Repeated 1-minute bursts of high-intensity interval training (HIIT) are more effective than conventional moderate, continuous exercise for improving aerobic fitness after stroke, according to a multicenter randomized controlled trial.

“We hoped that we would see improvements in cardiovascular fitness after HIIT and anticipated that these improvements would be greater than in the moderate-intensity group, but we were pleasantly surprised by the degree of improvement we observed,” Ada Tang, PT, PhD, associate professor of health sciences at McMaster University in Hamilton, Ontario, Canada, told Medscape Medical News. “The improvements seen in the HIIT group were twofold higher than in the other group.”

The results were published on August 7 in Stroke.

Clinically Meaningful

Researchers compared the effects of 12 weeks of short-interval HIIT with those of moderate-intensity continuous training (MICT) on peak oxygen uptake (V̇O₂peak), cardiovascular risk factors, and mobility outcomes after stroke.

They randomly assigned participants to receive 3 days per week of HIIT or traditional moderate exercise sessions for 12 weeks. Participants’ mean age was 65 years, and 39% were women. They enrolled at a mean age of 1.8 years after sustaining a mild stroke.

A total of 42 participants were randomized to HIIT and 40 to MICT. There were no significant differences between the groups at baseline, and both groups exercised on adaptive recumbent steppers, which are suitable for stroke survivors with varying abilities.

The short-interval HIIT protocol involved 10 1-minute intervals of high-intensity exercise, interspersed with nine 1-minute low-intensity intervals, for a total of 19 minutes. HIIT intervals targeted 80% heart rate reserve (HRR) and progressed by 10% every 4 weeks up to 100% HRR. The low-intensity intervals targeted 30% HRR.

The traditional MICT protocol for stroke rehabilitation targeted 40% HRR for 20 minutes and progressed by 10% HRR and 5 minutes every 4 weeks, up to 60% HRR for 30 minutes.

The HIIT group’s cardiorespiratory fitness levels (V̇O₂peak) improved twice as much as those of the MICT group: 3.5 mL of oxygen consumed in 1 minute per kg of body weight (mL/kg/min) compared with 1.8 mL/kg/min.

Of note, changes in V̇O₂peak from baseline remained above the clinically important threshold of 1.0 mL/kg/min at 8-week follow-up in the HIIT group (1.71 mL/kg/min) but not in the MICT group (0.67 mL/kg/min).

Both groups increased their 6-minute walk test distances by 8.8 m at 12 weeks and by 18.5 m at 20 weeks. No between-group differences were found for cardiovascular risk or mobility outcomes, and no adverse events occurred in either group.

On average, the HIIT group spent 36% of total training time exercising at intensities > 80% HRR throughout the intervention, while the MICT group spent 42% of time at intensities of 40%-59% HRR.

The study was limited by a small sample size of high-functioning individuals who sustained a mild stroke. Enrollment was halted for 2 years due to the COVID-19 lockdowns, limiting the study’s statistical power.

Nevertheless, the authors concluded, “Given that a lack of time is a significant barrier to the implementation of aerobic exercise in stroke clinical practice, our findings suggest that short-interval HIIT may be an effective alternative to traditional MICT for improving V̇O₂peak after stroke, with potential clinically meaningful benefits sustained in the short-term.”

“Our findings show that a short HIIT protocol is possible in people with stroke, which is exciting to see,” said Tang. “But there are different factors that clinicians should consider before recommending this training for their patients, such as their health status and their physical status. Stroke rehabilitation specialists, including stroke physical therapists, can advise on how to proceed to ensure the safety and effectiveness of HIIT.”

Selected Patients May Benefit

“Broad implementation of this intervention may be premature without further research,” Ryan Glatt, CPT, senior brain health coach and director of the FitBrain Program at Pacific Neuroscience Institute in Santa Monica, California, told Medscape Medical News. “The study focused on relatively high-functioning stroke survivors, which raises questions about the applicability of the results to those with more severe impairments.” Glatt did not participate in the research.

“Additional studies are needed to confirm whether these findings are applicable to more diverse and severely affected populations and to assess the long-term sustainability of the benefits observed,” he said. “Also, the lack of significant improvements in other critical outcomes, such as mobility, suggests limitations in the broader application of HIIT for stroke rehabilitation.”

“While HIIT shows potential, it should be approached with caution,” Glatt continued. “It may benefit select patients, but replacing traditional exercise protocols with HIIT should not be done in all cases. More robust evidence and careful consideration of individual patient needs are essential.”

This study was funded by an operating grant from the Canadian Institutes of Health Research. Tang reported grants from the Canadian Institutes of Health Research, the Physiotherapy Foundation of Canada, and the Heart and Stroke Foundation of Canada. Glatt declared no relevant financial relationships.

Marilynn Larkin, MA, is an award-winning medical writer and editor whose work has appeared in numerous publications, including Medscape Medical News and its sister publication MDedge, The Lancet (where she was a contributing editor), and Reuters Health.