What is threshold running?

Threshold running is running at or just under your second lactate threshold, the highest intensity at which the lactate your muscles produce is cleared as fast as it arrives. Above that line lactate accumulates and the effort can't be held for long. At the line, the effort is hard and it holds.

A tempo run is the coaching name for a continuous run at that intensity, so the two words name one effort. In the vocabulary of Norwegian singles, the same line is LT2 and the work just under it is sub-threshold.

Is a threshold run the same as a tempo run?

Yes, by the definitions of the coaches who named the session. A tempo run is a continuous run at threshold.

Jack Daniels wrote that tempo runs are steady, moderately prolonged runs that runners and coaches define differently. Ideally, he wrote, a tempo run "is nothing more than a steady 20-minute run at T-pace". T-pace is his name for threshold pace, and he describes the effort as "comfortably hard". Pete Pfitzinger, in his road racing and marathon books, defines tempo runs as continuous runs at lactate-threshold pace.

Some newer coaching writing puts tempo a step under threshold, somewhere between the two lactate thresholds, or describes the two words as points on a range rather than two separate sessions. That's a drift in how the word is used. The threshold is one line, and a run at it is the same effort whichever word the plan uses.

So when one plan says tempo and another says threshold, read them as the same session unless the plan defines tempo differently.

What's the difference between LT1 and LT2?

They're two different lines on the same lactate curve. LT1 is the intensity where blood lactate first rises above its resting level. LT2 is the highest intensity at which lactate production and clearance still balance, so blood lactate holds steady instead of climbing.

The older names are aerobic threshold for LT1 and anaerobic threshold for LT2, and first and second lactate threshold are the same two lines again. A 2023 review puts it this way: intensities up to the first threshold could be held for hours. The second threshold is the maximal lactate steady state, the highest constant workload at which production and clearance stay in equilibrium.

The threshold running you searched for is LT2. Sub-threshold means just under LT2, not just under LT1, which matters because the word threshold on its own could mean either line.

The lactate number at the second threshold is a convention rather than a constant. A 2009 review found 25 different threshold concepts in the literature, in three families: fixed lactate values, the first rise above baseline, and the steady-state or curve-shape methods. The fixed 4 mmol/L line is one of them. Measured directly in 165 recreational runners, the second threshold fell at 3.2 ± 0.6 mmol/L.

What heart rate zone is a threshold run?

The top of Zone 4 on the five-zone model your watch uses, and the Zone 3/4 line on the Norwegian five-zone scale.

In 165 recreational runners, the second lactate threshold fell at 90.6 ± 2.5% of maximum heart rate (women 91.7 ± 2.2, men 89.9 ± 2.4). Under heart rate reserve it fell at 87.0 ± 3.4% (women 88.1 ± 3.1, men 86.3 ± 3.4) in the same runners.

On the five-zone bands RunBeat and most devices use, Zone 4 runs 80 to 90% and Zone 5 starts at 90%, on either method. So the threshold sits at the Zone 4/5 line under percent of max heart rate and in the upper part of Zone 4 under heart rate reserve. That is the same place in the same runners, in different digits, because the two methods count their percentages from different floors. The HR zone calculator turns either percentage into beats per minute from your own maximum and resting heart rate.

The Norwegian scale is built on the thresholds themselves. Zones 1 and 2 sit below LT1, Zone 3 runs from LT1 to LT2, and Zones 4 and 5 sit above LT2. So the threshold is its Zone 3/4 line by construction. In a 2025 survey of endurance practitioners using a five-zone scale, the running zone-4 floor was placed at 86.6 ± 4.1% of maximum heart rate and the zone-5 floor at 92.3 ± 3.5%. Those are self-reported placements, from 298 respondents analysed for the boundaries, 101 of them runners.

"Zone 3" on the Norwegian scale and "Zone 4" on the device model name the same band, between the two thresholds. Both are conventions drawn on a continuous curve, which is why they can disagree on the digit and agree on the line.

What heart rate counts as sub-threshold, and the field test that finds your own threshold heart rate, are the next numbers to pin down.

What does threshold training do?

It has been shown to raise the lactate threshold in sedentary people, and it hasn't been shown to raise VO₂ max.

In the trial that held total work constant across four running intensities, 40 moderately trained young men trained for 8 weeks. The group running at 85% of maximum heart rate, the trial's lactate-threshold group, showed no significant change in VO₂ max and none in stroke volume.

In a 9-week trial of 48 well-trained endurance athletes (runners, cyclists, triathletes, and cross-country skiers), one arm trained 54 ± 7% of its hours at lactate-threshold intensity and none at high intensity. That arm showed no significant change in VO₂ peak. Its velocity or power at 4 mmol/L moved +1.4 ± 4.3%, not significant, against +8.1 ± 4.6% in the trial's polarized arm.

Whether it moves your threshold depends on who's training. A meta-analysis of 85 study groups from 34 studies found training near the lactate or ventilatory threshold an adequate stimulus for improving the thresholds in sedentary subjects. Conditioned subjects showed nonsignificant gains, and "a higher intensity may be necessary" for them.

So the controlled evidence that threshold work moves a trained runner's threshold is thin. Norwegian singles is one method built around sub-threshold work, and what its rules are and what its evidence shows are separate questions. Running a sub-threshold session by heart rate, rep by rep, is the next step after knowing where the line sits.

How does threshold running fit with Zone 2 and VO₂ max training?

Between them. A training week is built as an easy base with intensity on top, and threshold work is the middle layer. It is harder than the easy running that makes up most of the week and easier than the VO₂ max intervals at the top of it.

Most of the week is Zone 2 running, easy enough to recover from and to pile up. VO₂ max intervals sit at the top, run at 90 to 95% of maximum heart rate. Threshold runs sit under them, at an intensity you can hold far longer.

How much of the week goes to threshold has a ceiling. A systematic review of middle- and long-distance runners' training concluded that at least 70% of volume at low intensity and at most 30% at threshold and above is necessary to optimize endurance adaptation. Which of the three deserves more of your week is a separate decision.

Zone 2 and Zone 4 both run in RunBeat's Zone Training:

Zone Training in RunBeat RunBeat tells you to speed up or slow down to stay in zone, right over your music or podcast.