Sagittal
Frontal
Transverse
Figure 1: Examples of resultant ankle and knee joint moments (torque). The sagital view shows flexion and extension moments at the ankle and knee. The frontal view depicts ad-abduction moments at the knee and in-eversion moments at the ankle. The transverse view shows internal-external rotation moments at the knee and ad-abduction moments at the ankle.
of the surface. Energy storage is a function of surface stiffness and surface deformation
As a surface is loaded, it undergoes deformation and energy is input into the surface. However, as it is unloaded, some deformation, and therefore some energy, remains in the surface due to the time- dependent properties of the materials. Thus, in returning to its original shape, the work done by the surface on the athlete is less than the work done by the athlete to deform the surface. This energy that is dissipated in the surface is a material property that is common to all surfaces and can not be avoided. However, the magnitude of the energy dissipation can be influenced. Energy loss can vary widely between
different types of surfaces and even within surfaces constructed for similar purposes. Using drop tests, the majority of the energy input into an infilled surface is lost (about 85%). The sand/rubber mixture in the infilled turf surface absorbs a lot of energy and is not well suited to store and return energy. The energy lost from point elastic surfaces is lower than infilled surfaces (40-75%). There are large differences, however, between point elastic surfaces
Figure 3: Energy input, returned and lost in a sport surface. In each case the shaded region depicts the magnitude of the energy.
Figure 2 below: Measured knee joint moments (torque) for a subject performing a 180° rotational movement on two different surfaces. The magnitude of the knee joint loads are schematically represented by the size of the spheres and the percent difference between the two surfaces is also shown.
even when they are constructed for the same installation.
The absolute magnitude of energy loss measured during mechanical drop tests must be interpreted cautiously. Both mechanical and subject tests have been used to try to quantify energy return of sport surfaces. However, the correlation between the two methods is often low. In fact, the energy lost during mechanical drop tests may overestimate the energy lost during actual sport activities because the mechanical tests do not allow the surface to respond quickly enough, whereas the longer stance times provide sufficient time for the surface to almost fully expand to its original thickness. Energy return is only relevant if the magnitude is large enough to have an influence on performance. Typically, about 15-60% of the energy that is input into sport surfaces is returned. These magnitudes are in many cases substantial enough to influence athletic
performance. It has been estimated that the mechanical energy required for a forefoot running stride is 182 joule. A storage of 12 joule of energy in a sport surface represents about 6.5% of the energy required. Even if 50% of the energy is lost, a return of 6 joule is still
substantial representing over 3% of the mechanical energy per stride. If all of this energy returned aids performance, a 3% increase in performance relates to about 0.3 seconds in an elite level 100m race. This description is an oversimplification and it is unlikely that all of the returned energy goes directly into athletic performance, however, it demonstrates the importance of energy return from sport surfaces. In fact both empirical and theoretical studies have shown that energy storage and return in sport surfaces can result in real performance improvements.
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