1. When a foot strikes the ground the first motion in pronation begins at the calcaneo-astragaloid joint and then is continued by the astragalo-scaphoid and the other small joints in this region.
2. When the weight of the body is on the foot the application of the force of the contraction of the foot and leg muscles is not the same as when there is no weight on the foot.
3. Without weight bearing the force of contraction is applied mostly at the insertion of the muscles and the foot is consequently moved.
4. With weight bearing the foot cannot move and so the force is applied not at the insertion of the muscles but by their tendons at those points where motion is most easily possible.
5. With weight bearing it is possible to rotate outward or inward the tibia and fibula and, therefore, the astragalus and so to correct or increase pronation.
6. The adductor muscles of the foot are normally stronger than the abductor.
7. With weight bearing the force of the adductor muscles is applied not at their insertion but by their tendons at the internal malleolus and the sustentaculum tali, thereby tending to push these forward, upward and slightly outward and so correct pronation.
8. In weak and relaxed feet pronation may be most easily and beneficially corrected by preventing its occurrence at the calcaneoastragaloid joint and the beginning of weight bearing.
9. A simple way to accomplish this is by the use of a plate with a tilted heel, as described above.
10. The advantage of such a plate is that it puts the foot into the position best adapted for the proper use and exercise of the muscles.
11. Anything which will hasten the return of a normal muscle balance to the foot, be it plates, exercises or other methods should be used and this paper is written in the hope that by giving a clearer idea of the true action of the foot muscles treatment of any kind may be regulated to bring about the desired result of a normal muscle balance and the use of no apparatus.
Assistant to Orthopedic Surgeons to Out-patients., The Orthopedic Department of the Mass. General Hospital.