You might not need to block out hours for exercise after all. Researchers at the Norwegian University of Science and Technology say 30 minutes of high-intensity exercise a week can improve health, as long as the effort is hard enough to leave you noticeably out of breath.

That adds up to about 4.5 minutes a day, or roughly 10 minutes every other day, according to the researchers. They say the key is intensity, not duration.

If someone uses a heart rate monitor, their heart rate should reach about 85 percent of their maximum. Without equipment, the researchers say a simple test is talking. You should still be able to speak in short sentences, but not sing or comfortably keep up a continuous conversation.

“The biggest reported challenge regarding exercise is lack of time. But with intense, short workouts, this is no longer a valid excuse,” said Ulrik Wisløff, a professor at NTNU and head of CERG, a research group that studies the health effects of exercise.

Wisløff said cardiovascular fitness has a major effect on long-term health.

“Cardiovascular fitness is the best indicator of current and future health. Good cardio fitness reduces the risk of over 30 lifestyle diseases as well as premature death by 40 to 50 percent,” he said.

CERG first documented these effects in a 2006 study that analysed health information from 60,000 people. The university said later large studies from Norway and other countries reported similar findings.

The researchers said it is better to spread short, hard sessions across the week than do one longer workout.

“It is best to spread the sessions out a bit, because exercise also has an acute effect that lasts for one to two days, so that way you get the best of both worlds,” Wisløff said.

They said blood pressure and blood sugar control improve for 24 to 48 hours after a workout that is hard enough to make someone breathless for several minutes. Because of that short-term effect, experts recommend dividing exercise across two to four days a week when possible.

High intensity does not have to mean sprinting flat out or using the heaviest resistance on an exercise bike.

“No. Your own personal fitness level determines what gives you a high heart rate. If you are not very fit, simply taking a brisk walk may be enough. Having said that, you need to walk fast enough that you get quite out of breath. You can then increase the intensity as your fitness improves. Short intervals are effective. For example, this could be bursts of 45 seconds with 15-second breaks. Or like in Tabata workouts, with intense 20 second intervals interspersed with 10 second breaks. Otherwise, 4×4 intervals are recognized as highly effective for increasing oxygen uptake,” Wisløff said.

The researchers also said fitness cannot be stored up by doing extra exercise one week to make up for doing none the next.

“No. Fitness is something you have to maintain. Cardio fitness and strength decline quickly when not maintained, especially as you get older,” said Atefe R. Tari, a researcher and head of CERG’s initiative on exercise and brain health.

Tari said physical activity also benefits the brain.

“Physical health and brain health are closely linked, and cardio fitness is key here as well. Exercise leads to the formation of new brain cells,” she said.

Strength training also matters, especially for middle-aged and older adults, according to the researchers.

“We know that strength training is important, especially for middle-aged and older adults. There is limited research on how strength training affects lifespan, but a HUNT study exploring this is due to be published soon,” Wisløff said.

NTNU and CERG researchers have also developed a new measurement system called AQ, or Activity Quotient, which tracks exercise intensity through heart rate instead of focusing mainly on steps or workout time. The university said scientists built the AQ algorithm using population data from five countries, including Norway.

AQ can be measured through the Mia Health app, developed by NTNU and Sintef through the spin-off company Mia Health. The app connects with heart rate monitors, but users can also enter activities manually.

Researchers said AQ points build up when physical activity raises heart rate enough to make someone slightly out of breath. They found that people who reached at least 25 AQ points a week significantly lowered their risk of lifestyle-related diseases, and the strongest benefits appeared at 100 AQ points or higher.

In one study on AQ and health, the researchers analysed data from more than 0.5 million people. The university said the study found a strong link between higher AQ scores, better cardiovascular fitness and better health outcomes.

Wisløff and Tari are now urging Norwegian health authorities to put more emphasis on exercise intensity in the country’s official advice. They have also written the book Mikrotrening, which draws on research reviews suggesting that brief bursts of high-intensity activity may deliver greater health benefits than spending many hours each week on low- to moderate-intensity exercise.

“Norway could become a pioneer with regard to health benefits from micro workouts,” said Wisløff.

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