Pronation of the foot

Pronation is a natural movement of the foot that occurs during foot landing while running or walking. Composed of three cardinal plane components: subtalar eversion, ankle dorsiflexion, and forefoot abduction, these three distinct motions of the foot occur simultaneously during the pronation phase. Pronation is a normal, desirable, and necessary component of the gait cycle. Pronation is the first half of the stance phase, whereas supination starts the propulsive phase as the heel begins to lift off the ground.

The normal biomechanics of the foot are designed to absorb and direct the force occurring throughout the gait cycle, whereas the foot is flexible (pronation) and rigid (supination) during different phases of the gait cycle. As the foot is loaded, eversion of the subtalar joint, dorsiflextion of the ankle, and abduction of the forefoot occur. Pronation should not occur past the latter stages of midstance, as the normal foot should then supinate in preparation for toe-off. Abnormal pronation occurs when a foot pronates when it should supinate, or overpronates during a normal pronation period of the gait cycle. Approximately 4 degrees of pronation and supination are necessary to enable the foot to propel forward properly. In the neutral position, the foot is neither pronating nor supinating. If the foot is pronating or supinating during the stance phase of the gait cycle when it ought to be in the neutral position, a biomechanical problem may exist.

For ease of choosing corrective footwear, three types of pronation are recognized: neutral pronation, overpronation, and underpronation.

Some pronation, also called eversion, is natural in the body’s regular movement. However, overpronation and underpronation (supination) can be potentially harmful. Neutral pronation occurs when the foot experiences a normal, healthy amount of pronation instead of overpronating or underpronating. In healthy movement more of the toe area will be used when pushing off than in unhealthy movement. In neutral pronation the weight distributes fairly evenly among all of the toes with a slight emphasis on the big toe and second toe which are better adapted to handle more of the load.

...
Wikipedia