The last, a three-dimensional (Fig. 26-2) form on which the shoe is made, is considered by many to be the foundation for shoe production and development. Foot shape may vary with sports activities, and this is a major area of concern in the development of the last. The shape of the shoe toe box, instep, girth, and foot curvature are determined by the last. The biggest last variations occur in girth (or widest part of the forefoot) and in heel width.

Figure 26-2 Different lasts used in athletic shoes.

Rights were not granted to include this figure in electronic media. Please refer to the printed book.

From Reyatt T: The first step: know your feet. SHAPE Magazine, Nov 1992.

Straight and curved lasts

Most feet have a slight inward curve. Most sport shoe companies use a last that is curved inward approximately 7 degrees. The greater the curve, the more foot mobility is allowed, a benefit for the underpronator. The straighter the shoe, the more medial support it will provide; this can help to control overpronation.

Combination lasts

The term “combination lasts” refers to any last that varies from a standard proportional last to lasts that accommodate a combination of fitting or movement requirements.

Materials

Upper materials

Leather, rubber, plastic injection molding, soft nylon, mesh nylon, polyvinyl chloride (PVC)-coated fabrics, polyurethane-coated fabrics, and canvas have been used in the manufacture of uppers. Most uppers used in sports shoes are made of soft nylon, mesh nylon, leather, canvas, suede, and synthetic materials such as Kangoran.

Sole materials

Rubber is the most widely used sole material because of its versatility, durability, and performance. The most commonly used forms of rubber are a highly compressed molded form or a blown microcellular form. Carbon rubber and styrene-butadiene rubber are the two most common rubber compounds used in athletic shoes. Often used in running-shoe soles, black carbon rubber is the hardest wearing. Styrene-butadiene rubber also is hard and is used in tennis and basketball shoes.

Lasting Techniques

The most common methods of lasting used in shoemaking are slip lasting, board lasting, and combination lasting (Fig. 26-3).

Figure 26-3 Methods of lasting.

1 Slip lasting—A slip-lasted shoe is constructed by sewing together the upper like a moccasin and then gluing it to the sole. The last usually is forced into the upper, which then takes the shape of the last. A sock liner usually takes the place of an insole. This lasting method makes a lightweight and flexible shoe with no torsional rigidity.

2 Board lasting—The upper is placed over the last and fastened to the insole with cement, tacks, or staples. This construction promotes stability and torsional rigidity but decreases flexibility.

3 Combination lasting—More than one lasting technique can be used on the same shoe. Usually the shoe is board lasted in the rear foot for stability but slip lasted in the forefoot for flexibility. Combination lasting can offer customized features necessary for some athletes.

Bottoming Process

Bottoming is the process in which the sole components are attached to the upper. The upper determines the shoe fit and provides support, and the sole provides traction and cushioning.

The Outer Sole

The outsole is the most plantar surface of the shoe that makes contact with the ground and usually is attached to a midsole to form a complete sole. Most athletic shoes have outer soles of hard carbon rubber or blown rubber compounds. Blown rubber is the lightest outsole material but is not as durable as carbon rubber. Many outsoles are composed of both blown and carbon rubber, with blown rubber in the forefoot and midfoot and carbon rubber used in the high-wear area of the heel. Gum rubbers are hard wearing and grip well on most surfaces. PU is less versatile but also suitable for outsole material and seems to possess good durability. Nylon, leather, and PVC have specific outsole applications for certain sports.

Outer sole designs

Patterns can enhance stability and traction. They also can improve shoe lightness by exposing the middle part of the midsole, thereby eliminating part of the outsole and the associated weight. The design of the outsole (Fig. 26-4) can provide cushioning, traction, pivot points, flexpaths, and wear plugs.

Figure 26-4 Outsole patterns.

Rights were not granted to include this figure in electronic media. Please refer to the printed book.

From Reyatt T: The first step: know your feet. SHAPE Magazine, Nov 1992.

Outsoles are specific for surface, weather condition, and sport. Outsole options include:

• Wear-area reinforcement (running shoes).

