Take a look at the above picture. Some people, usually women, have knees that tend to “bend backwards” in stance. This is an overt sign of hyper-mobility, or excessive extensibility, of the joints.
The problem that often arises from this phenomenon is what we call “resting on your ligaments”. The woman in this picture can rely on this position to stand without using much muscle action at all. The quadriceps and hamstrings in particular do not need to engage in order to keep her erect. She is in fact resting on the posterior ligamentous structures of her knees.
One could argue that this is an efficient way of standing. It is efficient if you are considering how much energy the person is expending. The woman in the picture could probably stand in this fashion for an hour without much difficulty. The long term problem with standing in this fashion has to do with overall muscle strength.
In terms of long term health, we do want to use our functional muscles often. Stronger muscles that are utilized protect the joints by absorbing shock. Stronger leg muscles also allow us to ascend and descend stairs correctly, avoiding painful compensations.
If you are hypermobile in your knees, try “unlocking” them as you stand. This will force you to use your leg muscles to keep you standing. Build up your endurance slowly!