Redefining Intensity in Neurorehabilitation

High-intensity interval training (HIIT) is often associated with elite athletics or metabolic conditioning classes. But for stroke survivors, it may be one of the most powerful tools in our rehabilitation arsenal. Recent evidence highlights HIIT’s superior ability to promote neuroplasticity, improve cardiovascular health, and drive meaningful functional recovery. New articles demonstrate the training often outperforms traditional moderate-intensity aerobic training (MAT) protocols in several critical domains.

In the context of stroke rehabilitation, HIIT refers to structured bouts of activity such as walking at or near a patient’s maximum safe speed with brief recovery intervals. This model enables higher aerobic intensities than patients could otherwise sustain in continuous training, and as studies have shown, the payoffs can be profound.

What Does HIIT Really Look Like? Parameters That Define the Protocol

For clinicians unfamiliar with integrating HIIT into neuro rehab, it’s essential to understand what constitutes "high intensity." According to Boyne et al. (2023), HIIT for stroke survivors was defined by achieving an average heart rate above 60% of the heart rate reserve (HRR). Sessions included:

• 30-second bursts of walking at the individual's maximum safe speed

• 30- to 60-second rest intervals, either standing or seated

• A total session time of 45 minutes, performed 3 times per week

• A protocol spanning up to 12 weeks, with measurable gains seen as early as 4 weeks

Heart rate was monitored using the HRR method:

(HR_peak – HR_rest) × desired intensity + HR_rest

Patients were also monitored using the Rating of Perceived Exertion (RPE), with targets typically ranging from 15–17 (hard to very hard) on the Borg scale.

This framework allows patients to work at vigorous intensities in a sustainable and safe way, even when comorbidities or impairments would limit their tolerance to continuous aerobic exertion.

Why It Works: Physiologic and Neurologic Mechanisms of HIIT

One of the most compelling reasons to incorporate HIIT into neurorehab is its multi-system impact. At the neurological level, HIIT appears to enhance brain-derived neurotrophic factor (BDNF) and dopamine, key modulators of neuroplasticity. BDNF supports synaptic growth and cortical reorganization, while dopamine plays a crucial role in motor learning and reward reinforcement. These mechanisms are particularly important in post-stroke rehabilitation, where relearning movement and regaining function depend on the brain’s ability to adapt and rewire.

In addition to neural changes, HIIT drives cardiovascular adaptations that reduce the risk of recurrent stroke. As Crozier et al. (2018) noted, stroke survivors often have less than half the cardiorespiratory fitness (CRF) of age-matched peers and expend 40–50% more energy when walking. HIIT not only improves VO₂peak and ventilatory threshold, but does so with lower total training volume than traditional moderate-intensity protocols.

Furthermore, these gains are not just physiological, they translate to functional improvements that directly impact patients' independence and quality of life.

Functional Gains: From Gait Speed to Community Mobility

Functional outcomes are the ultimate litmus test for any rehabilitation intervention. Across both Crozier et al. (2018) and Boyne et al. (2023), HIIT produced greater improvements in several key measures:

1. 6-Minute Walk Test (6MWT): After 12 weeks, participants in the HIIT group increased their distance by 71 meters, compared to just 27 meters in the MAT group (p = .005).

2. Gait Speed: HIIT led to significantly greater improvements in both self-selected and fastest gait speeds, with gains evident by week 8.

3. Cadence and Stride Length: As reported by Pohl et al., HIIT yielded significant increases in both cadence and stride length when compared to progressive treadmill training or conventional therapy.

4. Functional Ambulation Category scores: These also improved more in HIIT groups, reflecting enhanced real-world walking ability.

For clinicians, these improvements mean less assistance needed for ambulation, fewer falls, and greater participation in community and home environments. And importantly, they demonstrate that training intensity is not just a “nice to have”, it's a necessary lever to pull in driving outcomes.

Safety, Feasibility, and Clinical Integration

Concerns around safety often limit the use of higher intensity interventions in neuro populations. However, both studies reported no serious adverse events related to HIIT protocols. Safety was ensured through:

• Pre-participation cardiovascular screening

• Monitoring of HR, BP, and RPE

• Use of body-weight support harnesses during treadmill sessions

• Supervision by trained physical therapists

While some sessions required brief modifications (e.g., reducing interval volume or intensity), adherence rates remained high, and participants tolerated the protocol well.

Parting Thoughts

High-Intensity Interval Training is a safe and effective intervention for stroke recovery. Research consistently shows that it leads to greater improvements in gait speed, endurance, and cardiovascular health compared to moderate training. With proper monitoring and structured protocols, patients tolerate it well. Yet in many clinics, HIIT remains underused, often due to uncertainty about parameters or outdated habits.

As physical therapists, we must start treating intensity as a core element of rehabilitation. It should be prescribed, progressed, and measured like any other intervention. When we under-dose intensity, we limit recovery. The evidence is here, the outcomes are clear, and our patients deserve care that drives the best possible outcomes.

References:

1. Boyne P, Billinger SA, Reisman DS, et al. Optimal Intensity and Duration of Walking Rehabilitation in Patients With Chronic Stroke: A Randomized Clinical Trial. JAMA Neurol. 2023;80(4):342-351. doi:10.1001/jamaneurol.2023.0033

2. Crozier J, Roig M, Eng JJ, et al. High-Intensity Interval Training After Stroke: An Opportunity to Promote Functional Recovery, Cardiovascular Health, and Neuroplasticity. Neurorehabil Neural Repair. 2018;32(6-7):543-556. doi:10.1177/1545968318766663

Disclaimer:

I am a current Doctor of Physical Therapy (DPT) student sharing information based on my formal education and independent studies. The content presented in this newsletter is intended for informational and educational purposes only and should not be considered professional medical advice. While I strive to provide accurate and up-to-date information, my knowledge is based on my current academic and clinical rotations and ongoing learning, not extensive clinical practice.