Runkeeper was placed on Earth by supreme beings of some sort. Its programmable, GPS-tracked interval runs make the iPhone into a serious, and entirely thoughtless, training tool. But Runkeeper’s ...

You’re looking to push every ounce of energy you have into your current HIIT (High-intensity Interval Training) workout. As the intervals pass you buy, the music constantly changes to fit your tempo.

We like interval training because you can pack a lot of effective exercise into a short amount of time. In fact, you can use it to do one of the most effective routines in only 20 minutes. Regardless ...