Many people will tell you that coffee is generally “a bad thing” before exercise. It dehydrates the body, causes false alertness and overall, doesn’t help matters.
However, studies have shown that the dehydration doesn’t actually happen as a result of coffee. It usually occurs as a result of things “getting moving” and electrolytes being lost. In fact, coffee (and caffeine in other forms) can actually improve your performance, both during a workout and when in recovery afterwards!
Provided it is consumed at the best time, and in the right way, caffeine can improve physical and mental performance, improving total distance and sprint speed in runners, for example.
Take a look at this Runners World article for more information.