**Aquatic High-Intensity Interval Training for Parkinson's Disease Study** This study involves an 8-week exercise program for people with Parkinson's disease (PD), a condition affecting movement. The exercise is called **High-Intensity Interval Training (HIIT)** and will take place in a pool. HIIT involves short bursts of intense activity followed by rest. The study will compare this aquatic exercise to usual care, which means no specific exercise program. The goal is to see if aquatic HIIT helps improve balance, physical function, and muscle health in people with PD. **Sarcopenia** refers to muscle loss that can happen as we age or due to disease. - **Key Points:** - The study lasts for 8 weeks with three pool sessions each week. - Participants must be between 55-75 years old and have balance difficulties. - Safety will be monitored throughout, with a follow-up until 12 weeks. Participants must have permission from their doctor and cannot have had a heart attack in the last 3 months. This study aims to see if water-based exercise is safer and more effective for people with balance issues.