We aimed to investigate the effects of isometric and combined concentric-eccentric (C-E) isokinetic types of exercise applied to the knee muscles in patients with knee osteoarthritis (OA) in an attempt to identify which type of exercise would be most effective in terms of pain relief, functional status, quality of life, and muscular strength. This was a controlled trial in which 71 patients were randomized into three groups. The first group was assigned to C-E, the second group to isometric isokinetic exercises, while the third, control group, was only given paracetamol. A visual analog scale for pain, Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) and Short From-36 (SF-36) for functional status and quality of life, and the isokinetic test for muscular strength were evaluated. Significant improvements were recorded in all groups for pre and post-treatment pain, the WOMAC and the SF-36 (P < 0.001). Among the exercise groups, improvement was significantly better in the C-E, isokinetic exercise group as compared with the isometric exercise group (P < 0.001). Isokinetic exercises are recommended as a treatment modality. However, the long-term effects of these exercises on pain and osteoarthritis progression need to be investigated.