Background <p>Cardiorespiratory fitness and vascular health are significantly impaired post-stroke. High-intensity interval training (HIIT) has emerged as a promising strategy to improve walking in people with chronic stroke, but broadly integrating HIIT into stroke recovery remains limited. One key barrier is the reliance on maximal exercise testing to prescribe HIIT, which is often not feasible in clinical settings. To address this gap, the Fitness After Stroke Trial (FAST) evaluates the preliminary efficacy of HIIT compared to moderate-intensity continuous training (MICT), using a validated submaximal exercise testing protocol to guide individualized exercise prescription in people post-stroke.</p> Methods <p>FAST will enroll 50 individuals with chronic stroke into a double-blind, two-arm, parallel-group preliminary efficacy trial. Participants will be stratified by lower extremity motor function and randomized to HIIT or MICT. Exercise sessions will occur three times per week for four weeks using a total body recumbent stepper. The primary outcome is estimated peak oxygen consumption from the Total Body Recumbent Stepper (TBRS) submaximal exercise test. Secondary outcomes include middle cerebral artery velocity at rest and during exercise, flow-mediated dilation and pulse wave velocity. Tertiary outcomes include walking speed and distance.</p> Discussion <p>FAST represents one of the earliest double-blind randomized trials to directly compare HIIT and MICT on a recumbent stepper in people post-stroke, using a validated submaximal exercise testing protocol to individualize exercise intensity. This study will generate preliminary effect sizes for key physiological and functional outcomes obtained with this modality, providing the critical data needed to optimize dosing strategies and power a future definitive trial.</p> Trial Registration <p>NCT05936008. Registered 7 July 2023, Study Details|Fitness After Stroke Trial|ClinicalTrials.gov.</p>