$1.99 from Conceited Software 
Cardio is boring. High-Intensity Interval Training (HIIT) is proven to burn up to 9 times as much bodyfat than regular cardio.
Interval Trainer is the perfect HIIT companion: it comes with a set of pre-defined workouts all using standard warm up, cool down, low intensity and high intensity interval durations.
Once the workout has started, Interval Trainer will not only keep track of the different intervals/parts of your workout, it will play different sounds and vibrate your iPhone when entering a cool down/warm up interval, a low or high intensity interval and upon completion of the workout.
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