We have long known that exercise is good for us. Hippocrates declared over 25 centuries ago that "if we could give every individual the right amount of nourishment and exercise, not too little and not too much, we would have found the safest way to health." There is now a wealth of evidence indicating that the benefits of physical activity extend across multiple organ systems and span the age spectrum.(1,2) Perhaps most compelling are data highlighting the potential of exerciseto prevent and attenuate chronic conditions related to insulin resistance and atherosclerosis.(3,4) Despite nationwide recommendations encouraging regular exercise for all adults, fewer than 1 in 3 Americans engage in routine physical activity.(5) Low adherence rates could relate in part to the fact that we still do not really understand how exercise works. For this reason, recommendations remain non-specific with respect to how frequent, how intense, how long, what type, and for whom exercise should be prescribed.(4).