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Less time in the gym doesn't mean you have to sacrifice fitness if you know this secret: interval training. Research shows that interval training — workouts in which you alternate periods of high-intensity exercise with low-intensity recovery periods — increases fitness and burns more calories over a short period of time than steady-state cardio (you know: just doing the same thing for your whole workout time).

So how do you get the most out of interval training, and how long should each push and recovery be? One of the many great things about interval training workouts is that there's no single hard-and-fast rule. Different lengths of work and recovery bring different benefits — and they're all good. (There are even HIIT apps.)