Caffeine is a powerful "ergogenic agent," meaning it promotes the ability of muscles to work. Studies show that caffeine boosts performance in both very short and very long athletic events, said Graham.
It used to be thought that caffeine worked by stimulating the release of sugar (glycogen) in muscles, but recent research suggests it helps muscles release calcium, allowing muscles to contract with more force. It takes only a medium cup of regular coffee for a 130-pound athlete to see a measurable improvement in performance, Graham added.