Your FTP Is an Estimate. Your Heart Rate Zones Are an Estimate of an Estimate.

Most cyclists set their entire training around FTP and FTP is a guess. Not a useless guess but derived from a short test and scaled by a fixed percentage. Then some cyclists derive their heart rate zones from that guess, which makes their zones a guess built on a guess. If you are training off FTP derived heart rate zones, you are two layers of estimation away from your actual physiology.

Our lactate testing removes both layers by measuring the thing itself. Here is how the estimation compounds, what the research actually says about how far off FTP can be and why measuring your metabolism directly fixes it.

FTP is a guess in the first place

FTP is meant to represent the power you can hold for one hour. Almost nobody tests it that way, because riding all out for an hour is brutal and hard to pace. So FTP is estimated from a shorter effort: a 20 minute test scaled to 95%, an 8 minute test or a ramp test with its own formula.

Straight away that is an approximation. You are not measuring your one hour power, you are running a shorter test and applying a percentage that assumes your physiology matches the average the percentage was built on.

The research is clear that FTP and your true lactate threshold are not the same thing. One study of trained cyclists did find a correlation between FTP and lactate threshold but the limits of agreement between them ran from roughly 45 watts below to 50 watts above the lactate value. That is a spread of almost 100 watts hiding behind a decent looking average. For an individual rider, FTP could be sitting well above or well below their real threshold.

It gets more specific. The size and direction of the error depends on your fitness. Research shows FTP gives a better estimate of lactate threshold in well trained cyclists but underestimates it in recreational cyclists. So the error is not even consistent and points in different ways for different riders and you cannot tell from the number itself which kind of rider you are.

Then heart rate zones are guessed from the guess

Having estimated FTP from a short test, many riders then set their heart rate zones as percentages of the heart rate recorded during that test. Garmin, for example, will calculate a zone as a percentage band of the average heart rate from your FTP effort.

Think about what that chain actually is. A short power test, scaled by a percentage, gives you estimated FTP. The heart rate from that same imperfect test, scaled by another percentage, gives you your heart rate zones. Your zones are now an estimate derived from an estimate and every bit of error in the first layer flows straight into the second!

And heart rate is the noisiest input you could build on. Your heart rate at a given power changes with sleep, hydration, caffeine, heat, altitude, illness, stress and for many athletes, where they are in their hormonal cycle. The heart rate you recorded on FTP test day was a snapshot taken under one set of conditions.

This is the same problem we wrote about for runners in Your Zone 2 Heart Rate Is Probably Wrong. The sport changes and the units change from pace to power, but the mistake is identical: zones set from a formula and a snapshot rather than from your physiology.

What our tests measure instead

A lactate test does not estimate your threshold from a surrogate. It measures the metabolic event directly.

During a graded ramp we take a small blood sample at each stage and measure blood lactate against power and heart rate at the same time. That lets us see the actual points where your metabolism changes: where lactate first rises above baseline (your aerobic threshold) and where it starts accumulating faster than your body can clear it (your anaerobic threshold). These are real physiological transitions, not a percentage of a 20 minute effort.

Because we are measuring the metabolism and reading your heart rate and power alongside it, we can tell you the power to hold and the heart rate to expect at each threshold, anchored to what your body is actually doing rather than to a fatigue dependent test result.

The heart rate variability problem gets handled too. Since your zones are anchored to measured metabolic thresholds rather than to a single test day heart rate, we give you power targets to train by, which do not drift with your sleep and hydration, and heart rate ranges to watch as a cross check. When your heart rate is running high for the power one day, you can see it, because the power target is fixed to your physiology and the heart rate is the variable, not the anchor. FTP derived zones have it backwards, building the whole system on the noisiest measurement.

Why this matters for your training

The practical cost of estimate on estimate zones is that you train in the wrong place without knowing it. If your FTP is overestimated, your threshold sessions are too hard and your easy rides creep into the grey zone, so you accumulate fatigue and never build the aerobic base. If it is underestimated, you leave adaptation on the table. Either way, high volume training built on wrong zones risks overreaching because the zones that are meant to protect your easy days are not actually easy.

Measuring your thresholds directly means your hard days are genuinely at threshold and your easy days are genuinely below it. For anyone doing serious volume, that difference is the difference between training that compounds and training that quietly digs a hole.

Testing for cyclists in South Wales

We now test cyclists on the bike as well as runners on the treadmill. We test at Heathwood Triathlon club in Cardiff, Team Rees in Caerphilly, Kulture in Bristol, with Swansea coming soon.

If you have been training off FTP and your zones have never quite felt right, this is the test that tells you whether it was you or the estimate. For how the same problem shows up differently for female cyclists, read FTP and the Female Cyclist. If you are deciding which test to book, read Lactate, VO2 Max, or Combined: Which Test Should You Book?.

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