• Cantilevered designs for shock absorption (running shoes).

• Pivot points (court shoes).

• Herringbone (court shoes).

• Suction-cup designs (court shoes).

• Multiclaw or stud designs (field shoes).

• Radial edges (court shoes).

• Asymmetric studs (field shoes).

• Traction and wear lugs (hiking and climbing boots).

Traction provided by the outsole is an important consideration in the design of a sport shoe and is directly related to the ability of the shoe to develop frictional forces with the playing surface. Traction needs depend on the specific sports needs. Too little traction may have a negative effect on athletic performance, and too much traction may put the athlete at risk for injury.¹

A running shoe should create a grip firm enough with the ground so that propulsion forces created by the runner will not be lost with push-off. Push-off has the highest traction needs; therefore the forepart of the outsole should provide the most traction. The outsole rubber used in running shoes usually is blown rubber (air injected to lighten it) or hard carbon rubber.

Cleated shoes must address a compromise between performance and protection of the athlete. Rotational traction, which is expressed by the torque about a normal axis that is developed to resist rotation of a shoe on a playing surface, must be reduced to decrease the incidence of injury while providing sufficient traction. Both cleat length and outsole material affect friction. Torg and Quendenfeld ¹ concluded that the increased rotational traction characteristics of some football shoes are related to an increase in number of significant knee injuries.

The necessity for lateral movement with court sports makes the traction characteristics of court shoes important. A flat outsole pattern develops the greatest frictional forces, whereas a herringbone pattern develops less.² With sprinting, initial ground contact is made with the front of the shoe. At foot strike a large horizontal velocity is created, resulting in a high braking force that can cause a backward slide. Anterior spikes help to prevent slipping. With jumping events, an athlete converts the large horizontal momentum of run-up to a vertical momentum at foot plant. The spikes prevent foot slip and allow the development of large propulsive forces necessary for long jump and triple jump.

With golf shoes, motion is primarily stationary with little horizontal velocity. Golf shoes provide a base of support that allows the performance of coordinated body movements needed in hitting the ball. A nonvertical alignment of the spikes prevents slipping in this sport, which mainly requires anterior and lateral forces.

Boating shoes require a large amount of natural rubber to prevent slippage on wet surfaces.

Midsoles and Wedges

Most of the recent advances in the athletic shoe industry have been made in midsole design and materials. The midsole and heel wedge are sandwiched between the upper and the outsole, attaching to both. These components provide cushioning, shock absorption, lift, and control.

Unit soles

Unit soles usually contain the outsole, midsole, and heel wedge as one unit. This design is used for roller-skate boots and for other sports in which the sole does not contact the ground. This design usually is heavy and provides little flexibility but excellent torsional rigidity.

Combination or prefabricated soles

Midsoles are manufactured from a combination of two basic materials: EVA and polyurethane. EVA is light, has excellent cushioning properties, and can be manufactured in various densities. The firmest densities in a multidensity midsole usually are designated by a darker color. These can be placed at critical points in the midsole to aid in motion control. PU is a denser, heavier, and more durable material than EVA. New forms of lighter PU are being developed.

Both EVA and PU are used to encapsulate other cushioning materials such as air bags (Nike and Etonic), gel (Ascics), silicone (Brooks), honeycomb pads (Reebok and Puma), and EVA (New Balance).

Some midsoles can be contoured to the foot and are referred to as more stable, anatomic midsoles.

The effect of shoe midsole composition on the amount of tibial strain produced with walking has been studied by Milgrom et al.³ Their study was designed to test the hypothesis that shoe sole composition can affect the level of bone strain and strain rates that can lead to a stress fracture. The sole materials tested were various polyurethane midsoles and one of polyurethane with embedded air cells. The sole composed of polyurethane with embedded air cells had significantly lower compression and shear strains and shear strain rates. They concluded that the polyurethane sole with the embedded air cells potentially could protect against stress fractures in a walking shoe.