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Short bursts of intense exercise may better improve cardiovascular fitness among stroke survivors than continuously exercising at a more moderate level, new research finds, adding to what's known about the role exercise can play in stroke recovery.

"Exercise is a really crucial part of rehabilitation," said study co-author Dr. Ada Tang, a physical therapist and professor of rehabilitation science at McMaster University in Hamilton, Ontario, Canada. "We know that exercise is beneficial to people with heart disease, but the body of research for how it can benefit people with stroke is much smaller. We need to promote exercise to people living with stroke and to the health professionals who guide them in their recovery."

Tang and her colleagues compared the effects of 12 weeks of high-intensity interval training, or HIIT, to moderate-intensity continuous training on cardiovascular fitness, risk factors and mobility among stroke survivors. The were published Thursday in the American Stroke Association journal Stroke.

"We found high-intensity exercise is better, but any type of aerobic exercise is good for improving heart health after a stroke," said lead author Dr. Kevin Moncion, a physiotherapist and post-doctoral fellow at McGill University in Montreal.

Every year, more than 795,000 people in the U.S. have strokes, which occur when a blood vessel carrying oxygen to the brain is blocked or ruptures. Nearly 1 in 4 strokes happen to people who previously had one. Research shows that controlling stroke risk factors – such as managing hypertension and diabetes through reduced sedentary time sedentary time and engaging in vigorous physical activity – is key to preventing a second stroke. Physical activity also has been shown to improve a survivor's mobility, walking, balance and mental state.

But how much and what type of physical activity can best aid in stroke recovery is still being studied.

In the new study, researchers analyzed the effect of the two types of exercise patterns on 82 stroke survivors with an average age of 65 and whose first stroke was six to 60 months prior to enrollment. Participants were randomly assigned to HIIT sessions or moderate-intensity continuous exercise three times a week for 12 weeks. All participants used a recumbent stepper, an exercise machine that simulates walking up stairs while seated.

The continuous exercise group used the machine for up to 30 minutes each session. The HIIT group used it for a total of 19 minutes, with 1 minute of high-intensity exercise followed by 1 minute of lower-intensity movement repeated 10 times each session. Exercise sessions for both groups included a 3-minute warmup and 2-minute cooldown.

Researchers compared the effects of each type of exercise on participants' cardiovascular fitness levels. To determine cardiovascular fitness, they measured VO2 peak – the maximum amount of oxygen a person's body can use during intense exercise – along with cardiovascular risk factors, including blood pressure, waist-hip ratio, arterial stiffness and mobility. Measurements were taken after 12 weeks of exercise and again after eight weeks of follow-up.

Both groups improved cardiovascular fitness, but people in the HIIT group improved their fitness level twice as much as those in the other group. Peak oxygen uptake increased by an average 3.52 milliliters per minute per kilogram of body weight (mL/kg/min) for people in the HIIT group, compared to 1.76 mL/kg/min for those in the moderate-intensity group.

Fitness levels remained high for the HIIT group even after eight weeks of follow-up, but they started to decline in both groups after the formal exercise programs ended. The HIIT group, however, remained above the threshold that was clinically significant.

Both groups showed similar improvements in mobility and no significant differences in cardiovascular risk factors.

"We were anticipating that HIIT would be beneficial," Tang said. "But the level of improvement was a pleasant surprise."

The findings make an important contribution to what's known about the role exercise plays in stroke recovery, said Dr. Mona Bahouth, an associate professor of neurology and medical director of the Brain Rescue Unit at the Johns Hopkins School of Medicine in Baltimore.

"Everybody can exercise in some form," said Bahouth, who was not involved in the study. "I feel strongly that we should start progressive exercise after a stroke, while the person is still in the hospital, even if the stroke is severe."

The challenge has been instructing people how much and how hard they should exercise after a stroke, she said.

"We use the term 'exercise' very loosely when we talk to patients, telling them they need to exercise 20 to 30 minutes a day," Bahouth said. "But that does not get at how intense that should be, what the target heart rate should be, whether it's safe for them to do at a certain level."

There are many factors to consider when determining how much a stroke survivor should exercise, she said. A major factor is how much a person was exercising before their stroke. Another is the severity of their stroke and what complications they are experiencing, such as whether the person is experiencing paralysis or dizziness.

Adaptive equipment allows patients to move at least some parts of their bodies with different levels of intensity, and therapists and nurses can assist those who need it, Bahouth said. Not moving at all can cause weight gain, muscles to atrophy, blood clots in the legs or arms, and poor cardiovascular conditioning.

While their study did not investigate the maximum intensity or duration for a stroke survivor, Moncion and Tang agreed that post-stroke exercise had no inherent limits.

"Pushing yourself is not dangerous, as long as you're in tune with your capacity," Moncion said.

Bahouth said stroke survivors should work with their health care team to develop an appropriate exercise plan.

Even if someone doesn't exercise immediately following a stroke, "it's never too late to start," Moncion said. "Recovery is a continuum. There's always room for improvement."