Any exercise is usually considered better than no exercise, but a new study indicates interval training — interspersing high and low speed levels during activities such as biking — is best at reversing age-related declines in muscle cells.

It has long been determined that the health benefits of exercise can include increased metabolism (the rate at which what we eat is converted to energy), a boosted immune system, and brain and heart health. But researchers in the U.S. recently set out to determine why exercise and which forms of it bring those benefits and how.

The researchers studied 72 men and women — half of them aged 18 to 30, and the other half 65 to 80. The volunteers had no chronic illnesses and were assigned to three different exercise programs:

High-intensity intervals of four-minute sprints on a bike, interspersed with light pedalling for three minutes.

Strength training with weights.

A combination of interval training and strength training.

Participants in the combination group were also sedentary for three months as a control.

In this week's online issue of the journal Cell Metabolism, Matthew Robinson, an assistant professor of kinesiology at Oregon State University, and his colleagues at the Mayo Clinic in Rochester, Minn., report that high-intensity interval training showed the biggest benefits at the cellular level, especially in the older adults.

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"It shows that the older adults still retain the ability to adapt to exercise," Robinson said in an interview. "The older adults were able to turn on a lot of the proteins, particularly the mitochondria, and that's good from an aging perspective."

As we age, the number of mitochondria — the powerhouses of our cells — in muscles tend to drop, and they don't function as well.

"What we were able to show is that with exercise training, particularly with this high-intensity aerobic exercise, you get pretty robust improvements in mitochondrial proteins along with their function."

The findings are important from a public health perspective, the study's authors said, given differences in how people respond to exercise. Some respond well, others more moderately and others not as well.

Older adults still retain the ability to adapt to exercise. - Matthew Robinson, researcher

Previous research has determined people with Type 2 diabetes who participate in interval walking — rather than just maintaining the same speed — get the most benefits in terms of improving fitness and reducing blood sugar, said Martin Gibala, a professor and chair of kinesiology at McMaster University in Hamilton. Gibala, who studies interval training, was not involved in the U.S. study in Cell Metabolism.

He urges anyone considering incorporating interval training in their fitness routines to first check with their physicians, as interval training involves pushing yourself a little harder at times during your workouts.

"Get out of your comfort zone," said Gibala said. "For some people whose only exercise is walking around the block, the advice to them is as simple as it sounds. Pick up the pace for a few light posts and then back off."

Comprehensive measures

He said that during interval workouts, which can involve working at intense levels, you may feel slightly out of breath and subjectively like your heart rate and breathing rate are higher.

Scientists in the Oregon-Minnesota study measured changes in exercise participants' muscle mass, strength and aerobic capacity. They also took muscle and blood samples to take detailed measures, such as what proteins were made and how quickly, as well as to see how well the body responded to insulin — a hormone made by the pancreas that allows the body to properly use sugar from carbohydrates in food.

High-intensity exercise in particular increased insulin sensitivity in all age groups, a positive change, said Robinson.

Gibala praised the researchers' comprehensive measurements and demonstrations in older people.

Strength training also helped to build muscle mass, the researchers found.

Canadian guidelines for healthy adults aged 18 to 64 recommend 150 minutes of moderate to vigorous exercise (such as walking quickly enough to talk but not sing) or 75 minutes of vigorous exercise (such as running or playing basketball) per week, along with two to three weekly sessions of strength training for muscle and bone health.