Train by your lactate thresholds: CAROL Bike now finds LT1 and LT2 from your heart rate variability

First things first: REHIT stays your number one

Nothing in this release changes our core recommendation. Reduced Exertion High-Intensity Interval Training (REHIT), two 20-second all-out sprints two or three times a week, remains the most time-efficient way we know to raise cardiorespiratory fitness and VO₂max. That is what the REHIT research by Metcalfe, Vollaard and colleagues showed (Metcalfe et al., 2012), and it is what thousands of CAROL riders see in their Fitness Score. If you only have ten minutes, spend them on REHIT.

Many of you told us you have more time than that, and that you want to add longer, easier endurance rides, the kind usually called Zone 2 training. We think that is a good idea, with one condition: it has to be done at the right intensity. Ride too hard and you collect fatigue without the adaptations you were after, the "junk miles" endurance coaches warn about. Ride too easy and not much happens either. The right intensity sits just below a physiological landmark called the first lactate threshold, and until now you needed a laboratory to find it. That is the problem this release solves.

Two thresholds, three intensity domains

Exercise physiologists divide effort into three domains, separated by two thresholds.

Below the first lactate threshold (LT1) is the moderate domain. Your muscles get most of their energy from fat and from carbohydrate burned with plenty of oxygen; lactate, the by-product of fast carbohydrate breakdown, is produced and cleared at the same rate, so it stays at resting levels. Breathing is comfortable, you can talk in full sentences, and you could keep going for hours. This is Zone 2.

Between LT1 and the second lactate threshold (LT2) is the heavy domain. Carbohydrate takes over as the main fuel, lactate rises but settles at a new, elevated level, and the effort is sustainable for a long time but no longer easy. Functional Threshold Power (FTP) sits near the top of this domain.

Above LT2 is the severe domain. Lactate and breathing climb without settling, and the clock is running: minutes, not hours. REHIT sprints live at the far end of this domain, and that is exactly why they work so fast.

Three intensity domains

Two thresholds divide every ride

Below LT1Moderate domain. Lactate stays at resting levels and you can talk in full sentences. This is Zone 2.

LT1 to LT2Heavy domain. Lactate rises, then settles at a higher level. FTP sits near the top.

Above LT2Severe domain. Lactate and breathing climb without settling. REHIT sprints live here.

LT1 is the first lactate threshold, LT2 the second. Both move with training, and both differ from rider to rider.

Training below LT1 is valuable because it is the intensity your body can repeat day after day while it builds the machinery of endurance: more and larger mitochondria in the muscle fibers, more capillaries to deliver oxygen, a greater capacity to burn fat and to clear lactate. Researchers such as Iñigo San-Millán and George Brooks have linked this capacity, often called metabolic flexibility, to better metabolic health, not only to better endurance (San-Millán and Brooks, 2018). Over months of training, LT1 itself moves up: you produce more power before lactate starts to rise.

For riders who combine REHIT with endurance work, the picture that emerges from the training literature is simple. Stephen Seiler's work on polarized training is the best-known example (Seiler, 2010): most of the time well below LT1, a little time well above LT2, and not much in between.

Why thresholds needed a laboratory, and why they no longer do

The reference methods for LT1 and LT2 are a lactate profile, which needs a series of finger-prick blood samples during a graded test, or gas exchange, which needs a face mask and a metabolic analyzer to find the equivalent ventilatory thresholds. Both are accurate, both are expensive, and neither is something you do at home on a Tuesday evening. Formulas based on a percentage of max heart rate or FTP are cheap but miss the individual by a wide margin: two riders with the same max heart rate can have LT1 thirty beats apart.

Over the last few years a third route has emerged, and it only needs an electrocardiogram-grade heart rate signal. Your heart does not beat like a metronome. The interval from one beat to the next varies by tens of milliseconds, and the pattern of that variation, your heart rate variability, changes with exercise intensity in a very specific way. At rest and in easy exercise the beat-to-beat intervals are strongly correlated with each other: a long interval tends to be followed by another long one, in a fractal, self-similar pattern. As intensity rises and the autonomic nervous system shifts from parasympathetic to sympathetic control, that correlation breaks down and the intervals become more random.

A measure called the short-term scaling exponent of detrended fluctuation analysis, DFA alpha1, captures this. Values near 1.0 or above mean strong correlation; 0.5 means uncorrelated, white noise. The research group around Thomas Gronwald, Bruce Rogers and Olaf Hoos showed in a series of studies from 2020 onwards that alpha1 falls smoothly during a ramp test (Gronwald et al., 2020), that the crossing of 0.75 agrees closely with the first ventilatory and lactate threshold (Rogers et al., 2021), and that the crossing of 0.5 agrees with the second (Rogers et al., 2021). In short, your heart rhythm announces both thresholds, and all it takes to hear it is a sensor that reports every beat to the millisecond.

DFA alpha1

What your heart rhythm says about intensity

~1.0Strongly correlated beat-to-beat intervals: rest and easy riding

0.75The crossing that agrees with the first threshold, LT1

0.5Uncorrelated intervals: the crossing that agrees with the second threshold, LT2

Thresholds as validated against laboratory gas exchange and lactate measurements (Rogers et al., 2021).

What the sensor has to deliver

That last point matters. The method lives on the beat-to-beat intervals, called RR intervals, not on the heart rate number. Optical sensors in watches and arm bands estimate heart rate from blood flow and smooth it heavily; they do not see individual beats well enough. A chest belt with electrodes reads the electrical signal of the heart directly and reports each interval with a resolution of about a millisecond. The CAROL H1 and belts such as the Polar H10 do this; the CAROL H3 arm band does not, and the app will tell you so.

