The short answer
The interval that started it: 4 x 4 minutes
In a well-known study, 40 moderately trained men were split into four groups and trained three times a week for eight weeks. Every group did the same total amount of work, at different intensities1:
| Group | Training | VO2max after 8 weeks |
|---|---|---|
| Long slow distance | Continuous running at 70% of max heart rate | Smaller change |
| Lactate threshold | Continuous running at 85% of max heart rate | Smaller change |
| 15/15 intervals | 15 s at 90% to 95% of max heart rate, 15 s easy | Up 5.5% |
| 4 x 4 intervals | 4 min at 90% to 95% of max heart rate, 3 min easy jog | Up 7.2% |
Same total work, different results: both interval groups raised VO2max significantly more than the two steady groups. The likely reason is time spent near your maximum. VO2max is mostly limited by how much blood the heart can pump at its peak2, and hard intervals hold you close to that peak for longer than you could manage in one continuous effort.
What the wider research says
One study is a starting point, not a rule. A meta-analysis of 28 controlled trials in healthy adults found that both continuous endurance training and interval training raised VO2max by a large amount compared with not training, with intervals a little ahead: about 5.5 ml/kg/min versus 4.9 ml/kg/min3. Another meta-analysis found that programs with longer intervals, of several minutes, produced especially large gains, with nearly every participant responding4.
So easy running works, especially for beginners. Intervals work a little better and take less time. The best plan uses both.
Why most of your running should still be easy
In a study of well-trained endurance athletes, a nine-week "polarized" block, mostly easy training with a smaller share of hard intervals and little in between, raised VO2peak by 11.7%. That was more than blocks built mostly on threshold work, mostly on high-intensity work, or mostly on high volume5. Easy running builds the aerobic base cheaply and leaves you fresh enough to run the intervals hard.
A sample week
This is the pattern our VO2max Engine program follows, for runners who already run three to four times a week.
| Day | Session |
|---|---|
| Tuesday | 4 to 5 x 3 to 4 min hard (about 90% to 95% of max heart rate), equal easy jog between |
| Wednesday | 30 to 40 min easy, conversational |
| Thursday | Threshold: 3 to 4 x 6 min at a "comfortably hard" pace, 90 s easy between |
| Saturday | 35 min easy, plus a few short uphill strides |
| Sunday | Long easy run, building from about 50 to 85 min over the block |
How hard is 90% to 95%?
Close to your 3K to 5K race pace. You can say a word or two, not a sentence, and the last minute of each rep is hard. Every rep should be about the same pace; if the last one is much slower than the first, start the next session a little easier.
How to progress
- Start with three hard reps and add one every week or two, up to five or six.
- Never put two hard days back to back.
- Every few weeks, run a lighter week to absorb the work.
- After six to eight weeks, repeat a 12-minute run test and compare.
Try this workout
4 x 4 minute intervals
Who it is for: Runners already running three or four times a week. New to running? Build an easy habit first.
- 1Warm up. 10 to 15 minutes of easy running.
- 2Main set. 4 x 4 minutes at 90% to 95% of max heart rate, close to 3K to 5K race pace, with 3 minutes of easy jogging between. This raised VO2max 7.2% in eight weeks 1.
- 3Keep it even. Every rep at about the same pace. If the last is much slower than the first, start easier next time.
- 4Cool down. Easy running or walking until your breathing settles.
- 5Each week. Once or twice, with most other running easy 5, and never two hard days back to back.
Want it planned week by week? VO2max Engine builds on this session.
Put it into practice
Sources
All research- 1Aerobic high-intensity intervals improve VO2max more than moderate trainingHelgerud et al., 2007. Medicine & Science in Sports & Exercise 39(4):665-671.
- 2Limiting factors for maximum oxygen uptake and determinants of endurance performanceBassett and Howley, 2000. Medicine & Science in Sports & Exercise 32(1):70-84.
- 3Effectiveness of high-intensity interval training (HIT) and continuous endurance training for VO2max improvements: a systematic review and meta-analysis of controlled trialsMilanović et al., 2015. Sports Medicine 45(10):1469-1481.
- 4VO2max trainability and high intensity interval training in humans: a meta-analysisBacon et al., 2013. PLOS ONE 8(9):e73182.
- 5Polarized training has greater impact on key endurance variables than threshold, high intensity, or high volume trainingStöggl and Sperlich, 2014. Frontiers in Physiology 5:33.
General fitness information, not medical advice. If you have an injury or a health condition, check with a professional before starting a new kind of training.