Muscle Strength Loss and Its Effect on Knee Cap Motion in Volunteers With Anterior Knee Pain

Background: \- Researchers are interested in how the muscles affect the movement of the knee cap. These muscles may be related to different kinds of knee pain that are not caused by an injury or a disease. Imaging studies such as magnetic resonance imaging (MRI) can be used to look at these muscles. To study these muscles and how they move, researchers will use MRI to look at healthy individuals and individuals with knee cap pain. Objectives: \- To study how changes to the muscles around the knee can influence knee pain. Eligibility: * Individuals between 18 to 55 years of age who have knee cap pain that cannot be explained by a specific injury or disease. * Healthy volunteers between 18 and 55 years of age. Design: * Participants will be screened with a physical exam and medical history. * This study requires two visits. Each visit will use standard MRI sequences to take images of the knee in motion and at rest. * On the first visit, the MRI scan will look at the knee in its natural state. Participants will move the knee up and down for 1 to 3 minutes at a time during the scan. * On the second visit, a local anesthetic agent will be injected into the muscle of the thigh. The anesthetic will block this muscle from generating force for 2 or 3 hours. Participants will move the knee up and down for 1 to 3 minutes at a time during the MRI scan.