In gyms everywhere, high-intensity interval training replaced hour-plus cardio sessions. The prevalence of these short, full-body circuits is most evident in the popularity of CrossFit's intense, minutes-long "workouts of the day." "Shorter workouts allow greater intensity," says Ryan Halvorson, senior editor of 'IdeaFitness Journal.' "That's why marathoners can be gaunt or plump, while avid CrossFitters tend to be lean and muscular."