Shoe Design

What every shoe designer should know.
Shoe design should be based primarily on the anatomy and the mechanical functionality of the human foot. The second consideration should be the intended use of the footwear which will determine the choice of materials and structure of the shoes. Finally, style and fashion should influence the outward appearance of the shoe without compromising its comfort.

Foot Anatomy
The feet are very complex structures with many interdependent components. The bones provide structural support. The heel bone or calcaneus adjoins the tarsal and metatarsal bones which are fairly rigidly connected to each other and do not have much movement at the joints. The metatarsals are joined to the phalanges or toe bones. The joints between the metatarsals and the phalanges form the ball of the foot which is very important for walking. Ligaments and tendons hold the bones together and control their range of movement. The Achilles tendon attaches the heel bone to the calf muscles and is the most important tendon for walking, jumping, and running. Nerves convey sensations from the sole and the toes, and control muscle movements. Blood vessels provide nutrients to all the structures of the foot.

Mechanical Functionality of the Foot
The functions of the feet are to bear the weight of the body and provide locomotion. While standing, the body is supported by both feet, and the weight is distributed between the foot pads and the heels. A human footprint shows precisely which parts of the foot support the weight of the body. The toes do not make much contact with the ground, but they provide balance.

Foot Nerves 

When walking, the complete weight of the body is placed on the heel during each step, and as the body moves forward, the weight shifts to the ball of the foot, and finally, to the toes. A running gait generally involves landing on the ball of the foot and not the heel. This allows the calf muscles to dampen the shock. Vigorous activities, like running or jumping, place additional pressure on the feet which may be equivalent to several times the body weight.

Mistakes in insole design
Adidas produces a variety of sport slide-style sandals under various trade names like Santiossage, Calissage, Adissage, and Chayassage. These sandals are light, have adjustable velcro upper straps, and have reflectorized strips to make them visible at night. The sandals also feature massage nubs throughout the footbed that are supposed to provide relief for tired feet. Unfortunately, the Adidas sports sandal illustrated here has an insole design with widely-separated deep indentations which do not provide uniform support for the load bearing surfaces of the foot, and the ridges that make contact with the foot put too much pressure on the nerves and muscles in the ball of the foot.

   

The basic mistake in the design of this insole is that the indentations are too deep (3 mm) and the ridges that support the foot are too far apart from each other (from 3 mm to 10 mm). In effect, the design reduces the area of the insole which supports the foot by 30% to 50%. The second problem is that the ridges supporting the weight of the body press directly against the nerves in the ball of the foot causing great discomfort when walking. It is useful to compare the insoles of a shoe to a bed of nails. When the nails are very closely spaced, a person can lie on a bed of nails without puncturing the skin because the weight of the body is distributed between many points of contact. If the nails were widely spaced, this would be very painful and dangerous.

The pictures above show the insole of the Adidas slipper, a charcoal rubbing of the design pattern, and an overlay of a footprint upon the insole design. The overlay makes it very evident that the largest gaps in the pattern are in the areas which should provide the greatest support. This is exactly the opposite of what is desirable. If the slipper designers had taken into consideration the shape of a human footprint, they would not have made this mistake.

The nerves of the foot are pressed uncomfortably by the highlighted insole ridges.

High heels and Pointed toes
Podiatrists (foot doctors) are kept in business by bad shoe designers, and by women who, for the sake of fashion, wear uncomfortable high-heel pointed shoes. Bad-fitting shoes are responsible for many types of foot problems such as blisters, calluses, corns, ingrown nails, bunions and hammer toes. Hallux valgus (bunion) deformities are caused primarily by pointed shoes which force the big toe into an unnatural position and cause misalignment of several bones in the forefoot. A hammer toe is a deformity of the second, third or fourth toes, in which the toe is bent at the middle joint, so that it resembles a hammer. Women's designer shoes with high heels and narrow toes force the foot to slide toward the front of the shoe causing the toes to become deformed from the pressure against the front of the shoe. High heels also cause posture problems because they shift the center of gravity forward and the spine has to compensate by bending backwards.

   

Low profile shoes
Shoes that don't provide enough room in the vertical direction will press the toe nails and the surface of the toes. The pressure and abrasion can produce ingrown toe nails and foot corns. Corns (heloma durums) are calluses that usually form over the joints and on the tips of the toes where the foot rubs against the shoe. Wearing thick socks can provide some cushioning to reduce the pressure on the foot and at the same time keep the inner lining of the shoe from rubbing against the toes. Sheer nylon hosiery reduces the friction from rubbing, but it does not provide cushioning.

 

Boots
Boots are designed to cover the foot and the ankle. Boots have different functional purposes. Boots can provide stability for the ankle to prevent sprains. Other types of boots can protect the legs against snake bites or needles of thorny bushes. Depending on their use, some boots may extend up to the knee or to the hip. These are the most common types of boots:

• Safety boots - steel toes protect the foot from being crushed by heavy boxes or construction equipment.
• Hiking boots - support the ankle, provide good traction, prevent sprains.
• Ski boots - support the ankle, can be released from the skis quickly to prevent leg injuries.
• Wading boots - keep the legs dry when fishing or crossing streams, and provide good traction on slippery stones.
• Cowboy boots - high heels provide stablility in the stirrups of the saddle for horse-back riding.

Gel insoles and Air soles
Gel insoles and soles with air pockets provide cushioning and minimize the shock of every step by distributing the weight over a larger surface of the foot. This decreases the pressure per unit area and makes it more comfortable to walk or run.

The Ideal Shoe
Since different shoes are worn for different activities, there is no such thing as a "perfect" shoe. A shoe that is perfect for hiking may not be a good shoe for running. The design of a shoe has to take into consideration the task for which it will be worn. A person involved in heavy lifting needs shoes with steel toes to protect the foot. A runner needs light, well-ventilated shoes with soles that minimize impacts. An office worker needs shoes that are comfortable and stylish. Regardless of the application, the design of a shoe has to consider, first and foremost, the anatomy and function of the human foot.