Zone 2, threshold, and VO2 max work are the three intensities a running week is built from. Easy running in Zone 2 makes up most of the week. Threshold work sits in Zone 4, at or just under the second lactate threshold, whether your plan calls it threshold or tempo. VO2 max intervals climb into Zone 5.
You train all three. The question is what to do on your hard days: threshold work or VO2 max intervals. That depends on your goal and your hours.
What does each kind of training change?
Only the intervals have been shown to raise VO2 max. Threshold work has raised the lactate threshold in people who weren't training, and easy running builds the volume the other two sit on.
Easy running in Zone 2 didn't raise VO2 max in the trial that held total work equal across four running intensities. Easy running buys you volume you can absorb, recovery between hard days, and consistency. In the one heart-rate-controlled trial of sub-elite runners, the group that kept more of its running easy improved a simulated race more.
Threshold training has raised the lactate threshold in sedentary people, and in conditioned subjects the gains were nonsignificant. It hasn't been shown to raise VO2 max.
VO2 max intervals in Zone 5 raised VO2 max in that same trial, and raised stroke volume with it.
Should your hard days be threshold or VO2 max intervals?
Intervals in Zone 5 if you're training to raise your VO2 max. Sub-threshold reps in Zone 4 if you're racing 5K to the half and want to hold a hard pace for longer.
If you're chasing a higher VO2 max, you go to Zone 5 because it's the only intensity shown to raise it. The session is the Norwegian 4x4.
If you're racing, your hard days go to sub-threshold reps in Zone 4, because a distance race is run on the speed you can hold at the threshold. A 2000 review of what limits endurance performance concluded that the speed at the lactate threshold is the best physiological predictor of distance running performance.
When your week has room for both, run both. In a 9-week trial of 48 well-trained athletes from four endurance sports, the group that ran mostly easy and added hard intervals gained most, both in VO2 peak and at the threshold.
You can run both hard days in RunBeat's Timed Intervals. RunBeat calls every rep and tells you which zone you're in:
Timed Intervals in RunBeat
RunBeat calls every work and rest phase, and your Apple Music playlists switch with it.
Does how much you run change which to train?
Yes. On about three hours a week the split between easy and hard stops separating the results. From five to nine hours a week there's a method built for the hours.
At about three hours a week, runners who put half their training between their two thresholds improved as much as runners on a mostly-easy plan, in 17% less time. For most recreational runners that band is Zone 4. VO2 max didn't change in either group. At that volume the shape of your week is yours to choose, and your hard day's intensity still follows your goal.
Norwegian singles is the method built for five to nine hours a week: three sub-threshold sessions in Zone 4, with every other run in Zone 2. It schedules no Zone 5 work until progress on the reps stops.
Above that, look at what elite runners do. In athletes training 10 to 13 times a week, about 80% of sessions are low intensity. World-class distance runners keep at least 80% of their running volume low intensity. A systematic review of middle- and long-distance runners concluded that at least 70% of volume at low intensity and at most 30% at threshold and above is necessary to optimize endurance adaptation.
Two shapes fit a week built on an easy majority: a Zone 2 base block with hard work on top, or an interval block on a base you already have.
Is Zone 3 bad?
Not in itself. The gray zone is two different intensities under one label.
The first lactate threshold sits in the upper part of Zone 3 for most recreational runners. On the Norwegian five-zone scale, "zone 3" is the band between the two thresholds, which is Zone 4. So when gray-zone advice means the band between the thresholds, it's talking about Zone 4, and sub-threshold work sits there on purpose.
In the one heart-rate-controlled trial of sub-elite runners, the arm that spent more of its time between the thresholds improved a simulated race less. Among recreational marathoners, more training in the middle and upper zones of a three-zone scale went with slower finishing times. That's a correlation across the group. And at about three hours a week, runners who put half their training between their two thresholds improved as much as runners on a mostly-easy plan.
A long run that sits in Zone 3 is race-specific work, a different session from the Zone 2 long run.
