This study aims to understand how treadmill exercise affects people with early-stage Parkinson's disease—a condition affecting movement. Participants, aged 40-80, will be split into two groups, each using a treadmill at different intensities (60-65% or 80-85% of their maximum heart rate, HRmax) four times a week for 12 months. The study will measure whether exercise slows down Parkinson's symptoms, using a scale called MDS-UPDRS Part III. **Parkinson's disease (PD)** is a disorder that affects the brain and leads to shaking, stiffness, and difficulty with walking, balance, and coordination. **MDS-UPDRS Part III** is a scale used to measure the severity of these symptoms. Participants will visit one of about 29 locations, most in the USA and two in Canada. They should not be taking any PD medications or have significant health issues. The study also explores how exercise affects mobility, cognition, and quality of life. - **Duration:** 12 months with possible follow-up to 24 months. - **Visits:** Regular treadmill sessions, four times per week. - **Eligibility:** No PD medication, ages 40-80, early-stage diagnosis. This study aims to discover if high-intensity exercise can better manage early Parkinson's symptoms.