Causes of pain when you straighten your knee generally include traumatic injury, overuse injuries, previous medical conditions, and other factors. - If you're new to training and haven't had a direct injury, you could have jumper's knee, runner's knee, and/or muscle weakness. - If you kneel often, play sports, or have health issues like diabetes, gout, etc, you could have knee bursitis. - If you're over 60 years old, overweight, or obese, you could have knee OA. - If you play sports, you could have jumper's knee, runner's knee, a knee dislocation, or a ligament/meniscus tear. In any case, it's best to check with a physical therapist or a doctor to diagnose and treat the cause of knee pain and rule out other conditions.