Knee pain, especially sudden pain in one knee can arise from overuse or abrupt injury (e.g., a collision during a football game). The knee joint carries the weight of your body and this weight increases when you move (e.g., walking, jumping, running). Moving quickly in unpredictable environments (e.g., fell running, football) might increase the chance of a strain (where the knee tissues have stretched but not damaged) from a fall or tackle. For these reasons, most athletes warm up before exercising or competing in sport and stretch before and afterwards. Sometimes it is the footwear of the athlete that requires adjustment to treat or prevent further injury. The causes of knee pain vary. Some less common examples might include tendonitis (i.e., inflammation of the tendon), housemaid’s knee (build-up of fluid around the knee joint) or a torn ligament. Ligaments connect bones at the knee joint and when torn require specific surgery. For example, anterior cruciate ligament (ACL) surgery is performed where new tissue is grafted onto a torn ACL.