How do you find your lactate threshold heart rate from your own runs?

What number are you looking for?

Your lactate threshold heart rate is your heart rate at the second lactate threshold. That is the hardest effort you can hold for a long time before lactate builds up fast in your blood. Lactate is a substance your muscles make all the time, and its level climbs as effort gets harder. Lactate threshold heart rate explains both thresholds and their many names.

A lab finds this number with drops of blood or a breathing mask. You can get close with a watch and one hard run. This page covers three ways to do it from your own runs.

1. A 30-minute solo test. One study compared it with a lab test.
2. A 20-minute test with a correction. It takes less time, and the shorter effort needs a small correction.
3. A race or a hard run you already did. It needs no new test, and it needs care when you pick the run.

How do you run the 30-minute test?

The coach Joe Friel describes a simple version^[1]. Start with some easy running to warm up.

1. Run 30 minutes alone at the hardest effort you can hold for the whole time. Friel says to run it with no training partners and not in a race.
2. Press the lap button on your watch at 10 minutes.
3. When you finish, read your average heart rate for the last 20 minutes. Friel calls that number an approximation of your lactate threshold heart rate.

Friel also warns that most people start too fast and then slow down, which spoils the number^[1]. Settle into a pace you can hold for the full 30 minutes.

Leaving out the first 10 minutes keeps the slow start out of the number, because heart rate takes a few minutes to catch up with your legs^[11]. A flat route or a track keeps the effort even.

How close does the 30-minute test get to a lab test?

One study put the two side by side. Twenty-seven competitive distance runners and triathletes ran a lab test that measured their threshold from drops of blood^[3]. The lab set the threshold at a blood lactate level of 4 mmol/L. That unit measures how much lactate is in the blood. The runners also did four tests that estimate the threshold, and one of them was the 30-minute time trial.

On average, the heart rate from the 30-minute test matched the lab heart rate. The pace matched too. For a single runner, the heart rate estimate had a typical error of about 8 beats a minute^[3]. The authors concluded that runners and coaches should consider the 30-minute test for both pace and heart rate at threshold.

Heart rate also climbs through the test, and that climb is part of the number. In a study of 18 runners, heart rate at their steady threshold speed rose about 8 beats from minute 10 to minute 30^[5]. Those are the same 20 minutes the test averages. A rise like that during the test is expected.

Can a 20-minute test do the same job?

A 20-minute test takes less time out of your week. The American Council on Exercise is a group that certifies fitness trainers, and it publishes one version of this test^[2]. It notes that fit people can hold a harder effort for 15 to 20 minutes than for 30 to 60 minutes. So the shorter test needs a correction downward.

You hold your highest steady effort for 15 to 20 minutes. You average your heart rate over the last 5 minutes. Then you multiply that average by 0.95. Its own example is a 20-minute bike test with an average of 168, which gives a threshold estimate of 160 beats a minute.

Cyclists use the same 95% step for power. A study of 15 trained cyclists compared 95% of their 20-minute power with a lab threshold found from several steady rides^[4]. For heart rate, the 20-minute method came out about 1% lower than the lab on average. For 95 in 100 riders, the gap stayed within about 8% either way of that average.

That study used bikes, and a bike number does not carry over to running. In 143 triathletes tested both ways, heart rate at the second breathing threshold came 6 to 9 beats lower on the bike^[15]. Run your test in the sport you train in.

Can you use a race instead?

A race is one long, hard effort that you already planned to run. Friel's test asks for something different. He says to run the test alone and not in a race^[1].

The length of the race changes what its heart rate means. A study of 211 competitive male runners recorded heart rate through races from 5 km to 100 km^[6]. In the 5 km and 10 km races, heart rate stayed above the second breathing threshold for 80 to 85% of the race. In the half marathon and longer races, the largest share of time came between the first and second thresholds. Faster and slower runners showed about the same pattern within each race distance.

A steady effort of about 45 minutes comes closer. Eighteen runners in one study ran all-out at fixed speeds for up to an hour^[5]. The fastest speed they could hold for 45 minutes closely matched the steady threshold speed found from their blood lactate. Heart rate matched as well, at 178 and 177 beats a minute on average.

So a race can give you a rough check. The average heart rate from a 5K or 10K race likely reads above your threshold heart rate. A solo test run the same way each time gives the cleaner number to track. Leave a few easy days between a hard race and a test, as running after a race explains.

