Almost every expert we spoke with recommended high intensity interval training for those who want to get fit fast. Why? Because HIIT workouts produce a big calorie burn in a short amount of time. Just because it’s short doesn’t mean it’s easy, though. Interval training will save you time, but it also requires that you put in three or four times the effort during the short 20- to 30-second bursts of movement that the workouts are made up of. “Do shorter HIIT sessions for 15-20 minutes,” says ACSM certified personal trainer Aileene Palm. “Combine cardio moves like jumping jacks and jumping rope with compound resistance training exercises that target more than one muscle, like squats and push-ups.” Certified Strength & Conditioning Specialist Lisa Reed, M.S. recommends four-minute long Tabata style workouts which she says incorporate high intensity interval training to improve cardio and strength fitness. Try one of her workouts here.