high intensity training means that u train for less time at a higher intensity, meaning that u put more stress on a muscle in a short period of time than it can handle
personally i've never tried these but i hear they take a lot out of you, after u do it u need more recovery time than a regular workout
I do it on off days, Sprint for 1 min, jog for 2, walk for 2 and repeat 3 times. Im pretty much dead after doing it.
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"You've got to block out all distractions when you train. Your focus has to be 100% into the rep. You've got to get into a zone. You know you're in the zone when guys in the gym look you in the eye and then quickly turn away 'cause they see the fire. You've got to be all business." - Dorian Yates