While it's tempting and natural to follow the reasoning that it must be something special that they do that results in their having legs like this, actually it's sort of like noticing that top scientists in the particle physics field have high IQ's, and assuming that their working with particle accelerators, or something else associated with their work, boosted their IQ.
Rather, if they had the genetics for low IQ they wouldn't BE top scientists in particle physics. So it's no wonder that all of them have exceptional intelligence.
Similarly, a person with genetics for weak and small legs is not going to rise to the top in this sport.
It should not be assumed that if you follow their training methods, your legs would be better than they are now. They might be smaller than they are now.