Lactate Threshold- The ultimate endurance test

  • Following the acid: creation to elimination
  • What is the threshold: pain and yelling coaches mostly…
  • How to test the threshold: a whole lot of testing
  • Sprinters vs. Milers: and how training affects the curve


“Man that’s gotta hurt!” said every coach, swimmer, and parent watching someone finish up the last 25 of a 200 fly. But why? Why is swimming fast/butterfly so painful? We talked about where lactic acid comes from and why it is a good thing we make it (see Energy Systems), but does it really have to hurt this much, or is there a way to get the same speed without all the acid?

Yes there is… and testing that ability is called finding the “lactate threshold.” This threshold is a really fancy way to determine endurance ability. When we start talking about how training improves this threshold, we will need to know exactly what is changing inside the body for this improvement to happen. Let’s start with tracking the acid, from creation to elimination.

Following the acid: creation to elimination

Let’s start from the beginning, and follow a molecule of lactic acid from creation to elimination. We have talked all about this before, but we never put it all together and focused in on the details. Remember that sugar is first processed in the Glycolysis system to produce pyruvate. Pyruvate then has to choose, either go into the mitochondria and be burned, or get turned into lactate.  During fast swimming, most pyruvate goes to the lactate (for reasons why, visit Energy Systems).

For our purposes, lactate and lactic acid are basically the same, so we will use the words interchangeably.

Tangent: lactic acid is the hydrogenated form of lactate. The H+ is acid, and can be buffered a little. The unit of buffering ability inside muscle cells is termed Slykes. The more slykes you have, the more you can buffer.

From here things get more complicated. Lactate can’t just build up forever, it has to either leave the muscle cell or get burned by the mitochondria just like pyruvate. In fact, both happen at the same time, and the better this process works, the longer you can hold a sprint and the better your lactate threshold will be.

About 75% of lactate is burned in mitochondria to produce energy. That means that at a fast sprint, you are actually burning more lactate than pyruvate! The other 25% is shipped out of the cell and into the blood where the lactic acid is buffered by compounds in the blood to produce carbon dioxide. The CO2 is then breathed out in the lungs, and the acid problem is solved. Here are some review pictures from our Lungs article.

Tangent: This CO2 is called non-metabolic carbon dioxide because it was not formed from burning anything directly. This is the basis behind measuring how much sugar vs fat you are using during exercise (see Lungs article for details).

Now lactate can kind of “walk” to the mitochondria or to the blood (“diffusion” if you would like to call it that), be we need to get rid of it fast, because we want to swim fast! So, in the membrane of the cell and mitochondria are special transporters that literally grab the lactate and throw it to the other side of the membrane, either into the mitochondria or out into the blood (like taking out the garbage).

Tangent: these transporters work better and adapt more when exposed to higher concentrations of lactate. That’s why coach makes you sprint 100s, because the only way to force the body to adapt these transporters is by flooding them with lactic acid…fun.

As great a system as this is, your body is actually even more intelligent. Since lactate can be used as a fuel, and it can be transported out of the muscle cell… why not have other muscles and organs eat up the extra lactate dumped into the blood? The liver, kidneys, and muscle cells not being used at the moment help pick out the extra lactate in the blood and burn it up themselves in their own mitochondria.

This is especially important when it comes to fast and slow twitch fibers that are really close to each other. The fast twitch fiber makes a lot of lactic acid (by definition, see Muscle Types article), and this lactic acid can quickly travel next door to the slow twitch fiber which has a lot more mitochondria which can “recycle” the “garbage” that was thrown out.

This way, we see that improving the mitochondria in slow twitch fibers by doing a lot of yards and endurance training will actually improve your sprinting ability because now the fast fibers are free to make as much lactic acid as they want, and it all (mostly) gets processed and eliminated with the adjacent slow fibers. That’s why coach makes you sprinters do distance sets! Because it helps you last longer in a sprint!

Tangent: Lactic acid is the most dynamic metabolite in the body during exercise. It has highest turnover known and is quickly created and eliminated. The goal of training is to improve turnover, we want to create lactic acid faster (that gives us power), and get rid of it faster too (that helps us last longer).


