Just after the heel strikes the ground when running the foot begins to pronate. Pronation is a movement that occurs as the weight of the runner moves from the lateral aspect (or outside) of the heel to the medial aspect (or inside) of the forefoot. This movement allows the foot and leg to adapt to the terrain and absorb the impact of the foot strike. A certain amount of pronation is necessary to run normally but too much or too little can contribute to injuries. Runners with low arches tend to “over-pronate”.
When the foot is in an over-pronated position the alignment of three major joints in the foot are less than optimal. This misalignment of the joints causes the foot to become structurally unstable, and, in turn the muscles, tendons and ligaments of the lower leg are forced to work harder in an attempt to stabilise the foot. This is why over pronators are vulnerable to ankle injuries. If you are a runner with a visibly low arch or have experienced recurrent or chronic overuse injuries, chances are you are pronating excessively.