Aerobic threshold: the test you can do on any trainer

Most riders know their FTP to the watt and have no idea where their aerobic threshold is. That is backwards, because the aerobic threshold, the point where your body stops being able to keep lactate at resting levels and starts working harder to cope, is the ceiling for the riding you do most of. Sit below it and you can go for hours. Drift above it and the easy ride quietly stops being easy. The good news is that you can find it without a lab, without a power meter, and on any trainer at all. This post gives you three methods, what each is validated against, and how much to trust the result.

What you are looking for

Physiologists call it several things: the first lactate threshold (LT1), the first ventilatory threshold (VT1), the aerobic threshold, the top of the moderate domain. Jamnick and colleagues' critique of intensity methods lists more than a dozen ways of defining it from lactate curves alone, which is why two labs can give you two answers. The practical definition is the intensity above which blood lactate first rises clearly above its resting level and breathing starts to deepen. Pallarés's group showed that in well-trained cyclists the ventilatory and lactate versions of this point sit close together and are reliable when re-tested, so the underlying thing is real even if its name is not settled. We cover the physiology in lactate threshold: LT1 and LT2 explained.

Two things about it are worth knowing before you test. It is lower than most people expect, often well under 70% of FTP in less-trained riders and closer to 75–80% in well-trained ones, and the ratio varies a lot between individuals, so a percentage of FTP is a poor substitute for a measurement. And it sits close to the intensity where fat oxidation peaks: Achten and Jeukendrup found that maximal fat oxidation occurred at an intensity just below the point where lactate began to rise. Finding one gets you most of the way to the other.

Method 1: the talk test

The oldest method and, surprisingly, one of the best validated. You ride at a steady intensity and read a passage of text aloud, thirty seconds or so, then answer one question: can you speak comfortably? Foster's group set it out formally: at each stage of a graded test the rider recites something and reports whether speech is comfortable, doubtful, or not. The last stage where speech is comfortable tracks the first ventilatory threshold; the stage where it becomes doubtful sits near the second. Rodríguez-Marroyo's group repeated this specifically in well-trained cyclists and found the same relationship, and Bok's 2022 review concluded the talk test is a valid and practical marker of the ventilatory thresholds across a range of populations.

How to do it on any trainer:

1. Warm up for ten minutes at an easy pace.
2. Raise the intensity in small steps every three minutes: a gear, a resistance notch, or 15–20 W if you have power. Cadence stays constant.
3. At the end of each step, read a paragraph aloud. Something familiar you can say without thinking. Thirty seconds of speech, not a sentence.
4. Note the last step where you could speak comfortably, in full sentences, without needing to snatch a breath mid-sentence. That step is your aerobic threshold, give or take one step.
5. Keep going a few more steps if you want the second threshold: it is where speech becomes broken, a few words at a time.

Weaknesses: it is subjective, it resolves only to the size of your steps, and people who are used to suffering tend to call "comfortable" later than they should. Do it on a day you are rested, and repeat it a week later. If both attempts land within a step of each other you have a usable number.

Method 2: heart-rate drift over a long steady effort

Below the aerobic threshold, heart rate at a fixed power settles and stays roughly settled. Above it, heart rate creeps upward through the effort even though the power does not change. Coyle and González-Alonso's review of cardiovascular drift describes the mechanisms: over a long effort stroke volume falls, partly from rising body temperature and fluid loss, and heart rate rises to compensate. The rate of that drift is much larger when the effort is above the threshold than below it, which makes drift a usable field marker.

The protocol needs a power meter or a trainer that reports power, and heart rate:

1. Pick a power you think is just below your aerobic threshold. If you have no idea, 65% of FTP is a reasonable first guess for a moderately trained rider.
2. Ride it for 60 minutes at constant power after a warm-up. ERG mode makes this easy; on a dumb trainer, hold the gear and cadence.
3. Compare average heart rate in the second half with the first half. A drift of under about 5% suggests the power was below threshold. A drift well above that suggests it was over. This is the same calculation as aerobic decoupling, which we cover in aerobic decoupling explained.
4. Repeat on another day, 10–15 W higher or lower, until you find the power where drift starts to grow. That is the threshold.

Weaknesses: heat, dehydration, caffeine, poor sleep and a fan that is too weak all drive heart rate up independently of intensity, and the 5% figure is a convention from coaching practice rather than a number from a trial. Indoors you must control the fan and the fluid, or every test will say "over threshold". It also takes several sessions to converge. On the other hand, each of those sessions is a perfectly good hour of endurance riding, so nothing is wasted.

Method 3: heart-rate variability during exercise

The newest method and the one with the most interesting science behind it. Heart-rate variability, the beat-to-beat irregularity in the heartbeat, does not just get smaller as you ride harder; its pattern changes. Rogers, Gronwald and colleagues work with a measure called DFA alpha 1, which describes how correlated successive beat intervals are and falls from around 1.0 at rest through 0.75 at the aerobic threshold and towards 0.5 at the second threshold. They tested the 0.75 crossing against gas-exchange thresholds in a lab and found it agreed with the first ventilatory threshold closely enough to be useful for training prescription.

What it needs: a chest strap that records beat-to-beat intervals accurately (optical wrist sensors are not good enough for this), and software that computes alpha 1 on a rolling window. Several phone apps and analysis tools do it. The test itself is a ramp: ride easy, increase intensity slowly, and note the heart rate and power at which alpha 1 crosses 0.75.

Weaknesses: the method is young, the agreement with lab thresholds is good on average and looser for individuals, and artefacts from a poorly fitted strap will wreck it. Some riders' alpha 1 never behaves as the model predicts. Treat it as a third opinion rather than the answer. We go into the wider topic in HRV for cyclists.

Which one to use

If they disagree, believe the talk test and the drift test over alpha 1, and believe the drift test on a cool day with a big fan over one on a warm day. If all three agree, you have a better number than most riders with a lab test, because you measured it on your own trainer in your own conditions and can repeat it whenever you like.

What to do with the number

Set the top of your easy zone a little below it, not at it, because the whole point of easy riding is to stay under the threshold with a margin and drift is always upward. Re-test every couple of months: the aerobic threshold moves with training more than FTP does, and it is usually the first thing to improve when a rider adds volume and the first thing to fall when they stop. If you have had a lab test, compare: a field number 10% off the lab's is normal, given how many definitions of the threshold exist.

In Moveee, the Power Profile estimates FatMax from your ride data, which will usually sit near the aerobic threshold you find here, and you can upload a lab test PDF to have LT1, LT2 and the zones read from it directly. Both of the sensor-based tests above run comfortably in the Indoor app: ERG mode holds the power for the drift test, and a paired heart-rate strap records alongside it. For the talk test you need only a trainer and something to read.

The short version

Your aerobic threshold is the most useful number you do not know. Read a paragraph aloud at rising intensities and note where it stops being comfortable; hold a steady hour and watch whether heart rate creeps; if you have a good strap, check where alpha 1 crosses 0.75. Agreement between two of the three is a number worth training by.