[CoD2] If user press F then sprint

[Toxys]

How can I change the player's speed if he press F key? I know how to detect if player pressed F but I can't change the player's speed:

		self endon("disconnect");

while(1)
{
    if(self useButtonPressed())
    {
    self iprintlnBold("Sprint activated!");
    }
   wait 10;
}