characteristics influence the interaction between the surface and shoe, which in turn influences the loading and moments that occur at the ankle, knee and hip joints. As an example, Figure 2 depicts the different knee moments associated with two similar infill surfaces but differing in friction. Surface 1 had about a 30% higher translational friction coefficient (m = 1.79) than Surface 2 (m = 1.36). Surface 1 resulted in 17 - 23% higher knee joint moments than Surface 2, which are likely due to the increased friction associated with the second surface.


A few epidemiological studies have


linked lower extremity injuries to high rotational resistance between shoe and surface. One study showed that injuries were more common under conditions where there was high rotational traction between the surface and shoe. Another study found a higher ACL injury rate with shoes that had higher torsional resistance. These studies provide important evidence that high rotational friction between shoe and surface is associated with injuries and suggest that lower rotational traction should be the goal. There is some further evidence for associating injuries to the friction between the surface and the shoe. It has been shown for tennis that those surfaces that allowed sliding (clay and synthetic sand, an artificial surface with loose granules) had three to five times less


Interaction


injuries than those surfaces that did not allow sliding (asphalt, synthetic surface, felt carpet and synthetic grill). There are two important frictional aspects when considering different sport surfaces: translational and rotational friction. In each case it is important to have an appropriate amount of friction. The friction between the surface and the shoe must be high enough to allow appropriate performance such as fast acceleration and quick directional changes, however, if the friction is too high the potential for high torsional and bending moments and injury exists. The optimal window for translational friction is difficult to determine and requires additional research. Similarly, optimal rotational friction characteristics are currently unknown. However, it is speculated that in general high rotational friction should be avoided. Also, it is important to understand what activities will be performed and what footwear will be used when selecting appropriate surface characteristics.


Performance


Each step, stride, jump, landing, etc. of an athlete requires the athlete to expend a certain amount of energy. If some of this energy can be reused, through energy return from the surface, the athlete can perform the same movement more efficiently. If the athlete expends the same amount of effort and performs


27


the same amount of work, his performance will be increased if energy is returned from the surface. As the athlete contacts the sport surface, energy is transferred from the athlete, through the foot and shoe into the surface. As the athlete leaves the surface, some of this energy can flow back in the opposite direction from the surface to the athlete. Thus, energy transfer to and from the surface can have a large influence on athletic performance. The magnitude of the energy returned from a sport surface to an athlete is a function of the amount of energy input into the surface minus the energy lost and is influenced by some physical constraints as well as different material and structural characteristics of the surface. (see Figure 3)


The work performed by the athlete on the surface results in deformation energy being input into the surface and is a function of the contact force. Large forces are exerted by athletes during sporting movements. Even simple movements like jogging can produce forces of over two times body weight and peak magnitudes can reach over ten times body weight for more intense activities like running jumps. The larger the force, the greater the potential for energy storage in the surface. Forces that athletes exert on surfaces are necessary for energy storage in the surfaces, however, the actual magnitude of the stored energy depends on the properties


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