Pronation is an inward motion of the foot during a footstrike. Pronating is a normal occurrence, up to an angle of about 15%. At higher or lower angles, corrective running shoes can help to prevent injury.
When I started reading about pronation, I found it helpful to compare running and walking movements to better understand pronation.
Walking:
When walking, the foot strikes the ground with the leg in a nearly vertical line coming down from the hip. The foot rolls with little sideways rotation, in a heel to toe movement.
Running:
While running the legs point at an inward angle, touching the ground closer to the mid-line of the body. This causes the feet to strike the ground with the outer edge of the heel and foot. The movement progresses along the outer edge of the foot, and the foot begins to roll inward. By the time the ball of the foot engages with the ground, the foot has rotated inwardly so that it is placed evenly on the ground in preparation for pushing off.
That, in a nutshell, is pronation!
What is pronation?
Testing your pronation with your wet footprint
Testing your pronation by looking at your running shoes
Testing your pronation by looking at your ankles
Examples: footprints of overpronators, normal pronators & underpronators
The best running shoes for overpronators
The best running shoes for normal pronators
The best running shoes for underpronators