This study looks at how stroke survivors with leg difficulties can improve their walking speed using high-intensity interval training (HIIT). **HIIT** is a type of exercise that involves short bursts of intense activity followed by rest. Researchers will check blood lactate levels, a substance that builds up in the blood during exercise, to see if it can predict who will improve. They will also look at **GABA**, a chemical in the brain that helps with nerve functions, and **CBF** (cerebral blood flow), which is the blood supply to the brain, to see if they help predict improvement. The study lasts 12 weeks with 36 training sessions. Participants' walking speed, balance, leg strength, and endurance will be measured. Blood lactate will be checked during some sessions with a finger prick. Eligible participants must be Veterans with a chronic stroke affecting their leg movement and cause slow walking. People with certain health conditions or implants that interfere with MRI scans cannot join.

• Duration: 12 weeks with 36 exercise sessions.
• Measures: Walking speed, balance, leg strength, endurance.
• Eligibility: Stroke survivors with specific conditions.