Why Two HIIT Workouts a Week Should Be Part of Your Routine

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It's been eight years since a team of Norwegian scientists from NTNU's Cardiac Exercise Research Group began collecting data on the effects of exercise training on all-cause mortality in older adults (70-77 years) for an ongoing longitudinal study called Generation 100.

To date, this is one of the largest (N = 1,567) randomized and controlled clinical trials to investigate how exercise affects mortality and morbidity in the elderly. The underlying research question for this study, which began in 2012, was simple: "Can exercise help people live longer?" After almost a decade, the results are in: These research findings (Stensvold et al., 2020) were published on October 7 in the BMJ.

The CERG team led by Dorthe Stensvold of the Norwegian University of Science and Technology found that although moderate-intensity continuous training (MICT) has many health benefits, high-intensity interval training (HIIT) twice a week was associated with even more health benefits and slightly longer life expectancy. As the authors explain: "[W]e observed a lower all-cause mortality trend after HIIT compared with controls and MICT." The YouTube video below sums up the findings of this study.

"First of all, I have to say that exercise, in general, seems to be good for the health of the elderly. And our study results show that on top of that, training regularly at high intensity has an extra positive effect," Stensvold said in a news release. "Among most 70-77-year-olds in Norway, 90 percent will survive the next five years. In the Generation 100 study, more than 95 percent of the 1,500 participants survived!"

"Both physical and mental quality of life were better in the high-intensity group after five years than in the other two groups," she added. "High-intensity interval training also had the greatest positive effect on fitness."

For this randomized controlled trial, one group of study participants was randomly assigned to do a "4×4" method of HIIT training twice a week; each high-intensity interval training workout included a warmup/cooldown period and took about 30 minutes total.

As you can see in the video above, the 4×4 starts with a 10-minute warmup, which is followed by four high-intensity intervals. Each 1-2 minute burst of high-intensity exertion at about 90 percent of maximum heart rate (a "hard" 16 on the 6-20 Borg Scale of perceived exertion) is followed by a 3-minute "active break" period at about 60 percent of maximum heart rate to remove accumulated lactic acid buildup. After the four intervals are complete, a 5-minute cooldown concludes each 4×4 interval training session.

The MICT group was instructed to work out two days per week for 50 minutes at a steady, moderate-intensity of about 70 percent of peak heart rate, which corresponded to a 13 (i.e., "somewhat hard") perceived exertion rating on the Borg Scale. Participants in the HIIT and MICT cohorts could choose whether they wanted to work out independently or participate in group, instructor-led training sessions.

Members in the third "control" group weren't offered organized training sessions or given specific guidance about exercise intensity or duration; they were simply advised to exercise in accordance with the recommended physical activity guidelines set forth by the Norwegian health authority. "This meant that the controls achieved an exercise dose at an intensity between the MICT and HIIT groups," the authors noted.

In terms of answering the million-dollar question: "Does this study offer definitive proof that exercise prolongs life?" Stensvold responded: "I'd like to answer with a clear and unequivocal 'yes,' because we believe that this is true. But training is probably not the only reason why so few of the Generation 100 participants died compared to what's expected in this age group."

Although HIIT training may not be a silver bullet that is causally proven to significantly increase longevity, the results of this study provide new evidence that over five years, high-intensity interval training can improve overall fitness, boost quality of life, and may reduce the risk of premature death more than moderate-intensity continuous training alone.

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"Our hope is that the national recommendations for physical activity will be modified to encourage older people even more strongly to do high-intensity training—either as their only form of exercise or to supplement more moderate training," Stensvold concluded.

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