Pronation as a term has received a bad rap from the running shoe industry, however, it may be the most critical of all foot functions. Unfortunately, the running shoe industry has attacked "pronation" for nothing more than a marketing ploy. Pronation is the natural motion of the foot rolling or tilting inward in order to dissipate shock. It occurs at different degrees and is best seen when running or landing from a jump. The foot lands on the outside border with the ankles turned inverted. As body weight settles down on the foot, the ankles roll inward and the foot rolls inward flattening out. Pronation cannot occur with arch support, especially the support of rigid orthotics. Support simply blocks the foot from rolling inward which causes the impact force to go up the body instead of being dissipated.
Transverse Arch Flattening
The ball of the foot is made of five very mobile metatarsal bones. An arch spans across these metatarsals known as the transverse arch. The foot naturally rolls across the ball of the foot starting at the outside and rolling inward to the big toe. To initiate the roll the foot touches down on the outside at the 5th metatarsal. This 5th metatarsal is very mobile and displaces easily away from the other four metatarsals making for a soft touchdown. At this point the transverse arch begins to flatten just like a leaf spring suspension system. This flattening widens the ball of the foot by about 15%. Rarely when fitted for a shoe do you account for the extent to which the foot widens. Which is why a "snug fit" can easily result in a stress fracture of a metatarsal bone.
Toes Splay Apart
As the five metatarsals widen upon bearing-weight, the five toes splay apart. They must spread even further apart than the metatarsals, with open space between each toe. The toes are responsible for forming a stable, wide base for the upright body. A second function of the toes is sensory proprioception. Upon touching down the toes spread across the ground so they can relay critical information to the brain, describing the level of landing surface and how the body will have to adjust its lean. Contrary to popular belief, the toes are the widest part of the foot, not the ball of the foot. Unfortunately, since fashion dictates shoe design instead of foot biomechanics, shoes are designed with tapered toeboxes, narrower than the ball of the foot.