Functional Threshold Power (FTP) was on the rise as a top cycling fitness metric even before Zwift arrived in 2014. Power meters were becoming more affordable, TrainerRoad had already been around for a few years, and cyclists were reading books like Training and Racing with a Power Meter.
But it’s safe to say Zwift’s arrival and explosive popularity helped make FTP an even more popular metric – the de facto fitness metric for cyclists, in fact. This happened because Zwift races used FTP for categorization, and Zwift was always watching, ready to give you that glorious popup:
But what is FTP, really? How is it best measured, and how should it be used? Let’s dig in!
FTP: A Definition
Many cyclists understand FTP to be the highest average power you can sustain for an hour, but that’s not actually what it is.
It’s called functional threshold power because it’s based on your lactate threshold. LT is the intensity level where lactate begins to accumulate in your blood. Ride above LT and lactate will accumulate, forcing you to reduce power within a few minutes or less. Ride below LT and you should be able to sustain your effort for a significant amount of time.
The confusion lies in defining that “significant amount of time”. Here’s a dirty little secret: we can’t all ride at our FTP for 60 minutes. Some may only be able to hold it for 35 minutes, in fact! As this post from TrainerRoad explains, “The pain tolerance and mental fortitude experienced under extreme discomfort are both things that must be trained and well paced.” What I’ve seen is that, generally speaking, more-experienced cyclists can sustain their FTP power for longer durations than less-experienced riders.
So what’s a good definition for FTP? I like what Zwift says in their blog posts: “FTP is the wattage you can stay below and sustain for longer durations, while going above it causes fatigue to occur very quickly.”
Testing/Detecting FTP
It’s important to know your FTP if you plan on doing any Zwift racing or training with power (indoors or out). Zwift racing relies on FTP for categorization (although that is changing with recent category enforcement improvements), and training with power typically involves executing particular intervals at a percentage of your FTP (such as a VO2 interval where you might ride for 4 minutes at 110% of FTP).
So how do you determine your FTP? Well, that’s our second dirty little secret – there is no standard method for determining your FTP! Crazy, right? But it’s true. Even Dr. Andrew Coggan, who is sort of the father of FTP, says the historic “gold standard” of lab testing isn’t always the best approach, and outlines several FTP detection options in this post, “all of which provide very similar estimates of your FTP”.
Let’s take a look at various methods of testing or detecting your FTP.
Software/Data-Based eFTP
Software/apps such as intervals.icu, XERT, TrainingPeaks WK05, and Strava will estimate your FTP (often referred to as eFTP) based on the power data you feed into their systems. Each of these tools estimates FTP in its own way, so each will deliver a different number. I don’t use WK05, but here are my current eFTP values, all based on the identical data:
- intervals.icu: 310W
- XERT: 312W
- Strava: 300W
In general, I’ve found that intervals.icu and XERT track together quite nicely.
Zwift’s FTP Estimator
Zwift estimates your FTP in a very simple way: it looks at your best 20-minute power in every activity, and if 95% of that 20-minute power is greater than the FTP value currently set in your profile, you’ll get the “FTP Increase Detected” popup.
Example: if your current FTP is set to 200W, then you do a Zwift ride where you average 220W for 20 minutes, Zwift will notify you of an FTP increase to 209W (95% of 220W).
This detection method is straightforward, but has two key drawbacks:
- It doesn’t take into account activities outside of Zwift.
- It doesn’t follow the common 20-minute test procedure, which includes a 5-minute all-out effort prior to the 20-minute effort, to deplete anaerobic power. Therefore, this method can overstate your FTP because of an increased anaerobic contribution.
ZwiftPower Category Detection
ZwiftPower, the website which hosts final race results for most Zwift races, has its own FTP detection in order to enforce proper rider categorization. It looks at your 20-minute power in Zwift races for the past 90 days, then slots you into a race category based on the average of your top 3 power results.
For example, here are my top three results currently (you can find these numbers under your ZwiftPower profile by clicking the “info” button next to “Minimum Category”):
ZwiftPower’s detection of rider FTP is flawed in a few ways, but its existence is necessary in order to reduce sandbagging, since Zwift doesn’t restrict which categories riders sign up for. (Again, this is changing now thanks to increasing adoption of Zwift’s category enforcement tools.)
