People who have a stroke often find it hard to do the things they did before. This can be caused by problems with arm movement. One in five people do not get any arm movement back after a stroke. Arm movements can be measured accurately in a laboratory, but it is very expensive and not easy to do in hospital. That means it is hard to tell if the arm is recovering to move like it did before the stroke or adapting to perform tasks in other ways. To tell if a treatment is working, the investigators are making a phone app to record arm movement, using the camera. The recordings will be turned into data showing movement difficulties and sent to hospital records for clinicians. Clinicians will see if movement changes, to help choose the best treatment. The investigators are looking for twelve stroke survivors to help test this app. * The session will be at King's College London, on Guy's Campus. * It will run for 2-3 hours. * Participants will wear a vest or tight-fitting clothes. * The investigators will place non-invasive markers on the participants arm. * The investigators will video simple movements such as drinking from a cup. * The investigators will also measure the same simple movements using the laboratory cameras. This will show us if our app can measure arm movement as well as laboratory tests. If they do, the investigators will know the app is accurate. In future this technology can improve recovery by correcting stroke survivors when they perform home exercises.