Posted by Steve Magness
That should satisfy most, but I know many are lookingfor a complex/scientific explanation. Since I'm lazyand don't feel like going into a big spiel onmechanics (maybe later), I'll give way to a recentarticle that I read that basically asks the questionof why certain athletes can run faster/slower compared to their track times. It's by Renato Canova and was published in New Studies in Athletics. Here are thepertinent excerpts in regards to mechanics and CC:
"The first quality that needs to be investigated isthe elastic reactivity of muscle fibres. As we know reactivity can be invoked when there is a firmsupport. On a very hard surface, the amortization phase of the running stride, the subsequent loading phase and then the elastic response are totally dependent on the athlete's muscle fibres, since the running surface absorbs little or no energy at all.This situation favours athletes with high elasticity and allows them to enhance technical efficiency, reduce energy cost and so maintain high speeds forlonger periods. When the running surface is elastic, for instance in the case of indoor tracks built on raised panels, the amortization phase is longer, the reactive response is delayed and its intensity reduced. Efficiency is not reduced, however, because the surface itself returns the stored elastic energy,thus creating a rebound effect on the touch down foot.
The situation is very different when, as is often thecase in cross-country races, the running surface absorbs energy but is not reactive. The foot sinks into the soft, muddy surface and elastic strength, both from the athlete's reactivity and the soil'srebound effect, becomes ineffective. In the case ofmost middle distance runners, a high muscle reactivity corresponds to a low general strength level, because the elastic component of the muscle is more developed than the contractile component. Consequently, the better class athletes tend to be handicapped, as opposed to athletes having greater strength and less elasticity - those who use a "low", almost "pulling", action."