Athletes have long known that caffeine boosts athletic performance by increasing muscle endurance, but until now, knowing how much to consume was largely a guessing game. Dr. Rob James, lead researcher at Coventry University in central England, has found the answer: Ingesting 6 milligrams of caffeine for every 2.2 pounds of body weight, just prior to exercise, increases muscle output in the legs up to 6 percent. For a 154-pound person, that's two cups of coffee. In researching mice and applying the results to humans, James was able to identify what, as he puts it, "could be the difference between an Olympic gold medal and an also-ran." Ingesting any less than that, he says, had no effect on performance.