Sources
- Festa L, Tarperi C, Skroce K, La Torre A, Schena F. Effects of Different Training Intensity Distribution in Recreational Runners. Frontiers in Sports and Active Living. 2020;1:70. doi:10.3389/fspor.2019.00070. PMC7739641. Full text read. 43 recreational runners recruited and 38 analyzed, 19 per arm, each with more than four years of training, 3.2 ± 0.5 hours a week before the study, and a half marathon raced the week before it began. Eight weeks on a polarized distribution (77/3/20 by time) or a focused one with half its time between the thresholds (40/50/10), the three zones set by heart rate at the first ventilatory threshold and the respiratory-compensation point, the two programs balanced for training impulse. Both arms improved velocity at VO2 max (+3.2 and +4.0%), at the ventilatory threshold (+4.0 and +3.2%), at the respiratory-compensation point (+5.7 and +3.4%), 2 km velocity (+3.5 and +3.0%), and running economy (−5.3 and −8.7%), polarized and focused respectively, with no difference between the groups on any parameter except total training time (29.9 ± 3.1 against 24.8 ± 2.0 hours). Conclusion, in the authors' words: "Focused Endurance Training obtains similar improvements than Polarized Endurance Training saving 17% of training time in recreational runners." VO2 max did not change in either arm. The three-zone model is the study's, not the five-zone model on a watch.
- Stöggl T, Sperlich B. Polarized training has greater impact on key endurance variables than threshold, high intensity, or high volume training. Frontiers in Physiology. 2014;5:33. doi:10.3389/fphys.2014.00033. PMC3912323. Full text read. 48 well-trained runners, cyclists, triathletes, and cross-country skiers randomized to four 9-week programs, 41 completing. Source for the polarized arm's gain, stated qualitatively above. One trial, and its athletes weren't all runners.
- Bassett DR Jr, Howley ET. Limiting factors for maximum oxygen uptake and determinants of endurance performance. Medicine & Science in Sports & Exercise. 2000;32(1):70-84. doi:10.1097/00005768-200001000-00012. PMID 10647532. Full text read. A review. The sentence above is its abstract's, verbatim: "The speed at lactate threshold (LT) integrates all three of these variables and is the best physiological predictor of distance running performance", the three variables being VO2 max, the fraction of it a runner can hold, and running economy. The review notes that the correlation was found across runners of differing ability and shrinks among runners of similar ability. A predictor, not a training effect: the review says nothing about threshold training raising that speed, and the controlled evidence that it does in an already trained runner is thin. Nothing else is taken from it.
- Esteve-Lanao J, Foster C, Seiler S, Lucia A. Impact of training intensity distribution on performance in endurance athletes. Journal of Strength and Conditioning Research. 2007;21(3):943-949. doi:10.1519/R-19725.1. PMID 17685689. Abstract read. 12 sub-elite runners over 5 months of heart-rate-controlled training, the study's three zones set by the ventilatory thresholds. Stated qualitatively above, no figure.
- Muniz-Pumares D, Hunter B, Meyler S, Maunder E, Smyth B. The Training Intensity Distribution of Marathon Runners Across Performance Levels. Sports Medicine. 2025;55(4):1023-1035. doi:10.1007/s40279-024-02137-7. PMID 39616560. Full text read. 151,813 marathons by 119,452 Strava-using runners; a three-zone model anchored on critical speed, with the boundary between its first two zones estimated rather than measured per runner. The relationship between more middle- and upper-zone training and slower times is a correlation.
- Campos Y, Casado A, Vieira JG, et al. Training-intensity Distribution on Middle- and Long-distance Runners: A Systematic Review. International Journal of Sports Medicine. 2022;43(4):305-316. doi:10.1055/a-1559-3623. PMID 34749417. Abstract read. 20 articles. The share above is the review's conclusion, in its "necessary to optimize" framing.
- Seiler S. What is best practice for training intensity and duration distribution in endurance athletes? International Journal of Sports Physiology and Performance. 2010;5(3):276-291. doi:10.1123/ijspp.5.3.276. PMID 20861519. Abstract read. The share counts sessions, not time, in nationally and internationally competitive endurance athletes training 10 to 13 times a week.
- Haugen T, Sandbakk Ø, Seiler S, Tønnessen E. The Training Characteristics of World-Class Distance Runners: An Integration of Scientific Literature and Results-Proven Practice. Sports Medicine - Open. 2022;8:46. doi:10.1186/s40798-022-00438-7. PMC8975965. Full text read. 59 world-leading athletes and 16 coaches of podium contestants; low intensity is the bottom two of the paper's seven zones. The share describes practice; the paper's session counts and weekly kilometers are not taken.