I don't mean mental reaction time in the sense of quickly weighing possible courses of action -- I mean literally the time it takes for you to register a change. The simple reaction-time experiment is to hold a ruler in front of someone, so it hangs down; they have a hand wrapped around (but not touching) the end of the ruler. You let go, they grab it, you measure how far it fell to see how fast they reacted.
The thing is, some people will start to grab it fast (high mental reaction time) and other people will actually touch it sooner (slow mental reaction time, faster physical reaction time). If you're running in a zig-zag pattern, this would mean that you would start turning faster, though other people might be able to execute a complete turn (start to finish) faster despite lower IQ.
And I would guess that the larger the muscles involved, the more important physical reaction is. So if you're talking about moving your fingers (as in the ruler example), then mental reaction is going to be more important, but if you're moving your whole arm or your legs, the physical reaction is more important.
The thing is, some people will start to grab it fast (high mental reaction time) and other people will actually touch it sooner (slow mental reaction time, faster physical reaction time). If you're running in a zig-zag pattern, this would mean that you would start turning faster, though other people might be able to execute a complete turn (start to finish) faster despite lower IQ.