What is the threshold: pain and yelling coaches mostly…

This concept has gone by many names: onset of lactate accumulation, respiratory threshold, anaerobic threshold… but the most accurate terms are probably “maximal lactate steady state” and “lactate threshold.” In the past, this concept was trying to find the point at which muscles switch from the aerobic (mitochondria) to the anaerobic system (glycolysis) as training intensity increased. This is not the right way to think about it. Remember that mitochondria and glycolysis work at the same time… all the time. But at different swimming speeds, we tend to use one more than the other. It’s more like a spectrum, rather than an on/off switch.

The lactate threshold is the maximum speed you can maintain without the lactate building up out of control. It is a SPEED… not a lactate level, that’s why we won’t talk about the numbers here. In order to achieve the highest speed possible without overloading with acid, you need create as little lactic acid as possible (aka… by using the aerobic system), and get rid of it as quickly as possible. In fact, since most people can generate a lot more lactic acid than they are capable of getting rid of, the lactate threshold is mainly set by how well you get rid of the acid.

Getting rid of the acid involves the two ways we spoke of earlier. Either beef up your mitochondria, or ship the lactate out the muscle faster which can be achieved by beefing up the transporters that are in the muscle membranes. (Building up some capillaries to increase blood flow doesn’t hurt either).

This can be a very confusing concept, but if we go to the pool and test it out slowly it will make more sense.

Let’s say you are swimming easy, like holding a pace of 60 seconds per 50. Lactic acid will be produced even now and go up a little. For the first 15 minutes or so, the acid will go up higher than expected, but then go down to a steady state, where it will hang out for a long time. Those first 15 minutes are a warm up period, where the muscles are not getting all the blood they can and the mitochondria are still “revving up” to their maximum ability (see Cardio article).

Now you speed up, holding about 50 second per 50. The lactic acid goes up again, just like before, and comes down again, except this time it is at a higher level than before. It is still in a steady state after the initial 15 minutes, neither going up or down.

You speed up again, this time to the fastest pace that you can maintain, about 40 seconds per 50. Now the lactic acid goes up even higher, but it is still held at a steady state. Things are not feeling very good right now, but you are holding on… just barely.

Finally, coach wants you going even faster, so you speed up to 38 seconds per 50. It is not much faster, but each 50 feels worse and worse, and a few minutes in you start slowing down and coming to a halt. The acid levels didn’t level out this time. Instead they just kept going up and up. Ouch…

This test is designed to find your lactate threshold, which in this example would have been the 40 seconds per 50 pace, since that is the fastest speed you could maintain without lactic acid going through the roof. At this pace (and every slower pace) your creation of lactic acid is equally eliminated, so the lactic acid remains at stead state. Once you started going faster, even just a little, your elimination of acid could not keep up with its creation, and so it just kept going up.

Tangent: that 15 minute warm up period may explain why milers can finish their mile races with so much speed. By the end of the race, their body has readjusted and they are capable to throwing down some crazy closing times.

All of this is like a bucket with a hole in the bottom and water pouring in from the top. The water is the lactic acid, and the bucket’s water level is how much acid is accumulating in the blood. At low swimming speeds the amount of water coming in is low, so the amount of water flowing out is slow and the water level is stable in the bucket. As you speed up, more and more water flows into the bucket, and more water falls out the hole. At some point, the amount of water going in is higher than the amount coming out, so the bucket eventually accumulates water and overflows.

Training does two things: it decreases the amount of water going in (decreases lactic acid production), and more importantly it pokes more holes in the bottom of the bucket (increases lactic acid elimination). This is assuming you are going the same speed of course!

There is one more part to the lactate threshold test (actually, it’s an add-on to the test, but we will say it is part of it). Testing the maximum lactate levels that can be achieved is useful and can tell us how good our anaerobic system (sprinting engine) is working. This is simple, swim ALL-OUT for a 100 or so, and then test the lactate levels again. This give us the peak lactate level.

Tangent: To truly max out your lactate takes a lot of pain and motivation. This is why racing in a meet is so good for you, because you never reach maximum lactate level in practice, you need the energy of a swim meet to get you the next level.


