I've actually already started implementing circuit training in workouts, because I have been looking for some better metabolic work, and I must say that the bits and pieces I have incorporated so far are pretty challenging and fun. (dieselcrew.com)
If you want to breathe hard, do metabolic work, or train your cardiovascular system, then you can swim, bike, run, row, or do another form of interval training. (bengreenfieldfitness.com)
most people are enamored with crossfit, but it's mostly just metabolic work with no rest. (marksdailyapple.com)