|Bukus high hurdle 1969|
Some people have very fast reaction times, some people can activate are large percentage of their muscles, and some people can have very fast reaction times AND activate a large percentage of their muscles. Combine that with great body proportions for your sport and you have a beast like Dick Butkus.
You might be asking then "Why are athletes today so much more athletic and explosive?" It's because they have a lot more muscle mass than athletes of the past. Adding muscle to your natural ability will just increase that explosive potential that you were born with. If you took any athlete from the past and packed on 20 or 30lbs of muscle on them, they will just as explosive of any athlete of today.
What's this all mean? It means don't waste your time with silly exercise programs that claim to increase your explosiveness. It's gonna do much more harm than good because of the wear and tear on your muscles and joints. Your time is better spent working out properly to get stronger muscles and then working on your skill level of the sport you play.