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Rider question

Why can’t I hold intervals just above my FTP?

Because FTP is an estimate of your threshold, not a measurement of it. It can land above your real limit, and a few percent past that limit, your body can no longer settle.

By Yarin Podoler · · 4 min read

The video version, 4:44, narrated by a synthetic voice. Watch on YouTube · Watch the Short

The session that comes apart

You test: 20 minutes as hard as you can hold, and your software takes 95% of it and hands you a new FTP. A few days later you do a session built on it, 3 × 12 minutes at 102%, barely above the number you just set. The first interval is hard but fine. Then you come apart in the second one, heart rate somewhere it has no business being, the target power sliding away no matter what you do with your legs. You almost certainly didn’t get much weaker in a few days. More likely, that number was never quite what you thought it was.

Sketch: 20 minutes as hard as you can, times 95%, becomes your new FTP. Then a session of 3 × 12 minutes at 102%, just above the FTP line: the first interval is hard but fine, and in the second, heart rate climbs toward its maximum and power slides away. “Much weaker in a few days?” is crossed out: the number was never what you thought.

What FTP actually is

Start with what FTP actually is. By its usual definition, it’s roughly the highest power you could hold for about an hour. Few riders ever test that directly, because an all-out hour is miserable, so we use stand-ins: 95% of a 20-minute effort, or a ramp test that takes three-quarters of your best minute. Each one turns an average across many riders into a rule, and your body may not match the average.

Sketch: FTP is roughly the highest power you could hold for about an hour, and few riders ever test it, because an all-out hour is miserable. So, stand-ins: 95% of a 20-minute effort, or a ramp test’s three-quarters of your best minute. Each is an average across many riders turned into a rule, and one rider sits off the line: your body may not match it.

Critical power: the line where things stop settling

Underneath, there’s a real boundary in your physiology, and one well-tested way to locate it is called critical power. Below it, oxygen use and blood lactate level off, even if they’re raised, and the chemistry inside the muscle finds a new balance. You’ll still tire eventually, but not because those numbers keep running away. Above it, none of them settle. Every minute takes you further from where you started, and the only open question is how long you last.

Sketch: critical power as a line. Below it, oxygen use, blood lactate and muscle chemistry each level off; you still tire, but these hold. Above it, none of them settle: oxygen use climbs to its maximum, lactate keeps rising and muscle chemistry keeps falling, until you stop. The only question is how long you last.

W′, the reserve above the line

Above critical power, the standard model gives you a limited amount of extra work to spend, called W prime. It’s measured in kilojoules, and its size differs from rider to rider. How long you last above the line is that amount divided by how far over you are, and it only refills once you drop back below, gradually.

Sketch: W′, a limited amount of extra work above critical power, drawn as a battery. It’s measured in kilojoules, and its size differs from rider to rider. Time above the line = W′ ÷ how far over: riding above critical power spends it, and dropping below refills it, gradually.

Why it’s the second interval

FTP’s stand-ins don’t measure that line directly, and studies comparing FTP with lab measurements find it isn’t reliably on it: the miss differs from rider to rider, in size and in direction. So take a rider with a 20 kJ reserve, a plausible size for someone trained, and say 102% put them 20 W over their line. The first interval spends about 14 kJ. Three easy minutes of recovery give back about 5, by the model’s numbers. So the second interval starts with about half the reserve, and runs out a little over 8 minutes in, which is what falling apart in the second interval feels like.

Sketch: a rider with a 20 kJ reserve, for whom 102% of FTP is 20 W over their line. The first interval spends 14 kJ, the recovery gives back about 5, and the second interval starts with about half the reserve. It empties a little over 8 minutes in, and the interval comes apart.

The experiment

So how do we know the line is real, and not just a number fitted to a curve? In the late 1980s, David Poole and colleagues found critical power for a group of volunteers, then had them ride right at it, aiming for 24 minutes. Their oxygen use, their breathing and their blood lactate all levelled off, and every one of them finished the 24 minutes without difficulty. Then they rode a small step above it, about 5% of the peak power they’d reached in a ramp test. This time none of it levelled off: oxygen use climbed to its maximum, lactate kept rising, and they stopped before the 24 minutes were up.

Sketch of the experiment by David Poole and colleagues in the late 1980s: a group of volunteers rode for 24 minutes at critical power, and a small step above it (5% of their ramp-test peak power). At critical power, oxygen use, breathing and blood lactate all levelled off, and everyone finished without difficulty. A small step above, oxygen use climbed to its maximum, breathing and lactate kept rising, and they stopped before 24 minutes.

What it means for you

So what does this mean for you? First, treat FTP as an estimate, not as your threshold. As a repeatable number to track over time, it’s useful. For intervals pitched just above it, the line that matters is one you’ve probably never measured, and a few percent can move you to the wrong side of it.

Sketch: first, treat FTP as an estimate, not your threshold. A rider’s FTP tests over a season: useful to track. But the line that matters, probably never measured, can sit anywhere in a band around FTP, and here a few percent puts an interval at 102% on the wrong side of it.

When an interval collapses

Second, when the second interval collapses, read it as information. The target may sit above your line, the recoveries may be too short or not easy enough, or you may have brought fatigue into the session. Try the target a few percent lower, and make the recoveries genuinely easy, because the reserve refills faster the further below the line you drop.

Sketch: second, a collapse is information. The target may sit above your line, the recoveries may be too short or not easy enough, or you brought fatigue into the session. The fix: three intervals a few percent lower, below FTP, with genuinely easy recoveries, because the reserve refills faster the further below the line you drop.

Watch what settles

And third, watch what settles. On a cool day, on fresh legs, if your heart rate and breathing are heading for your maximum by the end of an interval, instead of levelling off below it, you’re probably above your line, whatever the percentage says. I’ve had powers my chart called threshold feel almost easy, and powers it called endurance feel like work. The chart is only a starting guess.

Sketch: third, watch what settles. On a cool day, on fresh legs, heart rate and breathing through one interval either level off below your maximum or head for it; heading for max means you’re probably above your line, whatever the percentage says. The author’s chart called one power threshold, and it felt almost easy; another endurance, and it felt like work. The chart is only a starting guess.

That’s what’s underneath the watts.

Sources

The study

  1. Poole DC, Ward SA, Gardner GW, Whipp BJ. Metabolic and respiratory profile of the upper limit for prolonged exercise in man. Ergonomics. 1988;31(9):1265-1279. doi.org/10.1080/00140138808966766
  2. As described in: Poole DC, Burnley M, Vanhatalo A, Rossiter HB, Jones AM. Critical power: an important fatigue threshold in exercise physiology. Med Sci Sports Exerc. 2016;48(11):2320-2334. doi.org/10.1249/MSS.0000000000000939

Also in this article

  1. Borszcz FK, Tramontin AF, Costa VP. Is the functional threshold power interchangeable with the maximal lactate steady state in trained cyclists? Int J Sports Physiol Perform. 2019;14(8):1029-1035. doi.org/10.1123/ijspp.2018-0572
  2. Karsten B, Petrigna L, Klose A, Bianco A, Townsend N, Triska C. Relationship between the critical power test and a 20-min functional threshold power test in cycling. Front Physiol. 2021;11:613151. doi.org/10.3389/fphys.2020.613151
  3. Skiba PF, Chidnok W, Vanhatalo A, Jones AM. Modeling the expenditure and reconstitution of work capacity above critical power. Med Sci Sports Exerc. 2012;44(8):1526-1532. doi.org/10.1249/MSS.0b013e3182517a80

Written and researched by Yarin Podoler, a cyclist who wanted to know what’s underneath the watts.