One possibility is postural hypotension due to standing causing a generalized weakness due to dip in blood pressure. A feeling that knee joint is weak or giving away or unable to bear weight can be due to tear in mensicii. This causes pain worsening on walking and results in joint instability—that is you are not confident the knee joint will take your weight. Often the person feels a popping sensation on moving the joint. The other possibility is patellofemoral pain syndrome. This also causes pain on bending the knee, a grinding or popping sound and a feeling of knee giving away on walking. This happens due to overuse of joint, excess weight on joint, muscular dysfunction, foot deformities causing wrong weight transmission to knee and some biochemical changes. First physical therapy is started. Other treatments include knee caps and braces, taping the patella to reduce friction, anti inflammatory drugs, custom designed foot wear to correct arch problems if any and rest to the knee joint and applying ice packs. You need to avoid postures where knee is kept bent for long. Please consult a sports injury specialist or an orthopedic specialist for this. Take care!
The medical advice given should not be considered a substitute for medical care provided by a doctor who can examine you. The advice may not be completely correct for you as the doctor cannot examine you and does not know your complete medical history. Hence this reply to your post should only be considered as a guiding line and you must consult your doctor at the earliest for your medical problem.
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