The True Definition of Threshold

Your lactate threshold may not be what you think it is. Coach Trevor Connor explores the true definition of this physiological turn point.

A cyclist dressed in black rides out of the saddle against a black background

Lactate threshold: It’s a concept you probably think you’ve understood since your high school coach first yelled at you to “feel the burn.”

But it may not be what you think. Most see it as a switch, the point at which we transform from being oxygen-loving aerobic animals to lactic-acid-pumping anaerobic machines. Break this critical barrier and you have precious seconds before you’re lying on the ground gasping for air.

But it’s not that simple, says Dr. Iñigo San Millán, an exercise physiology researcher and the director of coaching at the UAE-Team Emirates WorldTour team. He is one of several physiologists redefining the concept of a “lactate threshold” and what is meant by threshold training.

As the personal physiologist of 2020 Tour de France winner Tadej Pogačar and previous podium finisher Joseba Beloki, San Millán knows that a turn point is one thing, but how long you can sustain such a level is an entirely different—and perhaps more important—indicator of ability.

When San Millán worked with ONCE team leader Joseba Beloki in the 1990s, the Spaniard barely registered above a Cat. 1 level in standard threshold tests. His threshold power was surprisingly low. It wasn’t until San Millán tested his ability to sustain relatively high power for a long time that he saw a Tour contender. It was a wattage that would crack a Cat. 1 within 30 minutes, but one that Beloki could sustain for a four-hour training ride.

A plethora of definitions

“Threshold” itself may be a misnomer, claim the authors in a 2009 review of the lactate threshold concept. Faude, Kindermann, and Meyer identified 25 different definitions in the literature, including anaerobic threshold, maximal lactate steady state (MLSS), respiratory compensation point, critical power, and functional power—to name a few. [1,2]

It’s so difficult to define “threshold” because, unlike what your high school coach would have you believe, it is not simply a point where we switch from aerobic metabolism to anaerobic. It’s a gradual transition. We’re almost always using both energy systems—we never stop consuming oxygen, and we produce lactate even when sitting on a couch. It’s just that the ratios change.

Therefore, San Millán believes the threshold concept is most appropriately used to identify what is sustainable for any given athlete. And what is sustainable, according to San Millán, depends on the length of the effort.

Former WorldTour pro Svein Tuft, who podiumed at the world time trial championships, was no stranger to riding at threshold. He echoes San Millán’s sentiment, believing that too many young riders seek a magic number.

“I’m going by a feeling that I know I can sustain,” Tuft says of his race efforts. “I might have to chase for 20 kilometers, but if I have to ride for 50 kilometers it’s a totally different feeling.”

Runners understand this better than anyone, which is why they talk and train in terms of their 5K, 10K, or marathon pace. This is because they may be able to sustain a blood lactate concentration of six to eight millimoles over five kilometers, but if they run a marathon at anything over two millimoles, they won’t finish.

Each pace is the athlete’s threshold for that distance, says San Millán.

As a result, many researchers are representing threshold not as a point but a range on the lactate curve (see Figure 1). This threshold range is also called the aerobic-anaerobic transition. The ends of this range are defined by two key metabolic events.

The lower metabolic event is often referred to as our aerobic threshold. It is the intensity where blood lactate levels start rising above resting levels. It is also the point where we use fat as fuel most effectively, according to San Millán. Riding below it, we rely almost entirely on slow-twitch muscle fibers and aerobic metabolism. Riding at “aerobic threshold” is what we often call zone 2 riding (in a five-zone model); it represents an intensity we can sustain for hours.

The upper metabolic event is what we traditionally think of as the lactate threshold. San Millán feels the term maximal lactate steady state (MLSS), commonly used in the literature, is more accurate because it is the highest level at which we can sustain blood lactate levels—though only for 15 to 30 minutes depending on our ability to clear lactate, according to San Millán. [1–3]

It is not a transition from aerobic to anaerobic energy—we’re still working aerobically. But it does mark key metabolic changes. According to San Millán, at this point we start relying on glucose for fuel, and fat utilization virtually stops.

So, why would an athlete need to know this?

According to the 2009 review, our threshold range, particularly MLSS, predicts our performance better than almost any other metric. [2] More importantly, our thresholds can be translated into heart rate and power zones that we can use to train by.

