Foot Pronation and its role in plantar fasciitis and running
Pronation, the movement that irritates the injured plantar fascia, is also important.
Pronation is how your body adapts to uneven surfaces and absorbs the force of gravity as you walk and run. During this, the foot moves in three directions:
The greatest stress on the plantar fascia occurs as you run up a steep hill, your knee is pushed further forward than your foot, your heel comes up off the ground, and the big toe is forced to flex. This movement tightens the fascia to its maximum tension. When this particular strain on the plantar fascia adds to the force propelling your entire body weight up the slope, the strain can stretch the plantar fascia to the point of failure.
The next movement to consider is the lateral tilt of the foot which adapts to sloping terrain. If you stand on the side of a hill and face downhill (not facing uphill ), your uphill foot will be pronounced and your downhill foot will supine. The difference is the direction of inclination of the heel bones. The medical or biomechanical terms for the internal and external tilt of the heel bones are "calcaneal inversion" and "calcaneal eversion".
Standing facing the slope, the downward foot is reversed. The ankle rolls outward, the heel bone tilts inward, with the base of the heel bone toward the midline. The foot is supinated, which decreases the tension on the plantar fascia. The uphill foot is reversed. The ankle tilts inward, the heel bone tilts outward, with the base of the heel bone away from the midline. The descending foot is pronounced, which increases the tension on the plantar fascia.
The last of the three forces associated with pronation is abduction. It is a rotation of the foot outward concerning the leg. Most humans only do this when pronating. Abduction is also more pronounced when a rotational force is applied. This usually happens when you change direction and push outward.