Cause and origin
One long-term sequela of maldistributions of stress is osteoarthritis of the midfoot. Unlike the forefoot, the midfoot actually forms an arch. When walking naturally, the longitudinal arch has evolved to absorb the momentum of the step, store the energy in the muscles and tendons and make it available to the body again during the push-off phase. This ensures that the person moves forward in an energy-efficient way. Efficient muscles and tendons are essential in order to fulfil this task.
Barefoot walking automatically trains these structures and keeps them functioning into old age. For this to happen, the push-off phase must be completed by the big and second toe when stepping. If this is not possible, osteoarthritis of the midfoot may develop at a later date.
The reasons for deviation from this ideal gait direction are development of splayfoot, support of the longitudinal arch and permanent squint positioning due to soft sole material.
Splayfoot is the most effective of these.
As explained before, because splayfoot is caused by "spherical" forms of insole, it has an effect on the midfoot very early on, initially moving the gait direction towards the fourth toe.
This means that the flexor in the big toe is put under more strain. During this phase, the longitudinal arch raises and the pressure when stepping moves outwards. This puts extreme strain on the navicular bone and metatarsal IV.