High-intensity interval training

Vigorous activity is usually defined as about 6 METs or more, six times resting oxygen use, while moderate activity begins at about 3 METs [7]. On the talk test, vigorous effort allows only a few words between breaths. Running, fast cycling, uphill walking, hard swimming and many competitive sports reach this level.

HIIT arranges vigorous effort into repeated work bouts, ranging from under a minute to several minutes, each followed by easier recovery. Sprint interval training uses very short, near-maximal sprints. In the Generation 100 trial, the interval sessions were set at about 90% of peak heart rate twice a week, compared with moderate continuous sessions at about 70% [4].

The World Health Organization physical activity guidelines recommend 150 to 300 minutes of moderate or 75 to 150 minutes of vigorous aerobic activity a week, or an equivalent mix, plus muscle-strengthening activity [1]. The two-to-one equivalence between vigorous and moderate time is the main reason HIIT is described as time-efficient.

The clearest effect of HIIT is on cardiorespiratory fitness. A 2024 umbrella review identified 24 systematic reviews with meta-analyses, representing 429 primary studies and 12,967 participants. Compared with no exercise, HIIT increased VO2 max by 3.25 to 5.5 mL/kg/min (weighted mean differences across reviews); compared with moderate continuous training, it added 0.52 to 3.76 mL/kg/min. The effect was seen in apparently healthy adults, adults with overweight or obesity, older adults and high-level athletes, and across formats including sprint interval training [2].

Most of the included reviews received moderate to critically low scores on the AMSTAR-2 appraisal tool, so the direction of the effect is consistent while the exact size of the advantage over moderate training is uncertain [2].

Because VO2 max is one of the strongest predictors of mortality in observational data [8], a faster rise in fitness per minute of training is the main proposed route by which HIIT could affect lifespan. No trial has shown that the additional fitness gained from HIIT translates into longer life.

Results in this section are reported as hazard ratios (the risk in one group relative to another; 0.80 means 20% lower). In the Nurses’ Health Study and Health Professionals Follow-up Study, 116,221 US adults were followed for 30 years with 47,596 deaths. Meeting the vigorous guideline of 75 to 149 minutes a week was associated with 19% lower all-cause mortality (hazard ratio 0.81, 95% CI 0.76 to 0.87), 31% lower cardiovascular mortality (0.69, 0.60 to 0.78) and 15% lower non-cardiovascular mortality (0.85, 0.79 to 0.92). The near-maximum association was at about 150 to 300 minutes a week, and higher amounts showed neither clear further benefit nor harm [3].

Among 55,137 US adults followed for a mean of 15 years (3,413 deaths), runners had 30% lower all-cause and 45% lower cardiovascular mortality than non-runners, corresponding to about three years of additional life expectancy. The association was similar across quintiles of running time, distance, frequency and speed, and running less than 51 minutes a week or at speeds under 6 miles per hour was sufficient [9]. Running was self-reported on a medical history questionnaire.

The Generation 100 study randomly assigned 1,567 adults aged 70 to 77 (790 women) in Trondheim, Norway, to two supervised sessions a week of HIIT (400 participants), two sessions a week of moderate continuous training (387), or advice to follow national physical activity guidelines (780), for five years. All-cause mortality did not differ between the control group and the two exercise groups combined. Mortality in the control group was 4.7%. Compared with controls, HIIT had a hazard ratio of 0.63 (95% CI 0.33 to 1.20), an absolute reduction of 1.7 percentage points, and moderate training a hazard ratio of 1.24 (0.73 to 2.10). Compared with moderate training, HIIT had a hazard ratio of 0.51 (0.25 to 1.02). None of these differences reached statistical significance [4].

Several features limit what the trial can show. At baseline, 87.5% of participants reported good overall health and 80% reported medium or high activity levels, deaths were few, and control participants chose to do more of their activity as high-intensity exercise than the moderate-training group, so the control group ended up exercising at an intensity between the two trained groups [4]. The trial is therefore a weak test of exercise against no exercise, and its HIIT result is a trend rather than a demonstrated effect.

Vigorous effort also occurs outside structured exercise, for example climbing stairs quickly, hurrying for a bus or carrying heavy loads. A UK Biobank analysis examined this vigorous intermittent lifestyle physical activity in 25,241 adults (mean age 61.8) who reported no leisure-time exercise and wore a wrist accelerometer; 852 died over a mean of 6.9 years [6].

Compared with no such bursts, 1.5 one-minute bursts a day was associated with 25% lower all-cause mortality (hazard ratio 0.75, 95% CI 0.66 to 0.85), the sample median of three bursts a day with 39% lower (0.61, 0.50 to 0.74), and the sample maximum of 11 bursts a day with 48% lower (0.52, 0.37 to 0.72). The median of three bursts a day was also associated with 48 to 49% lower cardiovascular mortality [6]. The study was observational with a short follow-up, and people able to make brief vigorous efforts may be healthier in ways the analysis could not fully capture.

Vigorous exertion transiently raises the risk of sudden cardiac death. In a case-crossover analysis within the Physicians’ Health Study, 21,481 US male physicians initially free of cardiovascular disease were followed for 12 years, during which 122 sudden deaths occurred. The relative risk of sudden death during and up to 30 minutes after vigorous exertion was 16.9 (95% CI 10.5 to 27.0) compared with other times. The absolute risk was very low, about one sudden death per 1.51 million episodes of vigorous exertion, and men who exercised vigorously more often had a smaller rise in risk during each episode [5].

The study included only men, so its figures do not directly apply to women. Its finding that habitual exercisers have a smaller transient rise in risk underlies the common guidance to increase intensity gradually over weeks rather than starting with maximal intervals. People with known heart disease or with symptoms such as chest pain on exertion are generally advised to seek medical assessment before vigorous training. Steadier aerobic exercise is described under Zone 2 training, and vigorous recreational activity under sports and active hobbies.