Interval Training Explained: VO2 Max, Threshold and Sprint Sessions

"Do intervals" is one of the most common pieces of training advice, and one of the least specific. An interval session could be 30-second sprints with four-minute rests, five-minute efforts at a pace you could race for an hour, or anything in between. Each one trains something different, and doing the wrong kind at the wrong time is a fast way to get tired without getting faster.

This guide breaks interval training down into its main types, explains what each one improves, how hard each should feel, how often to do them, and how to fit them around the easy training that makes up most of a good week. There are sample sessions for runners, cyclists, swimmers and team-sport players at the end.

What is interval training?

Interval training alternates periods of harder effort with periods of easier recovery. The recovery lets you repeat efforts at an intensity you couldn't hold continuously. Four 4-minute efforts at a hard pace add up to 16 minutes at that intensity, while most people could only hold the same effort for 8 or 10 minutes in one go.

That's the main point of intervals: more time at the intensity that drives a specific adaptation, without the fatigue of one long, all-out effort.

You'll see different names for the same ideas. High-intensity interval training (HIIT) usually means efforts near your maximal aerobic capacity. Sprint interval training (SIT) means all-out efforts, usually 30 seconds or less. Threshold or tempo intervals are longer and slightly less intense.

The main types of intervals

Threshold intervals

Your lactate threshold is roughly the fastest pace you can sustain without lactate building up rapidly in your blood. It's a strong predictor of performance in events from 10 km to the marathon, and in long cycling efforts.

Threshold intervals are long efforts, 3 to 10 minutes or more, at a pace that feels comfortably hard. You can say a few words but not hold a conversation. Example: 4 × 8 minutes with 2 minutes easy jogging between.

These sessions are tiring but not wrecking. Many coaches see them as the backbone of training for half-marathon and marathon runners.

VO2 max intervals

VO2 max is the most oxygen your body can use per minute, the ceiling on your aerobic engine. These intervals spend as much time as possible close to that ceiling. Efforts of 2 to 5 minutes at a pace you could hold for around 5 to 8 minutes in total, with recoveries similar to the effort length.

The best-known example comes from a 2007 study by Jan Helgerud and colleagues. Moderately trained young men did one of four programmes for eight weeks. The groups doing 4 × 4 minutes at 90 to 95% of maximum heart rate, with 3 minutes active recovery, or short 15-second intervals, improved VO2 max significantly more than groups doing long slow distance or threshold running at the same total work.

Short HIIT

Short intervals, such as 30 seconds hard and 30 seconds easy, repeated many times, can keep your heart rate and oxygen use high for long periods because the recoveries are too short to bring them down fully. They're popular with team-sport players and in group classes, and they suit people who find long hard efforts mentally tough.

Martin Buchheit and Paul Laursen's influential 2013 reviews on programming HIIT describe how changing rep length, intensity and recovery shifts the balance between aerobic stress, anaerobic stress and muscular load. Short intervals with short recoveries are mainly aerobic. All-out sprints with long recoveries are much more anaerobic and neuromuscular.

Sprint interval training

All-out efforts of 10 to 30 seconds with long recoveries of 2 to 4 minutes. Classic examples include 4 to 6 × 30-second all-out cycling sprints.

Work summarised by Martin Gibala and colleagues in 2012 showed that low-volume sprint interval training can produce surprisingly large improvements in aerobic fitness and muscle adaptations in a short time, even though the total exercise time is small. It's demanding, though. All-out efforts hurt, and they put a lot of load through muscles and tendons, particularly in running.

Strides

Strides aren't really intervals, but they're worth including. Short accelerations of 15 to 20 seconds, fast but relaxed, with full recovery. Done at the end of easy runs, they help running form and leg turnover without adding much fatigue. They're a good first step into faster running for beginners.

How much interval training should you do?

Less than most people think. As our Zone 2 training guide explains, Stephen Seiler's research on elite endurance athletes found that roughly 80% of their sessions are done at low intensity. Most of the rest are hard sessions like intervals.

For recreational athletes, that translates to:

• Training 3 to 4 times a week: one interval session.
• Training 5 to 6 times a week: one or two interval sessions.
• Training more than that: two, occasionally three in specific phases.

The rest of the week should be easy enough that you arrive at your interval sessions fresh. Hard sessions only work if you can actually push in them.

HIIT versus steady cardio

This debate generates far more heat than it deserves.

A 2015 meta-analysis by Zoran Milanović and colleagues compared HIIT with continuous endurance training. Both improved VO2 max, and HIIT produced slightly larger gains on average. A 2017 meta-analysis by Michael Wewege and colleagues, in people with overweight or obesity, found that HIIT and moderate-intensity continuous training produced similar reductions in body fat, with HIIT taking less time.

So HIIT is efficient, especially for VO2 max and when time is short. But it's also harder to recover from, harder to stick with, and can't be done every day. Steady training builds the aerobic base, is easy to recover from and can be done in large volumes. Most good programmes use both, in the right proportions.

How to pace intervals

The most common mistake is going too hard on the first rep and fading badly on the last. A well-paced session should feel progressively harder, with the last rep close to the same pace or power as the first.

• Use effort first: the 1 to 10 effort scale works well. Threshold around 7, VO2 max around 8 to 9, sprints 10.
• Heart rate lags: during short intervals, heart rate takes 1 to 2 minutes to rise, so it's a poor guide for reps under 3 minutes. Use pace, power or effort.
• Finish with one left: you should feel you could have done one more rep at the same quality.
• Adjust for conditions: on hot, windy or hilly days, go by effort rather than pace.

