Second guessing the lab results: was my FTP test accurate?

A couple of weeks into training with those Cyclinglab results, something's been bugging me. The 230W anaerobic threshold from the lab test felt accurate, but during training it feels different i.e. the training with associated power zones and corresponding heart rates don't really match us. I figured the numbers don't lie, but maybe don't tell the whole story.

When the training doesn't match the numbers

I've been following JOIN's training plan religiously, mostly Zone 2 work with some harder efforts mixed in. But something felt off during a recent workout. JOIN prescribed a 5-minute VO2 Max interval, between 265 and 290W, that's based on the 230W threshold from the lab. The associated heart rate zones we between 163 (threshold) and 192 (the max from the lab test) - I hit the power target in the sweetspot (275W avg), but never got close to the expected heart rate zones. In fact, the max value was just before the end 163 and averaged 158, well below my threshold. As such, it felt too easy for what was supposed to be an RPE 10 effort, it was more an 8.

That got me thinking. Was the lab test accurate, or did something affect my performance that day?

The context I forgot to mention

Here's what I didn't think much about at the time: two days before the Cyclinglab test, I had a pretty bad bout of diarrhea. Nothing dramatic, but enough to leave me feeling a bit off. Diarrhea obviously impacts your hydration levels and when you're dehydrated and your system is recovering, it probably affects impact performance, in some shape or form.

I mentioned it briefly to the trainer, but not really in a way that I felt it was a concern, and as such I don't think he felt it was a concern either. In hindsight, maybe that wasn't the best call. If I was still dehydrated, it affects a lot of things, like heart rate response and power output. Even if you feel okay, your body might not be at its full potential.

Testing the theory

After that workout where 275W felt ok, but not RPE10, I decided to experiment. I bumped my FTP setting in JOIN from 230W to 250W and went out for my next ride. Now everything felt much more aligned. The heart rate zones matched what the app expected, the efforts felt appropriately challenging, and the prescribed intensities made sense.

250W felt right in a way that 230W did not yet. Not too easy, not impossible - just the sustainable effort, where expected power zones and heart rates perfectly align in JOIN.

The plan moving forward

I'm not throwing out the lab results. They provided valuable baseline data and confirmed what I suspected about needing more aerobic base work.

For now, I've using 250W as my training FTP, but I know this needs proper verification,

The next step is setting up my Wahoo Kickr and doing a controlled FTP test, likely the same ramp test we did at Cyclinglab, so the results are more or less comparable (read on...)

Why this matters for Cape Epic

A 20-watt difference might not sound huge, but it changes the entire training approach. At 230W, you're in one category of capability. At 250W, you're in another. That affects everything from pacing strategy to realistic time goals for the race.

If 250W is more accurate, it means I'm not quite as far behind Jeroen as the lab results suggested. Still a gap to close, but maybe not as daunting. It also means my improvement targets for March might be more achievable than I thought.

Drum rolls - the results are in....

So, this morning I used my Wahoo Kickr and Zwift for the first time. I'd never used Zwift (or any other virtual training app) and I must say, I thought it was pretty fun. Driving around in a game-like looking world is probably something I'll do much more in the winter and I like it!

For the test, I selected a 20-min FTP test in JOIN, which with proper warming up and cooling down, takes almost an hour, and exported it to Zwift.
Long story short, my FTP was determined at 257W! Quite a big difference with the lab test we did on the 21st of August. Most likely that result was a bit under my level at that time, as gaining 27w FTP in only 15 days is quite out of the ordinary (if you read online about it). I really went all out on this test, my maximum heart rate hit an astonishing 196BPM at the end of the test, so then you know you're really giving it everything.

With the sync between Zwift and JOIN, my athletic profile in JOIN has been updated automatically, and the training program has been adjusted accordingly.

Now the question of course arises, how much can you improve how quickly? Well, JOIN has scheduled another performance test in 13 weeks, so in about 3 months, I can tell you more!

Until then, keep chasing!