Every Wednesday, Gretchen Reynolds singlehandedly influences how millions of Americans work out. In her Phys Ed column for The New York Times, she debunks myths, spurs conversation, and creates arguments among her readers by questioning widely held beliefs about exercise.

Expanding upon her popular columns, Reynolds tackles the questions we all have and (sometimes) ask about exercise. Consulting experts in physiology, biology, psychology, neurology, and sports, she uncovers how often we should exercise, how long workouts should be, how to avoid injury, and how to find the right form, routine and equipment for our goals.

She also reveals some surprising answers, like:

* 20 minutes of cardio at a time is enough to obtain maximum health benefits. (And in some cases, just six minutes is all you need.)

* Stretching before a workout is counterproductive. (It's better to just start easy, i.e., walk before you run.)

* Core strength is nice but not necessary. (A six-pack looks great but actua