Over pronation is when the foot naturally rolls inwards during a walking or running motion. Pronation at the foot is natural to help the body to absorb shock but over pronation is a problem which can cause issues with the arch at your foot and issues up the entire kinetic chain. In regards to the knee, over pronation at the foot forces the knee to internally rotate producing more stress. Remember the body is a kinetic chain, if your foot rolls inward during every step you take then so is your knee, creating shear forces on the knee joint that could possibly wear out at any time.
Over pronation at the foot can be caused by either foot instability or hip instability. It is much more common in women due to having weak hips. Remember the main causes for knee valgus includes weak hips as well. The major culprit muscle is a weak or under activated gluteus medius muscle which is a stabilizing muscle on the sides of the butt. Having a weak gluteus medius can cause the leg to roll inwards during every step.
At the time of heel strike during the walking or running gait pattern, the knee is usually fully extended or has about 20 degrees of flexion (bend), and the forces transmitted from the foot to the knee are very high. When extension or slight flexion is combined with internal rotation at the lower leg (stemming from over pronation at the foot) it is an extremely common positon for ACL tears. For an athlete with normal foot pronation it is much easier to recover from this vulnerable position quickly and easily. But for the athlete with excessive over pronation at the foot, this position is going to be more over pronounced and harder to quickly recover from. The foot rolls inward more in the athlete with the over pronation problem which produces excessive force up the kinetic chain into the knee. This causes improper force vector at the quadriceps and overtime can cause tracking issues at the knee cap creating more stress on the inside or medial side of the knee. The increased risk of ACL injury for this over pronated foot athlete happens every time they play sports. Eventually the shear forces on the knee in this single leg position described above give out and an ACL tear occurs.
The body’s first contact to the ground starts with the foot. By addressing over pronation problems in the foot and strengthening the gluteus medius and stabilizers in the foot to correct the over pronation problem it is easy to decrease foot pronation and allow the athlete to perform with less strain and pain to the knee and decreased risk of ACL injury.