or take 16 minutes (Borszcz 2018). About an hour describes the group average well, and any single rider may hold it for much less or much more.

Why it matters for training

Power zones are set as a share of FTP. The endurance zone runs from 55 to 75% of FTP, and the threshold zone from 90 to 105%. If your FTP is set 10% too high, every easy ride drifts toward tempo and every interval is harder than the plan intended.

FTP also feeds training load, because for a ride with a power meter, intensity is normalized power divided by FTP and load is hours times intensity squared times 100. An hour at FTP scores 100. A wrong FTP shifts the load of every ride, and everything built from load moves with it.

Testing or estimating

A test measures your power directly on 1 day, so the result depends on how that day goes. In 21 trained cyclists, 20 minute power changed by about 5% with the warm-up alone, from about 256 W after the 2 long warm-ups to 270 W after the shorter ones (Tramontin 2022). Riders in that trial also paced the test differently after different warm-ups.

An estimate reads FTP from rides you have already done. It uses many hard efforts instead of 1, so it needs no test day and updates as you train. Its weakness is that it only knows what you have ridden hard. If you have not pushed for a few minutes in a while, the estimate leans on older efforts.

The estimate uses a model called critical power. Your best power for a given duration falls as the duration grows, and the curve flattens toward a power you can sustain for a long time. In 13 under-23 professional riders, critical power estimated from their best 2, 5, and 12 minute race efforts matched a field critical power test (Spragg 2023).

How Peak Hybrid estimates your FTP

Peak Hybrid reads your FTP from power-meter rides, and e-bike rides stay out.

It fits a critical power curve to your best 2 to 15 minute efforts from the last 365 days, reads your 20 minute power off that curve, and multiplies by a factor of 0.90 to 0.95, 0.92 by default. Efforts under 2 minutes lean on anaerobic power, and efforts over 15 minutes are often below your best, so the fit uses the middle range. A 20 minute effort you actually held sets a floor, and the estimate re-anchors on it.

Each effort counts in full for 8 weeks. After that it fades 1% a week while you keep riding and 2.5% a week through weeks off the bike, and past 365 days it drops out. The 2 rates follow the direction of the detraining research, slow while you keep riding and faster through a break, since endurance fitness starts to fall within weeks of reduced training (Mujika and Padilla 2000). The exact rates are a starting choice, not a measured value.

Heart rate can act as a 2nd read. It needs a power-to-heart-rate line from 10 or more steady segments across 2 or more rides, and a steady effort of 25 minutes or more at 80 to 97% of your LTHR, both in the last 6 weeks. With those in place, heart rate can raise the estimate by up to 8%, and it only ever raises it. It stays off while your LTHR is an estimate.

The app shows the estimate as a band of about 5% either side, not a single point. A typed FTP always stays in force, and the estimate fills the field only when you leave it empty. When your typed FTP and your rides differ by more than 3%, the app offers to use the ride number or keep yours. The full method, with each formula, is on the method page.

How Peak Hybrid uses this

Your FTP sets the power targets on every ride and the load of every ride with power. On Strength for Cyclists, split squats and other lowering-heavy lifts come off the day before a hard or long ride, and those hard efforts feed your estimate. The plan details are on the plans page.