Per the NY Times:
"Now, Dr. Tarnopolsky and others report that caffeine increases the power output of muscles by releasing calcium that is stored in muscle. The effect can enable athletes to keep going longer or to go faster in the same length of time. Caffeine also affects the brain’s sensation of exhaustion, that feeling that it’s time to stop, you can’t go on any more. That may be one way it improves endurance, Dr. Tarnopolsky said.
The performance improvement in controlled laboratory settings can be 20 to 25 percent, Dr. Tarnopolsky said. But in the real world, including all comers, the improvement may average about 5 percent, still significant if you want to get your best time or even win a race."
Time to ban all that coffee and soda from the clubhouse now too, right?