In the comics, superspeed,- being the ability to move (in particular, run) at speeds typically at least as fast as something like a racecar), whenever it is shown, seems to tend to have the explanations as to how it works being something along the lines of:
- That it's a relative of the power of superstrength, or
- That it works by having interdimensional lightning flow through one's body, letting him break the laws of space and time.
Trying to come at it from a less vague point of view, though, if we (all impossibilities aside) start with the assumption (for the sake of argument) that a character has the power of superspeed as defined above, what's a more detailed and plausible-sounding explanation for it? Would increasing muscle strength (albeit quite exponentially) be all it would take? Would he have to be a cyborg who has high RPM servos instead of regular joints? The less "magic" an explanation requires, the better.
P.S.: Bonus points for explanations as how the other side of superspeed so commonly portrayed in comics could work,- being the ability to think so fast that you perceive everything around you as moving in slow-motion.