Some fitness fads come and go (we see you, Shake Weight), but others, like yoga or Pilates, aren’t going away anytime soon. Another trend that’s definitely here to stay? High-intensity interval training, or HIIT, which involves short bouts of extreme effort alternated with periods of rest. “HIIT workouts are quick and dirty, never lasting more than 30 minutes including a warm-up and short cool-down,” explains Gina Harney, certified personal trainer and creator of Fitnessista.com. “You’re working as hard as possible for a short amount of time and then reaping the rewards throughout the day.” While there’s no such thing as a fitness miracle, HIIT workouts come pretty darn close. Research suggests that HIIT can super-charge fat burning , boost metabolism , and improve cardiovascular fitness . Plus, the vigorous nature of HIIT ensures you’ll continue to torch even more calories for hours after your workout ends. Bottom line: HIIT is quick, effective, and the perfect way to train for anyone who’s short on time and wants to see results stat. (And who isn’t and doesn’t?) Intrigued? Get started with this exclusive upper-body workout from Harney’s new book HIIT It! In less than 30 minutes, these highly effective circuits will target and strengthen every muscle above your waist. All you need is a set of medium-weight dumbbells and your own bodyweight. To warm up, perform 5 minutes of moderate cardio (such as jogging, jumping rope, or cycling). Complete 3 rounds of circuit 1, then rest for 30 seconds to 1 minute. Boost your heart rate again by running or walking 1/4 mile. Then complete 3 rounds of circuit 2, followed by a cool-down stretch—and you’re done. To read more click here!