How to find your threshold pace for interval training

Once you know your threshold pace, you can plan your intervals. You know how fast each rep should be, and you notice when you're running them too fast. Getting the number takes one hard effort on a flat road. No lab.

What the threshold is

Think of it as the pace you could hold for about an hour if you had to. Just below it, the lactate in your blood rises a little and then settles. Above it, lactate keeps climbing, and sooner or later you slow down. A review of critical speed calls this boundary between heavy and severe intensity the critical speed. Races from 800 m up to about 10K are run above it, the marathon below it, and anything above it can only be held for a limited time (Jones 2017).

Sports science has produced plenty of threshold definitions. Faude and colleagues compared 25 of them and found that most correlate strongly with endurance performance, especially in running. Their reference is the maximal lactate steady state, the highest workload at which lactate stays stable over about 30 minutes, and several simple concepts estimate it usefully (Faude 2009). They all describe roughly the same zone. For everyday training, a practical estimate you can repeat is enough.

Three ways to find your threshold pace without a lab

A recent 10K race is the easiest source. As a coaching rule of thumb, 10K race pace sits close to threshold. A 10K is run just above critical speed (Jones 2017), so race pace lands close to threshold and a little above it. Race results hold up in research too. In a study of 27 runners and triathletes, the VDOT method, which works from race times, estimated the velocity at the lactate threshold with no significant difference from the lab test (McGehee 2005). Use a race you paced evenly on a flat course.

The 30-minute solo test comes from the same study. Warm up thoroughly, then run 30 minutes as fast as you can hold at an even pace. Your average pace for the last 20 minutes is your threshold pace, and your average heart rate over those same 20 minutes is your threshold heart rate. In McGehee's group, this time trial estimated both the velocity and the heart rate at the lactate threshold (4 mmol/L) with no significant difference from the lab. The Conconi test did worse in the same comparison (McGehee 2005).

Your best 20-minute pace from a recent hard session or race is the rougher option. The shorter the effort, the further the pace you can hold sits above critical speed (Jones 2017), and 20 minutes is shorter than a 10K, so this estimate tends to come out a bit fast. Treat it as a starting point until you have a race or a proper test.

The things that go wrong are predictable. Starting too fast is the classic one. You bank time early and pay for it late, and the average then says more about your pacing than your fitness. Hills and wind bend the pace in both directions, so pick a flat loop or a track on a calm day. Heat pushes heart rate up at the same output (Coyle 2001), which inflates the threshold heart rate. And a test on tired legs, the day after intervals or a heavy leg session, gives you a number from a bad day. Put an easy day or two in front of it.

Turning the number into running zones

With the number in hand, the other paces follow. Easy runs sit well slower. Tempo runs sit just below threshold: comfortably hard, sustainable for a long stretch. Threshold intervals run right at it, usually in reps of several minutes with short jogs between. VO2max intervals go faster, in shorter reps with longer breaks. Written as percentages of your threshold, the zones all move together when a new test changes the number.

A test beats a formula here. Jamnick and colleagues reviewed how well common intensity prescriptions separate the intensity domains. Fixed percentages of maximal heart rate or VO2max did it poorly, and thresholds from a test, lactate or ventilatory, did it better (Jamnick 2020). Two runners at the same percentage of maximal heart rate can be working in quite different domains. Zones tied to your measured threshold start from your own physiology.

Then there's the easy end of the scale. Seiler found that well-trained endurance athletes distribute their sessions roughly 80 to 20: about 80 percent easy, below the first lactate threshold, and about 20 percent hard. More hard sessions brought no further gain in that group (Seiler 2010). In practice, easy has to mean easy. Easy runs that creep toward tempo pace add hard work nobody planned, and the intervals tend to suffer for it.

This gets sharper when running shares the week with other sports. Heavy leg days and hard runs draw on the same legs, so the way you combine strength training and running deserves as much planning as the intervals themselves. If you're building a week around both, the article on hybrid training covers how strength and endurance fit into one plan.

When to find the number again

Your threshold moves as you train. Retest after a few weeks of consistent work, or earlier if your threshold intervals start to feel easier at the same pace. After a break of several weeks, test again before you go back to your old paces. And if a 10K is on your calendar anyway, that race can be the next test.

Running with Tahto works with a measured number or without one.

Heart rate makes a useful second measure, as long as you know its limits. At the same pace, it varies with daily form, heat, hydration, altitude and fatigue (Achten 2003). Over long durations and in the heat, it climbs at a steady output, a drift that gets stronger as you lose fluid (Coyle 2001). On a hot afternoon late in a long run, expect it to read high. And if you're tempted to skip the test, a maximal heart rate estimated from age can be off by about ten beats either way (Achten 2003).