Graph showing lactate threshold as the sweet spot between low-intensity aerobic threshold and high-intensity anaerobic threshold
Figure 1. A typical lactate-workload plot including the aerobic-anaerobic transition as a framework to derive endurance training intensities for different intensity zones. MLSS = maximal lactate steady state.

Of course, there are several zone models. Among coaches and in training tools such as TrainingPeaks, a five-zone system is popular. Physiologist Dr. Stephen Seiler developed a three-zone model—the polarized model—that is very popular among scientists. Zone 1 exists below aerobic threshold. Zone 2 comprises the range between the aerobic threshold and MLSS, or anaerobic threshold. Zone 3 consists of everything above MLSS.

As shown above, the five-zone model is very similar to Seiler’s three zones, it just further refines Seiler’s zone 1 and zone 3. In the five-zone model, those long, slow rides at intensities below your initial metabolic event fall into zones 1 and 2. Zone 2 is a small range just below your aerobic threshold up to your aerobic threshold. Zone 3 is the lactate threshold range between the two metabolic events (often called “sweet spot” or “no man’s land”), while zone 4 should roughly match up with your MLSS. Finally, zone 5 is that unsustainable effort that will have you gasping for air in no time.

RELATED: The Endurance Athlete’s Guide to VO2max and Lactate Tests

How to find your sustainable level

The quickest and most effective way to determine your thresholds is by visiting a physiology lab like San Millán’s. But if you don’t have the money or access, here are some guidelines to figure out your sustainable levels.

Be careful with power

The concept of functional threshold power (FTP) has become very popular in cycling. It’s simply the highest average wattage you can sustain for an hour. “I don’t think it’s very scientific, that term,” San Millán says. His concern is that it doesn’t show what’s happening in the body—the true metabolic cost. You may be able to suffer through 325 watts for one hour, but if your lactate levels rise through that effort, it’s not your true sustainable power. Using that number will lead to overtraining.

Use heart rate to assess your true sustainability

San Millán has thousands of data points from his lab to show that heart rate directly correlates with the lactate response. “It’s not old school at all. It’s a true physiological parameter,” he points out. Like lactate, it shows the metabolic cost of an effort. A true sustainable effort will be at a flat or very gradually rising heart rate (approximately one or two beats every 10 minutes).

Look at both watts and heart rate

This is not to say that power should be ignored. It is, of course, very powerful; it shows the ultimate mechanical output of your effort. And it’s that output that wins races. But the magic comes from a combined interpretation, according to San Millán. Heart rate shows the metabolic cost of a given wattage. For example, you may believe your FTP is 325 watts, but if you do an hour time trial at that power and your heart rate rises from 175 to 185 BPM in the course of 10 minutes, you know it’s not sustainable.

Do the distance

Remember that your threshold depends on the distance. So to determine your threshold, time trial your target distance. If it’s a short climb, then do a five-minute effort. If it’s a 40K time trial, then do a one-hour test.

And repeat it

Do the distance several times at varying wattages to find what’s sustainable. For example, if you do a 30-minute time trial at 325 watts and your heart rate doesn’t plateau, then wait a few days and try it again 10 watts lower. Keep repeating until you find the highest wattage you can hold while maintaining a stable heart rate.

Find your MLSS

Your MLSS is an important threshold because it highly correlates with performance—and because it is based on physiological changes that occur in your body at that specific intensity. The one problem with it is that it is hard to determine. The gold-standard method is to do repeated efforts of 30 minutes using the procedures explained above. Note that some studies have shown that despite a level blood lactate, heart rate can still increase up to 10 BPM between minutes 10 and 30. [4–7]

A multiplier in a pinch

If your target is a 40K time trial, doing a one-hour test to find your threshold may not sound very appealing—and multiple one-hour tests even less so. By multiplying your MLSS power by 95% you can estimate your one-hour power or FTP. However, San Millán warns it is not exact.

Training flat

Fortunately, our sustainable effort is very trainable. So once you’ve found the threshold for your target distance, you should do interval work at that effort. The length depends on your target distance, but if you’re trying to train efforts around your MLSS, generally five-minute or longer intervals are warranted. But just like your test, ensure a level heart rate during those efforts. Otherwise, you may be training too hard.

Trust your feel

Tuft recognizes the value of our threshold numbers but warns that we shouldn’t lose the ability to train and race by feel. “I think it can be very detrimental to base everything we do off this set number that we did in some physiology lab,” he says.


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