You ran a track test and got a threshold pace of 4:00/km. Your lab test or watch says 4:06. Neither is wrong. Critical speed reads faster than lactate thresholds in most runners, and the average gap is about 6 to 8 seconds per km.
That gap matters when you set tempo runs. Pick the wrong number and your “steady” 40 minutes turns into a grind.
Critical speed vs lactate threshold: two different lines
Critical speed (CS) is the fastest pace your body can hold with oxygen use staying steady. You find it from maximal efforts over several distances. Lactate threshold is the pace where blood lactate starts to climb faster than your body clears it. Labs find it with a stepped treadmill test.
One is a breathing line. The other is a chemistry line. They sit close together, but not on top of each other.
Think of a car engine. CS is the redline where the engine still runs smoothly. Lactate threshold is where the temperature gauge starts creeping up. You can drive past the gauge for a while. You can’t do it all day.
What the 2026 study found in 31 runners
Rainsberger and colleagues tested 31 runners, 52% of them women. Three groups took part: trained, highly trained and World Class. Their peak aerobic speeds were 15.5, 17.5 and 19.8 km/h. Everyone did a CS test and a lactate treadmill test about four days apart.
CS came out faster than both lactate methods. It beat modified D-max by 0.47 km/h and OBLA (onset of blood lactate accumulation) by 0.36 km/h. In short: CS read faster, and the gap didn’t care how fast the runner was.
The 95% confidence intervals were 0.16 to 0.78 km/h for D-max and 0.05 to 0.67 km/h for OBLA. Translation: the true gap could be small, but it’s very unlikely to be zero or negative.
How big is the gap in pace?
Speeds in km/h hide the real story. You train in minutes per km, so convert. These are my own conversions of the study’s numbers, not figures the authors reported.
pace (s/km) = 3600 / speed (km/h)
15.00 km/h = 240.0 s/km = 4:00/km
15.47 km/h = 232.7 s/km (about 7 s/km faster)
15.36 km/h = 234.4 s/km (about 6 s/km faster)
At a 4:00/km pace, the CS offset is about 6 to 8 seconds per km, or roughly 2 to 3% of speed. At a slower 4:17/km pace (14 km/h) the same 0.47 km/h gap is about 8 seconds per km.
Seven seconds per km sounds tiny. Over a 10 km tempo, it’s over a minute.
The gap for you can be much bigger
The average hides a lot. The study also gave limits of agreement, the range that covers most individual runners. For modified D-max, CS minus the lactate number ran from -0.54 to +1.43 km/h. For OBLA it was -1.40 to +1.96 km/h.
At a 4:00/km reference, +1.96 km/h means about 28 seconds per km faster. -1.40 km/h means about 25 seconds per km slower. So the two tests can disagree a lot for one person even when they agree on average. In short: don’t assume your own gap equals the average.
Why the MLSS test changes the answer
Maximal lactate steady state (MLSS) is the fastest pace where lactate stays flat for a long run. Two studies compared it with CS. They disagreed.
Nixon and colleagues tested 10 well-trained men. CS was 16.4 km/h and MLSS was 15.2 km/h. That’s 1.2 km/h faster, or about 7.9% of MLSS (my conversion). At 16.4 km/h the pace is 3:39 per km. At 15.2 km/h it’s 3:57. Lactate rose over time at only 0.5 km/h above MLSS, yet oxygen use stayed steady. CS marks where oxygen use stops being steady, not where lactate does.
de Lucas tested 8 male runners. CS was 15.2 km/h. Intermittent-running MLSS was 15.3 km/h. Continuous MLSS was 14.4 km/h. So CS matched the stop-and-go version and sat about 0.8 km/h (roughly 5.6%, my conversion) above the continuous one.
Hold that extra speed for an hour and it bites. Hold it for six minutes and it doesn’t.
Which number to set your paces from
Here’s a simple map. Offsets use a 15 km/h CS (4:00/km) as a reference. They mix results from different studies, so treat them as a rough guide.
| Method | Offset from CS | Pace at 15 km/h CS | Best use |
|---|---|---|---|
| CS, track or 3-distance test | 0 | 4:00/km | Ceiling for hard work, 3-8 min intervals |
| OBLA | -0.36 km/h | about 4:06/km | Tempo and cruise intervals |
| Modified D-max | -0.47 km/h | about 4:08/km | Tempo and cruise intervals |
| Continuous MLSS (de Lucas) | -0.8 km/h | about 4:13/km | Long steady threshold blocks |
| Continuous MLSS (Nixon) | -1.2 km/h | about 4:21/km | Long steady threshold blocks |
Steady tempo goes with the lower numbers. Hard intervals go with CS. You can read more on how the zone count shapes this in 3-zone vs 5-zone training.
Heavy long efforts also fade. See the 10% decay rule for threshold pace for what happens late in a session.
How to test critical speed
You have three practical options.
- Track, three distances. One published protocol uses 1200, 2400 and 3600 m, aimed at about 3, 7 and 12 minutes. Rest 30 minutes between efforts (Galbraith, 2011). The 2026 study used 1000, 2000 and 3200 m.
- 3-minute all-out test. One all-out effort that some labs use as a shortcut. It’s quicker, but check it against a time-trial result before you trust it for zones.
- Race results. Two recent hard efforts at different distances give a rough line.
Your testing surface changes numbers too. Track vs treadmill lactate testing covers that. And if your watch says one thing, here’s why Garmin’s threshold pace can run 25% off.
A quick case study: Dana, 36
Dana trains for a spring half marathon on 40 km a week. Her track CS test gave 15.0 km/h (4:00/km). She set tempo runs at 4:00/km, since that was her “threshold.”
Her 6 km tempos fell apart by km 4. Heart rate climbed and her breathing got ragged. This is an illustrative example, but it’s a common pattern.
She switched tempo to 4:08/km, the modified D-max style offset, and kept 4:00/km for 4 x 5 minute intervals. The tempo runs felt steady. The intervals stayed hard but repeatable. Same test, different job for each number.
Hard but steady is a skill. The right pace makes it learnable.
Mixing tests is where zones go wrong
Say you set zones from a lab lactate test in spring. In autumn you run a track CS test and drop the new number into the same table. Every zone just shifted 6 to 8 seconds per km, and nothing in your training changed.
Label the source. If you change methods, rebuild the table. If you want a close look at how two-zone and base models fit around these numbers, see how Zone 2 compares with LT1. Cyclists face the same split with power: critical speed vs FTP covers that case.
Where AthleteOS fits
AthleteOS keeps one threshold pace for you and builds every pace zone in your training plan from it. That’s the point: pick your test on purpose, enter that number, and your tempo days and intervals stay consistent. If you switch methods, update the number and the zones rebuild together. You can start a free trial here.
One caution on sources: these numbers come from study abstracts, so the details are worth checking in the full papers.
Lactate keeps rising at critical speed. Plan your tempo accordingly.