Call for emergency medical help. If you suspect you're having a heart attack, don't hesitate. Immediately call 911 or your local emergency number. If you don't have access to emergency medical services, have someone drive you to the nearest hospital.
The most common causes of knee weakness are osteoarthritis, sprain or strain, and chronic knee pain. Other possible causes, such as injury to the knee, are more rare. Osteoarthritis 22. Sprain or strain 17. Chronic knee pain 15. Knee ligament or meniscus tear 8. Fracture of the leg 4. Bursitis 2.
1 It often develops at the site of a previous injury. Other types of arthritis, such as rheumatoid arthritis, gout, and lupus, also can cause knee pain, swelling, and stiffness.  Osgood-Schlatter disease causes pain, swelling, and tenderness in the front of the knee below the kneecap.