Your CAROL Bike now checks the quality of the belt signal in the first minutes of every ride that uses it: the resolution of the intervals, whether every beat was received, and whether the intervals are consistent with the heart rate. Only a signal that passes is used for threshold analysis. If it does not pass, the ride continues on your FTP values and tells you why, so a bad reading is never silently turned into a wrong threshold.

Two practical tips from our own testing. Wear the belt snug, not tight: a belt pulled very tight across the chest can drop single beats in the 130 to 150 beats per minute range, exactly where LT1 tends to live. And wet the electrodes before you start; a dry belt gives a noisy signal for the first minutes, which is when the quality check runs.

What is new in app 3.9

CAROL Ramp Test

The Ramp Test has always measured your Maximum Aerobic Power (MAP) and estimated your FTP. With a chest belt suitable for HRV analysis it now also tracks alpha1 in real time, tells you when you pass LT1 and when you pass LT2, and saves both thresholds, in watts and in beats per minute, together with your max heart rate. One thing to know: LT2 arrives before the end. When you see the LT2 message, keep going to your limit, because MAP, FTP and max heart rate are measured from your final minute, and stopping at LT2 underestimates all three.

Metabolic Zone-Based Free Ride

Once a Ramp Test has given you LT1 and LT2, this ride builds six training zones from them, in power and in heart rate: recovery below 70% of LT1, Zone 2 from there up to LT1, tempo and threshold between LT1 and LT2, and two zones above LT2 for VO₂max and anaerobic work. Pick a zone and the bike adjusts the resistance to keep you in it, whatever your cadence. Your Classic Zone-Based ride, built on FTP percentages, is still there if you prefer it.

Metabolic Zone 2 Free Ride

This is the ride we are most excited about, because it addresses something a one-off test cannot. Your thresholds are not fixed numbers. Poor sleep, residual fatigue from yesterday's session, heat, caffeine or a cold all shift LT1 by a few percent, sometimes more, and a Zone 2 ride that ignores this is either too hard or too easy on the day.

So the ride starts with a mini ramp: three-minute steps from half of your usual LT1 power upwards, while the bike tracks alpha1 at each step. As soon as a step shows that you have crossed your threshold, the ride knows your LT1 of the day, eases off for two minutes to let heart rate and HRV settle, and then holds you at 90% of today's LT1. During the hold it keeps watching: if alpha1 drifts down, meaning you are closer to LT1 than planned, the power comes down a little; if it recovers, the power is restored. A heart rate ceiling just above your LT1 heart rate is there as a safety net. The whole identification takes about twelve minutes, and the ride then runs for as long as you like.

If you ride without a suitable belt, the Metabolic Zone 2 Free Ride still works: it uses your FTP to estimate Zone 2 and holds that power with a heart rate limit. It will tell you that it is doing so.

How confident should you be in the numbers?

We want to be precise about this. HRV-based thresholds are estimates, validated against laboratory methods in groups of athletes and recreational riders, with the kind of agreement you would expect from a good field method: close on average, with individual differences of a few percent. They are far better than a fixed percentage of max heart rate, and they are the only method that can be repeated at the start of every ride at no cost. They are not a laboratory test, and a laboratory test is not a daily measurement. Use them for what they are good at: setting your endurance intensity for the ride in front of you, and watching the trend over weeks.

A few things improve the reading. Do your Ramp Test rested, in the same conditions you usually train in. Pedal smoothly during the mini ramp; a pause or a sudden sprint resets the step. And repeat the Ramp Test every couple of months, or whenever the app suggests one, because the ride compares your LT1 of the day with your last Ramp Test and tells you when the two have drifted apart.

Also in this release

The bike app is now available in Spanish; you can choose the language on the registration screen. Under the hood, the Ramp Test, the ride upload and the heart rate sensor handling have been reworked, and the DFA calculation on the bike and on our servers now uses exactly the same method, so the thresholds you see during a ride are the ones that end up in your profile.

As always, tell us what you find. The thresholds feature has been through hundreds of test rides, including a good number on our own legs, and your rides will teach us more than ours did.

Team CAROL

Further reading

• Gronwald T, Rogers B, Hoos O. Fractal correlation properties of heart rate variability: a new biomarker for intensity distribution in endurance exercise and training prescription? Frontiers in Physiology, 2020.
• Rogers B, Giles D, Draper N, Hoos O, Gronwald T. A new detection method defining the aerobic threshold for endurance exercise and training prescription based on fractal correlation properties of heart rate variability. Frontiers in Physiology, 2021.
• Rogers B, Giles D, Draper N, Mourot L, Gronwald T. Detection of the anaerobic threshold in endurance sports: validation of a new method using correlation properties of heart rate variability. Journal of Functional Morphology and Kinesiology, 2021.
• San-Millán I, Brooks GA. Assessment of metabolic flexibility by means of measuring blood lactate, fat, and carbohydrate oxidation responses to exercise in professional endurance athletes and less-fit individuals. Sports Medicine, 2018.
• Seiler S. What is best practice for training intensity and duration distribution in endurance athletes? International Journal of Sports Physiology and Performance, 2010.
• Metcalfe RS, Babraj JA, Fawkner SG, Vollaard NBJ. Towards the minimal amount of exercise for improving metabolic health: beneficial effects of reduced-exertion high-intensity interval training. European Journal of Applied Physiology, 2012.