How to test the threshold: a whole lot of testing

So… you could test your threshold like we did before, and swim at ever increasing speeds for 30 minutes at a time, but that takes a long time and it is real hard to do with other people in your lane. That’s why other ways of testing the threshold were developed. One of the more popular ways is to do a set of 5 x 300 swims with a minute rest in between each. Each 300 gets faster by about five seconds, with the goal being maximum speed by the last 300. For instance, you would do the first 300 in about 4:00 minutes, the next in 3:55, 3:50, 3:45, and last one fast one for a 3:33. During each minute of rest, a professional (I mean it this time, someone who knows what they are doing) takes a small blood sample and measures your lactic acid levels. These 300 times create a graph that looks something like this:

To get a peak lactic acid, you need to make sure you do that on a separate day, when you acid levels start at zero. That way you won’t get a false reading. But this maximum lactate level can be added to the graph too.

Notice how at first the graph starts as a straight line, with acid levels increasing at a reasonable rate compared to the speed we are going. But after the third 300, something changes. The line starts to curve upward, and exponentially increases compared to the speed. This is called the inflection point, and this is where we will find our threshold. There are many ways to “estimate” the threshold from this graph, but it’s not that important for us who just want to understand the basics.

This is a very useful graph because now we can see how training and other things affect our swimming. Say you repeat the test in a month after practicing doubles and racing every weekend, what would we see change? The line on the graph would shift to the right. This means you are holding a higher speed in the water while maintaining the same lactic acid. This is an objective way to measure how well your endurance training is going. As the line shifts to the right, you notice that each 300 is now faster, but the lactate levels didn’t increase compared to the intensity.

There are other reasons for this inflection point on graph too that help explain why it happens. As swimming intensity increases, recruitment of more fast twitch fibers which, by definition make more lactate and use less mitochondria, occurs. These fast fibers dump a lot of acid into the system, causing problems with its elimination (maybe this is why coach tries to tell you to “relax” the first half of a race).

Epinephrine and glucagon (coming up in Hormone article) release during heavy exercise greatly increases rate of sugar burning and sugar release from liver. This leads to even more lactate production. As more and more blood is being shunted to the muscles being used, less blood is reaching the liver and kidneys, which reduces their ability to remove lactate from blood too. Even performing the threshold test in the morning rather than the afternoon will decrease performance by about 2% (don’t say I didn’t try to get you out of hard swimming in your morning practice…)

There’s more… the threshold is stroke specific. This may seem obvious, but remember in our Motor Unit article we talked about how we use slightly different muscle fibers for each stroke. That means training freestyle all the time won’t help improve your backstroke threshold as much. The threshold is also pool specific… and I don’t mean yards to meters. I mean if you tested the same person in a 25 meter pool vs a 50 meter pool, their average performance “decreases” by over 5%, which is a lot more than expected!

Tangent: Peak lactate levels can change too. Being on a low carb diet will decrease the peak lactate you can achieve by 25%, and being on a high carb diet will increase it by 25%. Those are big numbers to worry about and shows just how important good nutrition and recovery are.

Oh…you don’t have a lactic acid analyzer on your pool deck? No trained physiologist to collect the blood either? Well… there are other ways to test the threshold, in fact, it’s not as hard as you think.

Since the lactate threshold is basically the fastest pace you can maintain with constant swimming, that’s what the tests look like. For instance, swimming constantly for 30 minutes and then calculating the threshold from the total distance covered is called the T-30 test (Timed- 30 minute swim). The T-3000 and T-2000 are also verified by scientific studies. These tests involve swimming a timed 3000 or 2000 and using the time to calculate the threshold. Even the T-1000 has been shown to be accurate, but according to the great Ernest Maglischo (a famous swimming scientist and coach) that is too short a distance, and I happen to agree (disagreeing with Ernie is a brave thing…).  Whatever test you use, make sure you do the same test throughout the season since switching up tests may give a different number, and we want consistency above accuracy.

Lactate Threshold Testing (T-30)

Lactate Threshold Testing (T-3000)



Threshold (in seconds/100)


Time (s)

Threshold (in seconds/100)














































How about measuring your threshold during practice without using test sets? That’s where heart rate and color coded training comes in. Some coaches use different colors to signify swimming intensity levels. These are really just different ways to describe where your swimming speed should end up on the graph. Some colors are below the threshold, one color at the threshold, and some colors above. The idea is that you instinctively choose the right speed for the right training.

