One of the best measures of running fitness is lactate threshold (LT), which is the running speed or heart rate at which lactate — an intermediate product of aerobic metabolism in the muscles — begins to accumulate rapidly in the bloodstream. If your training program is working, two things are sure to happen. One is that you will run faster at the point where your blood lactate level spikes. The other is that your heart rate at this threshold will increase (i.e., your lactate threshold heart rate will move closer to your maximum heart rate).
In addition to being useful as a measure of running fitness, lactate threshold is also useful for establishing individual intensity zones for training. That’s because LT happens to fall at a moderate intensity level. Efforts that are more than a little faster than the pace or heart rate that corresponds to LT are defined as high intensity and offer a different set of benefits than moderate-intensity training. Efforts that are more than a little slower than the pace or heart rate that corresponds to LT count as low intensity and offer yet another set of benefits.
Lactate Threshold Training and Testing
Training right at or very near LT intensity is a potent way to build running fitness. Once a week or so you should hit this intensity with things like tempo runs (e.g., 20:00 @ LT between warm-up and cooldown), fast-finish runs (e.g., 35:00 easy + 10:00 @ LT), Fartlek runs (15 x 1:00 @ LT/1:00 easy between warm-up and cooldown), and long runs with tempo surges (15 miles easy with 4 x 5:00 @ LT sprinkled in).
In order to know whether your LT is improving, and to train in LT-based zones, and to target your LT in certain workouts, you need to know the running pace, heart rate, and/or power that corresponds to your lactate threshold. Exercise scientists determine lactate threshold in a laboratory environment. In a typical LT test, a runner starts running at a low speed on a treadmill and is then required to run incrementally faster until the point of failure. At each step, the person leading the test takes a small blood sample from the runner’s fingertips and measures its blood lactate concentration.
After the test is completed, the collected data is used to create a graph in which the blood lactate concentration is plotted against pace and/or heart rate. Lactate threshold is pinpointed where the blood lactate concentration begins to increase rapidly. In a typical trained athlete, that point corresponds to roughly 85 percent of maximum heart rate and falls somewhere between 10K and half-marathon race pace.
The protocol just described has some obvious disadvantages. It requires special equipment and expert assistance, it’s relatively costly, and it’s invasive. For all of these reasons, it can’t easily be done with optimal frequency (once every six to eight weeks) by most runners. Fortunately, there are do-it-yourself alternatives to lab-based lactate threshold testing that work quite well. Let’s take a look at three of them.
The Time-Trial Method
The time-trial method of determining lactate threshold pace and heart rate can be done on a treadmill, on a running track, or on any other flat, smooth surface that’s conducive to fast running. It also requires some means of measuring time elapsed and distance covered as well as heart rate. Be sure to conduct this test on a day when you are not fatigued from recent hard training.
Begin with several minutes of easy jogging to warm up. When you’re ready, start tracking time, distance, and pace on your treadmill or watch and run for 30 minutes at the fastest pace you can sustain for that amount of time. Be careful to avoid the common mistake of starting too fast and then slowing down toward the end of the time trial due to fatigue, which will produce an inaccurate result. When you get to 10 minutes, note your heart rate.
At 30 minutes, stop and note your heart rate again. Calculate the sum of your heart rate at 10 minutes and your heart rate at 30 minutes and divide by two. That’s your LT heart rate. Your LT pace is your average pace for the entire 30-minute effort, assuming your pace was fairly steady.
Note that LT test results obtained on the treadmill often don’t translate well to outdoor training because A) most treadmills are poorly calibrated and B) heart rate is lower on the treadmill yet perceived effort is higher, so runners can’t go as fast. If you choose to test on a treadmill, make sure the machine has been calibrated recently and keep the belt at 0 percent, as raising it to 1 percent as runners are often told to do will only exacerbate the indoor/outdoor pace discrepancy.
A 2005 study by scientists at East Carolina University found that this method of determining LT heart rate and pace is very accurate. Its downside is that it’s hard — equivalent to running a half-hour race without the added adrenalin from the actual racing environment.
The Race Time Method
We know that a runner’s lactate threshold pace is a strong predictor of his or her race times. But it also works the other way around: Your race times can be used to estimate your pace at lactate threshold.
I often use coach Greg McMillan’s Running Calculator for this purpose. Simply enter a recent race time in the relevant field and press the “Submit” button. Near the top of the results page, you will see “vLT” with some numbers next to it. That is your approximate lactate threshold pace.
To determine your LT heat rate, warm up and then accelerate to your LT pace on a flat, smooth surface. Wait for your heart rate to plateau and note it. That number is your LT heat rate.
The High-Tech Method
The wearable technology revolution has given runners a variety of new options for DIY lactate threshold testing. As a coach, I have found some more of these options to be reliable than others. For example, I’ve learned to completely tune out LT pace and heart rate estimates generated by Garmin’s Auto Detect feature. Just this morning I received a notification informing me that the device used by a client of mine detected a new LT pace of 7:33/mile, which is significantly slower than his current marathon pace. Garmin users (and I am one) should follow the brand’s Guided Test protocol to get LT estimates they can trust.
More trustworthy still is the Stryd run power meter’s Critical Power feature. Critical Power is not quite the same thing as lactate threshold, but it’s very close and, more important, extremely consistent. Recently, the Stryd app used by another client of mine generated a predicted time for an upcoming 30 km time trial based on her current Critical Power estimate that precisely matched the prediction I came up with through my own calculations.
What’s Your Lactate Threshold?
Do you know your lactate threshold pace, heart rate, and/or power? You should. Fortunately, getting accurate measurements without expert help (or blood!) has never been easier. Choose your preferred DIY method and start tracking your fitness and training more effectively today.