Functional Footwear defined
1- Foot-shaped design: A shoe should mimic the ‘fan-shape’ of a healthy unshod foot i.e. the widest part of the shoe should be the distance from the base of the great toe to the tip of the smallest toe (the toe-box). ‘Wide’ shoes that are not foot shaped are just as harmful to foot function as narrow shoe-shaped shoes.
2- Flat sole: The weight–bearing area of the sole should be flat to the floor to provide maximum surface area.
3- No toe-spring: The toes can only perform their stabilising role if they are in contact with the ground.
The thickness of the sole will vary depending on the activity (forces acting on the foot) and the terrain. In activities that require maximum sensory feedback, a thin sole is appropriate, but in activities that expose the foot to large, repetitive loads (marathon running) or unstable, irregular substrates (trail running), a thicker sole to provide more cushioning or increased traction is appropriate.
The ‘Goldilocks’ principle applied to sensory feedback. It is important to remember that if the human brain receives too much sensory input it will be perceived as potentially harmful and be felt as pain in the organ being stimulated. Too much light is felt as pain in the eyes, too much noise is felt as pain in the ears and too much "ground feel" is felt as pain in the feet.