If you want a more “objective” way to measure threshold training, heart rate is not a bad choice. Holding a heart rate of roughly 90% of your maximum heart rate (max HR = 220 – your age, + or – 12) correlates to about your lactate threshold. For most of us, that’s about 175 beats per minute. Now remember… heart rate is a fickle thing, and many things affect it (see Cardio article) like pool temperature, hydration status, and type of sport (upright vs swimming) so don’t rely too much on it. Not to mention finding your maximum heart rate can be tough and variable as well. The best thing to do would be to run a single T-3000/2000, and every 500 or so stop just long enough to measure your heart rate. That should give you a good idea of your threshold heart rate. Even if you are in better shape later on in the season, that heart rate should still correlate to your threshold, regardless of how fast you are going (just the opinion of this writer FYI).

Tangent: Lactate threshold is about 70% of your VO2 max.

Sprinters vs. Milers: and how training affects the curve

Let’s compare our two favorite types of swimmers: sprinters and milers. Remember, sprinters are very very very rare in swimming and a true sprint event is only about 40 seconds long, so everyone who races anything over that is pretty much in the miler group. If we made a sprinter and a miler perform five 300s with faster and faster paces for each 300, then took lactic acid samples after each 300, we would get a graph that looks like this for each swimmer.

Notice how the miler’s line is pushed way to the right. That means they can hold a faster pace while staying at the same lactate level (aka… higher threshold). Compare now to the sprinter who struggles to maintain any kind of speed without their lactic acid going through the roof. Swimmers want great thresholds, so we want the line to be as far to the right as possible. But speed is important too, so let’s put our two swimmers up on the blocks and run a 100 all-out, then we will measure their peak lactate levels.

The sprinter won! But why, their threshold is so low. Well if we look at the graph, the sprinter’s lactate levels went way higher than the miler. Because of that you would assume that this level of acid would shut down the sprinter and slow them down. And that is true, but the race didn’t last long enough for the acid to really build up. Instead what it shows is that the sprinter is capable of producing a lot of power from their glycolysis system, and this results in a lot of lactic acid. The miler on the other hand does not have a well developed glycolysis engine, so they struggle to go fast or make a lot of lactic acid.

In practice, you have probably run into these two types of people. The sprinter wonders at how the miler can just repeat 200s within a couple seconds of their lifetime best, and the miler marvels at how fast the sprinter can crush a 25. And at the same time, both swimmers can’t understand how the other one fails to perform like they do. Remember, a lot of these differences are genetic. Milers probably have a lot of slow twitch fibers which don’t make a lot of acid, but also don’t fatigue, and sprinters are big balls of fast twitch fibers which do nothing but produce massive power and lactic acid.

So what if each of these two types of swimmers trained like their opposite, what would happen? Well in real life each would probably improve in what they trained, the miler would get better and sprinting and the sprinter would get better at distance events. That’s the great part of the swimming machine, it adapts. As far as the lactate threshold and peak lactate levels go, they would shift to mimic the training. A rightward shift in threshold for the sprinter, and left shift for the miler. Peak acid ability would decrease in the sprinter, and increase in the miler too.

If we zoom back in on the molecular level, we would notice changes there too. For the miler, their glycolysis system would improve with higher concentrations of those specific enzymes, as well as improved lactate transporters to help get rid of lactic acid faster. These changes would improve their speed and lactate peak. There may also be a decline in their mitochondria’s size and ability which will lower their threshold.

The sprinter would see the opposite change, with larger mitochondria, better capillary flow, and fewer glycolysis enzymes. This would result in a better threshold, but a lower lactate peak.

So is there a way to have your cake and eat it too? Yes… it’s called taper, and we will spend a whole article on it later. The goal of training is to find a way to balance these “opposing” forces, or better yet to find a training regiment that will improve both to their maximum capability. That’s what makes training so fun, it is all a huge experiment that requires constant analysis and adaptation. Good luck!


Karl Hamouche- Swim Smart founder

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