Do you know that feeling that your mind still wants to run fast, but your legs don’t? That feeling that your legs get heavier each step you take? That moment your whole body starts to protest, and just wants to stop running? Well, that’s the moment you crossed your Lactate Threshold. But what is this threshold, and why does it matter in (trail) running?

Roughly you can see your Lactate Threshold as the fine line between running at a pace you can keep on running forever, and a pace you get tired very quickly.

Running a marathon in 1 hour

Let’s give you an example. Joshua Cheptegei is the world record holder on the 5 kilometers, with a time of 12 minutes and 35 seconds. That’s 2 minutes and 31 seconds per kilometer. If he would run a marathon sustaining this pace, he would finish in 1 hour, 46 minutes and 12 seconds. However, the (official) world record in the marathon is 2 hours, 1 minute and 9 seconds, set by Eliud Kipchoge. That’s fifteen minutes slower, or with 2 minutes and 52 seconds per kilometer 21 seconds slower per kilometer.

The reason is simple: we can’t sustain a high speed for a long period of time. The longer the distance, the slower our pace. Otherwise Usain Bolt, world record holder at the 100 meters sprint with 9.58 seconds, would have run the marathon easily within 1 hour and 15 minutes.

Hitting the wall

So if we run too hard, we hit the wall, run into the man with the hammer, are in the red zone, or however you want to say it. So the trick is to find the pace we can sustain for the desired amount of kilometers. That pace is the pace of your Lactate Threshold.

Every pace higher than your Lactate Threshold pace your fatigue accelerates. Every pace lower, you can sustain for a longer period of time. The higher you push above your threshold, the quicker you get tired. The further you stay under your threshold, the longer you can sustain your chosen pace.

Finding your Lactate Threshold

I like the KISS principle; Keep It Simple Sweetheart, so I’m just telling you what you need to know to run better, faster and with more ease. The role lactate plays in your body, I prefer to leave to the scientists. If you want to know all about that, you’re best off reading the articles written by George A. Brooks, professor Integrative Biology at Berkeley, University of California.

For now, if you want to know your Lactate Threshold precisely, you have to do a test in an exercise physiology laboratory, where they will take different blood samples to see the amount of lactate accumulating during your training sessions. The downside of that test, besides the costs, is that you probably will run on a treadmill, which is something completely different from running outside on the track, road or trail.

Using wearables

Another option is to buy a portable lactate analyzer, and take the blood samples yourself. That way you can run the way you normally run, and will have a better idea of what your Lactate Threshold is under normal circumstances.

However there are easier methods. The easiest is just to use your smart watch. Lots of sport watches can detect your Lactate Threshold either through a guided workout, or automatically during a normal run. They will use your heart rate data across a range of paces. The downside of this, is that it’s an estimation of what your Lactate Threshold will be. The nice thing is, you will mostly be given a precise heart rate level in beats per minute and a running pace.

Do your own test

If your watch doesn’t have a Lactate Threshold test, don’t worry. As long as you have a watch that can track the distance you run, you’ll be fine. If not, you want to find a loop where you know the precise distance off. For example a 400 meters running track.

Now warm-up for a couple of minutes. After that run for 30 minutes at a pace you can just sustain for those 30 minutes, and keep track of the amount of meters you’re running. If you’ve done that, all that is left to do, is some calculations.

1. First take the amount of meters you’ve run, and divide them by 1.800 seconds (as 1800 seconds equals 30 minutes). The result tells you how many meters you have run per second. We call the outcome A.
2. Divide 1.000 meters by A (the outcome from point 1). We call this outcome B.
3. Divide B by 60 seconds/minute. This will give you your minutes per kilometer.

For example you run 6.000 meters in 30 minutes.

1. 6.000 meters divided by 1.800 = 3.33 meters/second. Outcome A.
2. 1.000 meters divided by 3.33 meters/second (A) = 300 seconds/kilometer. Outcome B.
3. 300 seconds/kilometer (B) divided by 60 seconds/minute = is 5 minutes/kilometer. That’s your Lactate Threshold.

A few calculations

Let’s take an example with less nice numbers. Let’s say you run 4.500 meters in 30 minutes. This will mean:

1. 4.350 divided by 1.800 = 2.42 meters/second (A).
2. 1.000 meters divided by 2.42 meters/second (A) = 413 seconds/kilometer (B).
3. 413 seconds/kilometer (B) divided by 60 seconds/minute = 6.89 minutes/kilometer.

Okay, that’s not possible. You can’t have 6 minutes and 89 seconds. This is because we have to put a metric number into time. Well 89 relates to 100 as 53.4 relates to 60. See: 100 divided by 100, times 89 = 89. So 60 divided by 100, times 89 = 53.4. So your Lactate Threshold is 6.53 minutes/kilometer.

This test isn’t 100 percent accurate. However, research done at East Carolina University (US) shows that the results you’ll find with this test are (almost) the same as the ones you’ll find with an exercise test in a laboratory.

How to use your Lactate Threshold

Let’s go back to example one, in which we could run 5 minutes per kilometer. Or in other words 12 kilometers/hour. If I’m running a 10 kilometer race, and I know that I can only sustain 5 minutes/kilometer for 6 kilometers, I know I should definitely run a little slower to make sure my legs still can keep going after 6 kilometers.

However, if I do only a 3,5 kilometer race, I know I can easily run faster than 5 minutes/kilometer, because I know I can run 6 kilometers at that pace. How much faster, or how much slower, is something you have to find out, by trying different speeds in training or during a race. The more you run, the more you get to know yourself.

Heart rate versus minutes/kilometer

So far I’ve been talking about pace, about minutes per kilometer. That’s fine for road runners and track runners. Most road races are more or less flat. In trail running it’s different. Hills, mountains, sand, river crossings and mud make it hard to keep an even pace. So for trail runners it’s easier to know your Lactate Threshold in heart beats per minute instead. Most wearables will provide you with this number.

An exercise test in a laboratory can give you a heart rate as well. If you want to use the 30 minute test, do the test as described above and find your pace (minutes per kilometer). A few days later, go for another run. Warm-up again first, and then run 20 minutes at your Lactate Threshold pace. Your average heart rate of those 20 minutes is your Lactate Threshold heart rate.

When you go for a trail run, no matter if you run uphill or downhill, keep that heart rate in mind.

Changes over time

Last, but not least: your Lactate Threshold isn’t a fixed number. It can increase or decrease over time. The more you train (properly) the higher your Lactate Threshold, the faster you can run for longer periods of time. However, if you stop running, or stop pushing yourself in training, it will decrease. So where your Lactate Threshold now might be 5 minutes/kilometer (as in our example) in the future it might be 4.30 minutes/kilometer.

How you can improve your Lactate Threshold, is something I’ll explain in another article. For now, do the test, use your results wisely and keep on running.