The research project will investigate the extent to which a smartphone camera sensor tool can help predict and measure knee stiffness and pain after Total Knee Replacement Surgery (TKR) and how a tool such as this could be implemented into the NHS. Total knee replacement (TKR) is a frequent procedure undertaken in England and Wales, with more than 100,000 conducted each year. Although most patients have a successful outcome following their TKR, approximately 10-20% of patients are dissatisfied, predominantly because of pain and knee stiffness. A method to detect early problems with pain and stiffness could facilitate earlier referral to non-surgical treatments, which are effective in preventing the need for manipulation under anaesthetic (MUA). Here the investigators will validate and provide proof of concept for a smartphone camera sensor tool that measures knee range of motion alongside symptoms of pain for use in the home setting. The study will comprise of 3 stages; 1. We will conduct 45 minute online interviews comprising of (1) people who have had total knee replacement surgery, (2) healthcare professionals and stakeholders. 2. We will invite 30 participants who are 5-9 weeks post TKR and 30 participants who have had no previous musculoskeletal injuries to attend a session at the university. The lab testing will be conducted at the VSimulator, a biomechanics research lab at the Exeter Science park, and at the teaching labs on St Lukes Campus, Exeter. Here participants be asked to answer 8 questionnaires and have some of their movements measured. 3. Participants will be asked to repeat the 'timed up and go' and the 'sit to stand' tests in their homes and record them using a mobile device. The study is funded by the NIHR Exeter Biomedical Research Centre grant and sponsored by the University of Exeter.