I agree about it all being genetics. I've never heard the term "twitch" but I do know what you're talking about. Typically runners, basketball, tennis players, anything that requires a lot of endurance have a completely different type of muscle than body builders, power lifters, gymnists, football or baseball players would have which require a lot of explosive strength. As a rule of thumb, you can often tell by the shape of the muscle. Typically endurance muscles are somewhat short, narrow and have a longer tendon. While not particularly strong, they can go all day. Explosive muscles tend to be longer, wider and have shorter tendons. In other words they seem to stretch all the way to the joint. They're not as fast and don't have the endurance to run distances, but they can run sprints pushing a truck. Its not impossible for someone to do both, but they will always excel at the one best suited for their muscle type. But it really does always come down to genetics.

That probably a lot more info than you wanted. Please excuse.