Testing Your Lactate Threshold Heart Rate (LTHR)

What LTHR is

Your LTHR is the heart rate Runsense reads from your best 20 minutes of a hard, steady 30-minute effort, and it is the number your heart rate zones are built on. It sits at the boundary between zones 4 and 5, and the guide on heart rate zones covers the bands. It comes from an effort you ran and not from a population formula for maximum heart rate.

"Lactate threshold" is not one thing. A 2009 review located 25 different lactate threshold concepts and described two typical breakpoints: the intensity at which blood lactate first rises above baseline, and the highest intensity at which lactate production and elimination are in equilibrium¹. A threshold number means little until you know which concept it comes from, so Runsense defines LTHR by the field test below, and it means the same thing each time it is measured. The guide on lactate threshold covers the concepts and what they predict.

The 30-minute field test

The standard do-it-yourself method is a 30-minute time trial. Run it alone, as hard as you can hold for the full 30 minutes, and average your heart rate over the last 20 minutes. Most people go too hard in the first few minutes, so start a shade conservative.

In a 2005 study of 27 competitive distance runners and triathletes, heart rate from a 30-minute time trial did not differ significantly from heart rate at a lactate threshold measured directly in a criterion test, with a standard error of the estimate of 8.0 beats a minute. The criterion was a blood lactate of 4.0 mmol/L, and the authors concluded that coaches and runners should consider the 30-minute time trial as a way to estimate the heart rate at the lactate threshold². Runsense reads the trial slightly differently, as described below, so this supports the test and does not validate that exact reading.

Here are the steps Runsense gives you. Heat is on the list of reasons to wait, because, as a 2015 review notes, in the heat heart rate rises out of proportion to the work being done³:

• Start your watch when you begin the warm-up and keep that one recording going to the end of the test.
• Warm up for 15 minutes: start very easy and build so the last few minutes are real work you could still speak in full sentences through. A few 20-second strides at the end are fine.
• Then run 30 minutes as hard as you can hold *steadily*. Start a shade conservative and lift the effort a little every 5 minutes, so you are going all out for the last 10 to 15. Going out too hard and fading reads back low, so pace it like a hard but even race effort.
• Run solo, on flat, uninterrupted ground, well rested, and do not watch your heart rate during the effort. Runsense reads it from the recording.
• When the 30 minutes are up, end the workout and cool down on a separate recording.
• If it is really hot, you are under the weather or you cannot get a clean flat route, skip it and reschedule.

When to test, and when not to

Run a test when a new number can change how your zones are set. A threshold reading sets the zones for a whole block of training, so the test belongs where a new number can still do that work.

Come to it fresh, the way you would come to a race. Runsense's rule is that the two days before a test hold no hard work: easy running, a walk, mobility or a short strength routine are all fine. When Runsense puts a test on your plan, it eases any hard session of your coach's in those two days to an easy run, and if you do your own intense training, such as CrossFit, your coach takes it into account when choosing the day. A test run on tired legs reads your threshold low, and a low number mis-sets every zone built on it.

No race anywhere near it. Runsense never puts a test within 21 days of a race, before it or after it, whether it is your goal race or one you have just run. A test is a 30-minute maximal effort. In the weeks before a race it cuts into the race's preparation, and in the weeks after one it reads a recovering athlete low.

You need to be able to run 30 minutes without walking, because the test is 30 minutes of hard running and nothing shorter is the same test.

Test from steady training. A near-maximal half hour belongs in running that has been consistent for a few weeks: not in your first weeks of running, and not straight back from a break, an illness or an injury, when it asks for your hardest effort before your legs are ready for it. Get a couple of weeks of steady running back first, then test.

A hard stretch inside another run is not a test

Your watch has recorded plenty of hard minutes that were never a threshold test: the climb in a long trail run, a surge in a race, the last mile of a tempo. A number read off one of those stretches would anchor your zones to an effort you never held steadily. Late in a long run there is a second problem: a 2001 review found that in prolonged exercise at a constant rate, heart rate rises progressively as stroke volume falls, and the fall starts after 10 to 20 minutes⁴.

Runsense reads your history for runs that were a test in all but name. A run counts when it is an hour or less; its hardest twenty minutes are on flat ground and start after at least ten minutes of warm-up; its best half hour is within five beats of its best twenty minutes; and those twenty minutes are within ten beats of the hardest twenty minutes your watch has recorded in the last six months. A steady easy run on flat ground can have the shape without the effort, so it does not count. Only a run like that can be filed as your threshold from your history; everything else is an observation about your training, and your coach will say so.

How the result is read

Runsense sets aside the first 15 minutes of the recording, your warm-up, and takes the highest 20-minute average heart rate in the half hour that follows. The usual do-it-yourself version averages the last 20 minutes. When heart rate climbs and then holds, the highest 20 minutes is close to the last 20, it leaves out the opening minutes while heart rate is still rising to the work, and a cool-down recorded into the same file cannot pull it down.

Runsense counts a reading as measured for the training block it was set in. A number you entered, one estimated from a run, or a test from an earlier block sets your zones until a test in your current block replaces it.

Let Runsense run it with you

Your coach considers putting a test on your plan when you are two weeks or more into a training block, you have written to your coach in the last five days (a run alone does not count), your last two weeks of recordings include a run with heart rate detailed enough to read a test from, and no test this block has measured your threshold. When it is the moment, your coach adds the test itself and tells you what it changed and why. The test lands at least three days ahead, so the easy days before it are still to come, and, if you have a goal race, at least six weeks before it, so you get weeks of training on the new zones. You can always ask your coach for a test yourself.

It goes in place of one of your coach's upcoming runs, never your long run, a race, a rest day or a session of your own. The full steps are on the test in your plan, word for word. Run it as written and Runsense reads your threshold straight from the recording, then proposes the number for you to confirm before it sets your zones. Change your mind beforehand and tell your coach: the test comes off and everything it replaced goes back on your plan, and your coach will not put another on unasked until your next training block.

If you have had a lab test

A lab test raises intensity step by step while blood lactate is sampled, and the threshold is then picked from the lactate curve, as a 2009 review describes¹. Because many threshold concepts are in use, a lab number depends on which one the lab used. A lab test takes an appointment and costs money, and Runsense does not require one.

If you have had one, enter the number in the app's settings under Heart rate, in the Lactate threshold row, and your zones update straight away. Runsense counts only a test it reads from your own recording as measured, so your coach can still put the field test on your plan, and its reading replaces the number you entered.