When looking at the knee, unless there’s some sort of direct traumatic event, knee pain typically isn’t because there’s something wrong with the knee itself. Rather, there’s often a dysfunction above and/or below the joint such as the hip or ankle causing pain to be felt at the knee.
If you recall the joint-by-joint approach, the primary role of the foot is to provide stability and the role of the ankle is mobility (remember the knee is a stable joint). If one or both of these is dysfunctional, it can create compensation patterns and lead to issues all the way up the kinetic chain.
The foot provides stability and support to transfer forces up the chain when walking, running, jumping, etc. Within this stability role, the foot also provides a lot of neuromotor control for things like balance and coordination.
If stability and motor control in the foot is compromised it can lead to pain and dysfunction in other joints like the knee. Without proper control, one big thing that can happen is the foot over-pronates and flattens too much which causes the knee to collapse inward placing extra strain on it. The most common reason stability and motor control is compromised is due to previous injury.
Having adequate mobility at the ankle is also important, especially ankle dorsiflexion or the ability to move the foot up towards the shin. A generally accepted range is about 20 degrees of dorsiflexion.
There are many reasons this motion can become limited such as tight calves, joint restrictions, lack of use, osteoarthritis, or frequently wearing high heels. However, the most common causes are post-surgery or previous injury.
The body will compensate to make up for the limited mobility. This may be again, over-pronating at the foot or giving in at the knee and is collapsing towards the midline.