While you're right about humans not being the fastest animals on the planet, adding power from the upper body does not increase the power available. The muscles in the legs and glutes are our largest, strongest muscles. An athlete can max out his aerobic capacity using only his legs; so using additional muscle groups to generate power will decrease the wattage available to the legs by an equal amount.

Interestingly, a trained human runner can run a horse to death. The horse is faster, but must rest between sprints while the human draws closer and closer. The race doesn't always go to the swift.