Thus, the main aim of the study is to evaluate two modes of exercise therapy focusing on exercise dosage; 1) exercise therapy with an exercise dosage lasting 80 - 90 minutes duration at each treatment compared to; 2) low dosage exercise therapy (conventional exercise treatment) lasting 20-30 minutes at each treatment) in patients with long-term knee osteoarthritis. All patients in both intervention groups will receive three treatments a week for 12 weeks making a total of 36 treatments. Physiotherapists at four different intervention centers will treat patients from both exercise interventions. Assessments will be performed at inclusion, after end of treatment, and at six and 12 months follow up. Objective tests (20 meter walk test, sit to stand in 30 seconds and maximum number of single knee flexion/extension in 30 seconds) is tested out at inclusion and at end of treatment. The testers are blinded regarding intervention group. Questionnaires are filled out by the patient at inclusion, end of treatment, at six and 12 months follow up. To assess when possible clinical interesting changes start to occur during the three month treatment period, patients are filling out KOOS (function) and VAS (pain) after every 6th treatment making a total of six measurements during the intervention period.