Muscle weakness is an inevitable consequence of any serious knee problem.  Pain and mechanical instability prevent the normal use of the joint which is followed by shrinking, or atrophy, of the surrounding muscles. Muscle weakness can lead the knee to feel vulnerable and unstable. 

It usually takes many months of hard work to restore the muscles to full power and function following knee injuries and more major knee surgery.  Physiotherapists can often help speed up this rehabilitation process.