How Effective is HIIT?

High Intensity Interval Training (HIIT) is anaerobic training that allows you to cut cardio duration without sacrificing the cardiovascular benefits.  HIIT primarily focuses on anaerobic pathways with numerous HIIT ratios that can be utilized such as a 1:1 (e.g., 30 sec of work to 30 sec of active recovery beginner Interval), a 2:1 (e.g., 20 sec of work to 10 sec of active recovery Tabata or 60 sec of work to 30 sec of active recovery intermediate Interval), or a 3:1 (e.g., 45 sec of work to 15 sec of active recovery advance Interval).  The beauty of HIIT is that it not only cuts your workout duration, but in a lot of ways is way more effective than longer duration endurance cardio, especially when it comes to fat burning.  But, to be effectively performing HIIT you have to be intensely working out in the "work" phase.
During exercise, higher levels of intensity cause an increase in respiration to allow larger volumes of air to move into and out of the lungs to facilitate increased delivery of oxygen and the removal of carbon dioxide.  Simple, basic, you work harder resulting in you breathing harder.  And as with everything, this simple process has a name...minute ventilation or Ve.  There are two deflection points in the otherwise linear increase in Ve and that is VT1 and VT2.  When you are exercising below VT1 you are able to carry on a conversation fairly normally.  This is not very intense exercise and below VT1 is where a lot of long duration, steady state, endurance running/exercising is.  At or just above VT1 is where carrying on a conversation is a little more difficult and speaking is slightly uncomfortable.  Exercise just below VT2 represents the highest intensity an individual can sustain for approximately 20-30 minutes.  Speaking is very broken and choppy just below VT2 and not at all at or above VT2.  Intensely working at VT2 and above is where HIIT should be on the "work" phase.  Active rest should be between VT1 and VT2 allowing your body to recover from the work phase without lowering your heart rate too much. 

Recent research shows that the cardiovascular adaptations that occur with HIIT are similar, and in some cases superior, to those that occur with continuous endurance training (Helgerud et al. 2007; Wisloff, Ellingsen & Kemi 2009).