Strength and Balance in Gender- and Age-Matched Controls

Inclusion Criteria: * Over the age of 59 * Gender and Age-Matched (within 3 years) of main protocol participants Exclusion Criteria: * Parkinson's disease diagnosis * Significant cognitive deficits. * Another neurological or orthopedic deficit that in the investigator's opinion would have a significant impact on gait and cognition (e.g. stroke, fracture). This study involves one visit to Oregon Health \& Science University. The study visit will last approximately 2 hours. Measurements: 1. The participant's balance will be assessed by having them stand on a platform and completing mental tasks such as naming the alphabet backwards. They will be asked to have their eyes open on some tasks and closed on other tasks. For safety, they will wear a harness like those used by rock climbers. 2. The participant's ability to move and balance will be measured by putting six sensors on their bodies (both wrists, both ankles, chest, and back). Each sensor is about the size and weight of a deck of cards. To measure movement and balance, participants will rise from a chair, walk three feet, turn around, walk back to the chair, and sit down. 3. The participant's strength will be evaluated using a machine that resembles a stationary exercise bike. They will have one leg and foot strapped onto the machine. Participants will then be asked to flex and extend their leg as quickly as possible five times. This will be repeated with the other leg.