During the first half of the stance, the midfoot and the forefoot do the majority of the work, in comparison to the rearfoot.
The natural movement associated with the rearfoot eversion is a combination of vertical and medial midfoot displacement.
This means that a tight arch will have you compensate with the rearfoot. The arch undergoes significant deformation for about 75% of the whole stance phase. A genuine good reason to work on the mobility and flexibility of your foot!
In order to make it up for the fixed heel bone in contact with the ground, the tibia has to rotate internally and the knee goes into flexion.
While pronation is normal for most runners, prolonged pronation may increase the risk of injury. If pronation continues after the midstance, a mechanical dilemma occurs at the knee joint. The tibia wants to externally rotate and the knee wants to extend, but because the foot has achieved more internal rotation than necessary, the hip starts to excessively rotate in order to keep good joint alignment at the knee.
Excessive contact pressure on the kneecap may then occur and put the runner at risk of cartilage deterioration and anterior knee pain.