Warm-up and cool-down

A proper warm-up makes the first rep feel better and lowers injury risk. A good template:

1. 10 to 15 minutes of easy running, cycling or swimming.
2. A few mobility drills or dynamic stretches (leg swings, skips, lunges).
3. 3 to 4 short pickups at the target interval effort, with full recovery.

Cool down with 10 minutes of easy movement afterwards.

Sample interval sessions

For runners

For cyclists

• 3 × 10 minutes at threshold, 5 minutes easy between.
• 5 × 4 minutes at VO2 max effort, 4 minutes easy.
• 30-second hard / 30-second easy for 10 minutes, twice, with 5 minutes easy between.
• 6 × 30-second all-out sprints, 4 minutes easy.

For swimmers

• 10 × 100 m at threshold pace, 15 seconds rest.
• 8 × 50 m fast, 30 seconds rest.
• 5 × 200 m hard, 30 seconds rest.

For team-sport players

• 15 seconds hard / 15 seconds easy for 8 minutes, two to three sets with 3 minutes rest.
• 6 × 30 m sprints with a walk-back recovery, two sets.
• Small-sided games, which provide high-intensity intervals with a ball.

Building intervals into a training block

Progress interval sessions gradually, like any training. A simple progression over six weeks for a runner doing one session a week:

Keep strength training going alongside intervals. Our strength training guide explains how to schedule lifting around hard sessions.

Hill intervals and fartlek

Not every interval session needs a track or a stopwatch.

Hill intervals

Running hard uphill builds strength, power and aerobic fitness with less impact than flat sprinting, because you can't land as hard. Find a hill of moderate gradient that takes 30 to 90 seconds to run up. Run up at a hard effort, jog or walk down to recover, and repeat 6 to 10 times. Hill sessions are a good way to introduce intervals for beginners and returning runners.

Fartlek

Fartlek, Swedish for "speed play", mixes faster and slower running without fixed distances. On an easy run, pick a landmark and run hard to it, then jog until you've recovered, then pick another. It's less structured than intervals, which makes it a good option when you want some intensity without the pressure of hitting splits.

Intervals for beginners: your first month

If you've never done structured intervals, start gently. You need a few weeks of regular easy exercise first, so your muscles, tendons and heart are ready for harder efforts.

"Comfortably hard" means you can say a few words but wouldn't want to chat. You should finish each session feeling you could have done more.

Intervals for older athletes

Intervals are valuable for masters athletes, because VO2 max and top-end speed decline with age and high-intensity work helps slow that down. A few adjustments help:

• Longer warm-ups: 15 to 20 minutes before hard efforts.
• More recovery between hard sessions: often 72 hours rather than 48.
• Lower-impact options: hill repeats, cycling or rowing intervals reduce stress on joints.
• Regular strength training: to protect muscles and tendons from the extra load.

If you have a heart condition or other health concerns, talk to your doctor before starting high-intensity training.

Signs you're doing too much

Intervals are stressful by design. The benefits only come if you recover from them. Watch for:

• Easy runs feeling much harder than usual.
• Resting heart rate noticeably higher than normal for several days.
• Poor sleep, low mood or irritability.
• Failing to hit paces you managed easily a few weeks ago.
• Persistent muscle soreness or niggles.

If several of these appear together, take an easier week with no intervals at all. Fitness won't disappear in a week, but pushing through accumulated fatigue often leads to illness or injury.

Common mistakes

• Too many hard sessions. Three or four interval sessions a week leaves most amateurs exhausted and stuck.
• Easy days too hard. If easy runs creep into moderate effort, you won't recover enough to push in intervals.
• Going out too fast. The first rep should feel controlled.
• Skipping the warm-up. Your first rep will feel terrible and injury risk rises.
• Same session every week. Vary the type of intervals to keep improving.
• Under-fuelling. Hard sessions need carbohydrate. Our fuelling guide covers what to eat beforehand.

The bottom line

Intervals are the sharpest tool in endurance training. Used in small amounts, on top of a solid base of easy training, they improve speed and aerobic capacity faster than almost anything else. Overused, they leave you tired and flat.

Pick one interval session that matches your goal, do it once a week, pace it so the last rep is as good as the first, and keep the rest of your week easy. That's most of what you need.

Frequently asked questions

Sources and further reading

1. Buchheit M, Laursen PB. High-intensity interval training, solutions to the programming puzzle: Part I: cardiopulmonary emphasis. Sports Medicine, 2013;43(5):313-338. DOI PubMed
2. Milanović Z, et al. Effectiveness of High-Intensity Interval Training (HIT) and Continuous Endurance Training for VO2max Improvements: A Systematic Review and Meta-Analysis of Controlled Trials. Sports Medicine, 2015;45(10):1469-81. DOI PubMed
3. Helgerud J, et al. Aerobic high-intensity intervals improve VO2max more than moderate training. Medicine and Science in Sports and Exercise, 2007;39(4):665-71. DOI PubMed
4. Gibala MJ, et al. Physiological adaptations to low-volume, high-intensity interval training in health and disease. The Journal of Physiology, 2012;590(5):1077-84. DOI PubMed
5. 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):276-91. DOI PubMed
6. Wewege M, et al. The effects of high-intensity interval training vs. moderate-intensity continuous training on body composition in overweight and obese adults: a systematic review and meta-analysis. Obesity Reviews : An Official Journal of the International Association for the Study of Obesity, 2017;18(6):635-646. DOI PubMed