Can you find it in runs you have already done?

Your watch may already hold a run you can use. Look for a run where you went hard and steady on your own for about 30 minutes or more. Read the average heart rate from minute 10 to the end of the hard part. That works the same way as the 30-minute test. The run has to be one unbroken effort at your hardest steady pace.

An everyday tempo run reads lower than the test. You held back on that run, so its heart rate stayed below your hardest steady effort.

An interval session cannot do this job. The breaks between repeats let you run harder in each repeat than you could in one unbroken effort. Ten well-trained runners in one study found their fastest steady-lactate pace with and without short breaks^[7]. That pace was 15.3 km/h when they ran 5 minutes and rested 1 minute. It was 14.5 km/h with no breaks. A threshold number taken from intervals reads too high.

A bend in your heart rate line cannot do it either. The Conconi test is an old field test. It looks for the speed where heart rate stops rising in a straight line. Only 9 of 14 trained runners in one study showed a bend at all^[8]. Where it showed up, it came at 186 beats a minute on average, against 172 at the lab threshold. It appeared near 95% of max heart rate. The authors concluded that the test does not find the lactate threshold.

Why does a lab test give a different number?

Lab tests differ from one another too. A lab step test raises the speed every few minutes and reads lactate at each step. The length of each step changes the answer. In 8 well-trained runners, the speed at a lactate level of 4 mmol/L was 12.9 km/h with 6-minute steps and 14.4 km/h with 3-minute steps^[9].

The lab's steadiest answer takes several long efforts. The maximal lactate steady state is the fastest effort where blood lactate stays flat. In one lab study of 100 adults, finding it took several 30-minute efforts at fixed workloads^[13]. A step test is quicker and gives a different kind of number.

Heart rate also drifts upward during a long effort at one speed. Fifteen trained young runners held one steady speed for 40 minutes^[10]. A lab step test had put that speed at 88.3% of their max heart rate on average. After 40 minutes the same speed read 92.2%. A field test averages your heart rate through a long effort, so it carries some of that climb. A short lab step does not.

So the lab and the field measure related things in different ways. Each number is most useful when you repeat it the same way and compare it with your own last result.

How does your number compare with Diastra's Threshold band?

Diastra's Threshold band runs from 87% of your max heart rate up to one beat under 92%. To compare your test with the band, divide your test number by your max.

Say your max is 190 and your 30-minute test gives 171. That is 90% of 190, so it lands inside a Threshold band of 166 to 174 beats a minute. It also lands near the research average. Blood tests on 165 recreational runners put the second threshold at 90.6% of max heart rate on average^[12].

Your own number can land outside the band. In 100 adults, the lab steady threshold landed anywhere from 75% to 97% of max heart rate^[13]. In the 165 runners, a guess from the average for each sex came within about 9 beats of the real second threshold for 95 in 100 runners^[12]. The average miss was about 3 beats with a measured max. It grew to about 7 beats when the max was estimated.

So the max under the percentage matters as much as the test. A max guessed from age can miss your real max by a wide margin, and every percentage drawn from a wrong max is off by the same share. Finding your max on Apple Watch shows where to look in your own workouts.

Treat the band as your starting target for a threshold run, and check it against how the effort feels. If your test lands below the band, a run held inside the band will feel harder than your test effort did.

How do you know your threshold is improving?

Watch your pace at the same heart rate. If you run faster at your threshold heart rate than you did a few months ago, your threshold has moved.

Pace is the clearer sign. In the 165 runners, fitter runners did not have their second threshold at a higher share of max heart rate^[12]. So a higher threshold heart rate is not a sign of fitness on its own. The pace you hold at that heart rate tells you more.

Repeat the test the same way each time. Use the same route, the same warm-up and a similar time of day, so the two numbers compare. A test on a hilly loop one month and on a track the next gives two numbers that do not compare. Pick similar weather too, because heat raises heart rate at the same effort^[14].

What does Diastra do with your threshold?

Diastra reads every workout your Apple Watch saved in Apple Health. It finds your max heart rate from your own workouts, a guided test or a number you type, and it says which one it used. Your Easy, Threshold and VO₂max bands are drawn from that max. On a threshold run the watch holds you in your Threshold band with a tap when you drift out, and only the minutes that held are credited. Over the weeks Diastra shows whether your training is working. Getting fitter. Holding steady. Slipping.

Threshold runs covers how to train at this effort once you know where it is.