When people stand, and when they try to stand tall and straight, they have an inevitable tendency to brace the legs, which involves the hyper-extension of the hip joint, the knee joint, and the fixing of the ankle. This tendency is there all the time, stimulated at the drop of a hat, and that is why we need the constant reminder of the opposite.
Walter Carrington (“Thinking Aloud” – Knees Going Forward and Away p160)