High Intensity Interval Training is becoming more and more popular these days.  You can see it with the boom of Crossfit.  People are getting more into HIIT because they say you get more out of their workout.  Getting better results in burning calories and weight loss while building muscle.  Here is an introductory video on HIIT.  Go ahead and watch it!  They call it the  20 minute work out, can you fit that into your life?