MY husband and I were riding our bikes not long ago, and when we were about a mile from home, we did our usual thing. We call it the sprint to the finish: ride as hard and as fast as we can until we reach our driveway, racing to see who could get there first. We pulled up, slammed on our brakes and hopped off our bikes. A neighbor was walking by and said, "How did you do that?" "I just put on my brakes," I told him. No, he said, he meant how could we just stop like that without cooling down? The cool-down, Dr. Tanaka said, "is an understudied topic." "It's an idea we can't get rid of," he said.