I have sore knees time to time an they get weak an kinda bend an make me lose balance what is the cause of this?

Muscle inhibition. Knee pain can be due to anything from muscle imbalance (most commonly) and arthritis to nerve irritation at the level of the back. When the body senses pain, it can sometimes inhibit the surrounding muscles, making the area weak. You should be evaluated by a sports medicine physician to determine the cause of the symptoms. Regardless, p.T. Is helpful to develop appropriate rehab program.