The problems with this method of detection include:
- It’s only based on Zwift activities (not a full picture of your data)
- It’s only based on your last 90 days of Zwift races (again, not a full picture of your data)
- It’s only based on your 20-minute critical power, which can overstate your FTP for the same reasons Zwift’s estimates can overstate it.
- Perhaps the biggest flaw, though, is that it can easily under-estimate your power. If you are a strong rider in your category and aren’t being pushed hard enough to turn in 20-minute maximal efforts in races, ZwiftPower will detect an artificially low FTP value which lets you stay in that low category. Sort of a self-fulfilling prophecy! (For more on this topic, read “Are Crit City Races Redefining ZwiftPower Rider Categories?“)
Zwift’s FTP Tests
Zwift’s built-in FTP tests are both very close to the standard Coggan 20-minute test format. Both their “FTP Test” and “FTP Test (shorter)” are solid FTP test formats and the most accurate way to test your FTP in Zwift, especially if you are an experienced rider who can pace themselves well for a 20-minute maximal effort.
Zwift’s Ramp Tests
Zwift’s Ramp Tests start you at low power, then increase the power incrementally every minute until you aren’t able to push further. Ramp Tests are an excellent first FTP test because they’ll give you a ballpark figure without the need to worry about perfect pacing or a 20-minute sufferfest.
The downside of Ramp Tests is reduced accuracy. To be precise, anaerobically strong riders will be able to push further into the red (and thus get an inflated result) while aerobically strong cyclists may actually see artificially low results.
The Ramp Test trades accuracy for convenience, and in some situations, that’s a smart trade. As Shayne Gaffney from GC Coaching says, “If you have absolutely no clue what your FTP is, or are returning from injury or a long hiatus off the bike, you can use the ramp test results to predict a sustainable pace for the 20 minute FTP test.”
Lab Testing
Riders can pay to visit a lab for a lactate threshold/FTP test in which blood samples are drawn at various effort levels in order to determine that line where your body isn’t able to clear lactate sufficiently, leading to increased accumulation in your bloodstream. Coggan says there are two drawbacks to this method:
- “Few people have access to such testing on a regular basis”
- “Power at LT as determined in this manner is often significantly below what athletes and coaches tend to think of as a ‘threshold’.”
Outdoor FTP Tests
Some riders find it hard to produce the same numbers indoors as they do outside (Hunter Allen talks about this more). These riders might prefer outdoor tests, especially if they’re wanting an accurate FTP number for outdoor riding.
So head outside for an FTP test! Just make sure you follow a valid protocol for your test and plan your route so traffic and road issues don’t negatively impact your results.
My preferred outdoor method is to follow Coggan & Allen’s protocol as outlined in Training and Racing with a Power Meter, and time it so I’m on a steady 20+ minute climb with no stoplights or bad traffic for the 20-minute test section.
Coggan/Allen FTP test protocol:
- 20 minutes endurance-paced warmup
- 3×1-minute on, 1-minute off fast pedaling (100+ rpm)
- 5 minutes easy
- 5 minutes maximal TT effort
- 10 minutes easy
- 20 minutes maximal TT effort
- Cooldown
Implications for Zwift Racing
With so many methods to test/detect FTP, your results can vary significantly depending on where you look. Take me, for example. I did a Zwift Ramp Test last Friday that pegged my FTP at 318W. Yet Strava says it’s 300W, ZwiftPower detects it at 295W, and XERT says 312W!
That’s a difference of almost 0.3 w/kg, which is a big deal if you’re on the cusp of a race category.
For your personal use, my advice is to find a test/detection method and stick with it. I’ve been happy with both intervals.icu and XERT because they automatically detect my eFTP with no need for actual testing. But you’ll need to find what works best for you.
When it comes to Zwift racing, my varying FTP numbers and the varying lengths of time people can ride at FTP clearly highlight a serious flaw in FTP-based categorization. That’s one big reason why I’m happy to see more and more races using Zwift’s new category enforcement tools – because it leads to fairer racing overall.
Yes, we all want results-based categorization – and that will come! But for now, it’s a big step forward to move beyond FTP-based categorization and into a new setup (category enforcement) that uses a uniform system for determining minimum race categories.
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