Your brain can't (currently) handle any faster muscles than we have. Climbers often experience something called "sewing machine leg" when they're tired; it's a case of our nervous system not being fast enough to control our leg muscles so you get an oscillation (a characteristic of systems that do not have fast enough feedback, or do not have properly damped feedback.)
Our bones and ligaments can't handle much more in terms of pulse loading than they do now.
You'd rupture blood vessels with the G forces needed to do that.
And your peripheral nervous system, and skeletal system, and ligaments. And your circulatory system to handle the heavier loads, which means higher volume heart and more capable lungs. Which means a bigger chest and a faster metabolism, which means a modified digestive system to process food faster and better islets of Langerhans to better regulate the higher levels of blood sugar.
It is _very_ hard to change just one thing.