Bekele and Gebreselassie work on this stufff all the time and have done for years. Don't assume that they arent' aware of the things I am saying on this thread, these are very old ideas, older than me, even older than you.
Don't assume that in 20 years time athletes will be lighter, why would they? Bekele isn't the same build as other East Africans with the same height as him.
Bolt has obviously changes his approach to training. His transformation into a 100m runner has shocked other sprinters, especially his ability to start fast, which is a problem for tall sprinters.
Physical talent is extremely common, please don't make inane comments about genetics.
Now on the subject of muscle tension in that book I haven't read, I agree this is very important. The right muscle tension gives us a high power output generating a longer faster stride and also super economical power output. When combined with good endurance, we can do amazing running.
The most important factor here though is the endurance added to the muscle tension, because high muscle tension is easy to generate, but not to sustain. That is where World Records come from and will continue to be so in the future.