" Next comes a husky boy in baggy shorts. “Bring it on in, Doug,” Duncan says. “What’d you get?” “Nine minutes.” “Flat?” “Yeah.” “Nice work.” When Michelle and Krissy finally saunter over, Duncan asks for their times, but Michelle’s watch is still running. Apparently, she didn’t hit the blue button. Krissy did, though, and their times are the same. She holds up her wrist for Duncan. “Ten twelve,” he says, noting the time on his clipboard. What he doesn’t say is “It looked like you two were really loafing around out there!” The fact is, they weren’t. When Duncan downloads Michelle’s monitor, he’ll find that her average heart rate during her ten-minute mile was 191, a serious workout for even a trained athlete. She gets an A for the day. "

― John J. Ratey , Spark: The Revolutionary New Science of Exercise and the Brain