Improve VO₂Max with HIIT

High-intensity interval training (HIIT) alternates demanding exercise with rest or light activity. Some studies have found larger increases in VO₂Max than with moderate continuous exercise matched for exercise volume. However, the intensity is high, and study protocols cannot be recommended unchanged to everyone. This page separates the conditions tested in research from general safety principles.

What is HIIT?

Interval exercise repeats intense efforts separated by short recovery periods. A review distinguishes near-maximal HIIT from sprint interval training (SIT), performed above the workload that elicits maximal oxygen uptake. Studies mainly involving healthy people have reported increases in VO₂Max and muscle mitochondrial content with both approaches. Adaptation depends on duration and frequency as well as intensity (MacInnis & Gibala, 2017).

Study protocol 1: 4 × 4 minutes

One study assigned physically active male university students to four exercise methods, matching total work and frequency over 3 sessions a week for 8 weeks. The analysis included 40 participants with a mean age of 24.6, with 10 in each group, exercising on an inclined treadmill. The results and safety cannot be assumed to apply equally to inactive people, older adults or people with medical conditions (Helgerud et al., 2007).

• Study protocol: Four 4-minute running intervals at 90–95% of maximum heart rate, with 3 minutes of light jogging at 70% between intervals. Including a 10-minute warm-up and 3-minute cool-down, each session took 38 minutes.
• Results: VO₂Max relative to body weight increased from 55.5 to 60.4 mL/kg/min. The increase was significantly greater than with continuous running at 70% of maximum heart rate or at 85% (the lactate threshold), matched for total work.

A review of 53 randomized controlled trials (1,514 participants) in adults without disease classified interval length, total high-intensity work per session and intervention duration separately. “High volume” meant at least 15 minutes of high-intensity work per session, excluding recovery; “low volume” meant 5 minutes or less. Advantages over moderate continuous exercise appeared in categories such as long intervals of at least 2 minutes and high volume, but results varied by population. This does not establish that combining these conditions is optimal for everyone (Wen et al., 2019).

Study protocol 2: short intervals

• 15 seconds / 15 seconds: In the same study, participants repeated 47 intervals of 15 seconds running at 90–95% of maximum heart rate, followed by 15 seconds of active recovery at a speed corresponding to 70%. Including a 10-minute warm-up and 3-minute cool-down, this took about 37 minutes. VO₂Max relative to body weight increased from 60.5 to 64.4 mL/kg/min. The increase was greater than with continuous running, but the difference in improvement between this group and the 4 × 4-minute group was not statistically significant (Helgerud et al., 2007).
• 20 seconds / 10 seconds × 7–8 bouts: This was the study behind the format commonly called “Tabata.” Participants used a cycle ergometer at a workload with an oxygen demand corresponding to approximately 170% of VO₂Max. Among 7 participants completing the 6-week intervention, VO₂Max increased by 7 mL/kg/min and anaerobic capacity by 28% (Tabata et al., 1996). The author's later account describes 4 days a week of 7–8 bouts of 20 seconds of exercise and 10 seconds of rest. The remaining day combined 30 minutes of continuous exercise at 70% of VO₂Max with 4 bouts that did not reach exhaustion (Tabata, 2019).

Although the repeated 20/10-second intervals took about 4 minutes, the overall intervention was not “just 4 minutes every day.” Warm-up, cool-down and the continuous exercise performed on another day need to be considered separately. The same timing does not guarantee the same effects if the workload or activity differs. In Wen et al. (2019), short HIIT intervals of 30 seconds or less improved VO₂Max compared with no exercise, but their advantage over moderate continuous exercise was unclear.

Understanding the study intensity through heart rate

The 4 × 4-minute study targeted 90–95% of measured maximum heart rate. An age-based estimate—208 − 0.7 × age (Tanaka et al., 2001)—also exists, but can differ from an individual's actual value. The following calculation illustrates the percentages; it is not an exercise target for you.

Example: at age 40, estimated maximum heart rate is 180 beats/min. Its 90–95% range is 162–171 beats/min, and the 70% recovery level is 126 beats/min.

• An age-based estimate or the highest value recorded by a smartwatch alone cannot establish a safe exercise intensity. Do not push yourself excessively to find your maximum heart rate.
• Heart rate rises with a delay after exercise begins. It is normal not to reach the target at the start of an interval.
• Beta blockers, some calcium-channel blockers and alpha blockers affect heart rate. If you take medication, consult your doctor first about appropriate intensity indicators. We do not recommend replacing a heart-rate target with a self-selected “hard” or “very hard” effort to perform HIIT (MHLW 2023, INFORMATION 4).

Frequency and duration in the studies

• Helgerud et al. (2007) used 3 sessions a week for 8 weeks. Tabata et al. (1996) used 5 days a week for 6 weeks, combining different sessions within the week. Neither study established a starting exercise dose for the general public.
• In a review, longer interventions tended to be associated with greater increases in VO₂Max (Milanović et al., 2015).
• If tiredness after vigorous exercise interferes with daily life the next day, the intensity or amount was excessive. Rest and do less next time (MHLW 2023, INFORMATION 4).

Exercising safely

• Always warm up. For exercise of a certain intensity, warming up helps prevent injuries and medical events.
• Do not stop abruptly. Cool down for 5–10 minutes afterward to help prevent dizziness or fainting.
• Symptoms that mean stop. Stop immediately if you develop chest pain, cold sweats, palpitations, unusually severe tiredness, dizziness, unsteadiness or severe joint or muscle pain.
• Check today's condition. If any daily-check item applies—such as fever, feeling sluggish, feeling unwell after insufficient sleep or chest pain—do not push yourself; rest.

The guidance above comes from MHLW INFORMATION 4. See Starting exercise safely for details.

A review of 23 studies comparing HIIT and moderate continuous exercise in outpatient cardiac rehabilitation for people with coronary artery disease or heart failure included 1,117 participants, of whom 547 were in HIIT groups. There was 1 major cardiovascular event related to HIIT, equivalent to 1 event per 17,083 sessions (Wewege et al., 2018). These results were obtained under medical supervision and do not establish safety when exercising alone.

Separate study protocols from your own starting point

If you do not exercise regularly, the basic approach is to begin at low intensity for short periods, then increase gradually while checking how you feel and your next-day tiredness. The studies described here did not test general starting rules such as how many weeks to wait before HIIT or how many intervals to begin with.

Whether to add HIIT, and at what workload, depends on your exercise habits and health. If you have a medical condition, take medication or have symptoms that concern you, consult your doctor first. If you are unsure how to start, also consult an exercise professional.