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.
It works the other way around too: muscle weakness puts extra stress on the knee joint, causing arthritis-like symptoms or making existing arthritis worse. Weakness and stiffness in the legs can quickly cause a downward spiral of immobility. Movement hurts, but lack of movement makes things worse.